Tag: charging

Tech Tips

Testing, Recovery and Charging a Sealed System

October 16, 2018 By No Comments

There are many appliances with compression refrigeration circuits that do not have ports installed for testing, recovery, charging and evacuation.  These can include window units, PTACs, fountains, refrigerators and much more.

This presents a challenge any time you suspect or know there is a refrigerant circuit issue. How can you diagnose and make a repair when you can’t connect?

First, when an appliance is a sealed circuit DONT OPEN IT UNLESS YOU HAVE A GOOD REASON TO DO SO.

If you do need to open the circuit, first look for factory “process tubes” often on the compressor or on the lines near the compressor.

You can then either add a piercing valve to the tube or you can often pinch off the tube with a pinching tool, Leave the tool in place, cut the process tube/stub near the end and solder / braze in a Schrader to the end.

Common pinch off tools

Once the new port is added you simply remove the pinching tool and form it slightly back into shape. It does not need to be completely re-rounded because the size of these types of systems will generally be small so recovery speed will not be a huge factor.

You can then recover the charge and once the charge is fully recovered you can add in additional soldered/brazed in ports.

Another method is to use a piercing line tap valve to access the system like the ones shown below.


These should also only be left on as a temporary measure to get the refrigerant recovered. You then must repair the “pierced” hole, usually by putting a tee schrader in the same spot where the piercing was.


Once you have good, solid access points in place (don’t forget to flow nitrogen), now you can proceed to evacuation, weighing in a factory charge and then performing further diagnosis.


If done correctly the appliance will have no leaks at the ports and will be much easier to service next time.

Keep in mind when working on systems with very small charges that hoses can hold a lot of refrigerant. You may consider using Smart Probes with tees and only one short 1/4″ hose from the charging tank to the suction probe to reduce losses.

Also make sure the hose is well purged before starting the charging process. Keep the air and moisture out.

— Bryan

Tech Tips

What Should My Subcooling Be?

August 24, 2018 By 4 Comments

 

Testo 570 Premium Manifold

This is the article you read BEFORE you call and ask a senior tech what your subcool should be, or the one you send to a junior tech when the call and ask you.

So what is subcooling? (or subcool as many call it)

Subcooling is a measurement of temperature DECREASE of a liquid below its saturation (mixed liquid/vapor) temperature at a given pressure.

For example, water boils at 212° Fahrenheit at sea level (atmospheric pressure of 14.7 PSIA). If water is at 212°f and at atmospheric pressure at sea level you can be sure it is at saturation, which means it is either in the process of boiling or condensing. If you measure that same water and it is at 202° you can be sure that it is fully liquid and that it is no longer in the process of either boiling (changing from liquid to vapor) or condensing (Changing from vapor to liquid). Because the water is at 202°  instead of 212° we know it is liquid and we can also say it is subcooled by 10°. This 10° of subcooling PROVES that not only is it fully liquid but that it has given up more sensible heat energy enough to drop 10° below the boiling temperature at that pressure.

With refrigerant, we measure the subcooling between the condenser and the metering device and it gives us a lot of information. It not only tells us whether or not the line is full of liquid it gives us indications of refrigerant charge as well as condenser efficiency when viewed in conjunction with the condensing temperature (high side saturation temperature). Now be careful, like with all measurements, it is only as accurate as your tools, it must be taken using liquid line pressure and temperature (Line between the condenser and metering device) NOT discharge line pressure and temperature (line between the compressor and the condenser) AND you must have a good connection to the port. I can’t tell you how many times green techs have called me with “crazy” readings only to find out their hose was not depressing the Schrader core fully.

So what should it be?

Generally speaking 10° – 12° of subcooling at the outlet of the condenser coil is most common but you must look for the proper design subcooling for the particular system you are working on. Some systems will require subcooling readings of up to 16° for maximum efficiency and capacity.

Many techs will say that subcooling  is how you “set a charge” on a TXV / TEV / EEV metering device system

Subcooling is one of many factors you consider when setting a charge but you first need to make sure that your equipment is properly matched with the correct metering device. The air flow is set in properly, the blower, air filter, condensing coil and evaporator coils are clean and WHENEVER adding or removing charge use a scale so you can monitor your progress.

While it is true that subcooling is the primary charging measurement on a TXV /TEV / EEV system, subcooling is important to check on every system, every time you connect (whenever possible).

Negative Subcooling isn’t possible if the liquid line temperature and pressure are taken at the same point. What is possible is to have a miscalibration of your tools that make a zero subcooling look like a negative subcooling.

Zero Subcooling means that the refrigerant in the liquid line is a mix of liquid and vapor, this is not an acceptable condition except in cases where the system is designed to inject discharge gas into the liquid line on purpose to increase liquid pressure (headmaster).

Low Subcooling is an indication that not enough refrigerant is contained or “packed” in the condenser. This can be due to undercharge, poor compression, or a metering device oversized or failing open (overfeeding).

High Subcooling is an indication that more than the designed amount of refrigerant is “Backing up” or “packed” into the condenser.  This can be caused by overcharge, restriction (such as a contaminated line drier or kinked liquid line) or an undersized or failing closed metering device.

Keep in mind, the subcooling can often read in range on a system that still has issues. Many times this is because the previous tech simply “set the charge” by subcooling without fully testing all aspects of the equipment such as airflow.

— Bryan

Tech Tips

What Should My Superheat Be?

August 21, 2018 By 2 Comments

The most common and often most frustrating questions, that trainers and senior techs get goes something like this. “What should my ______ be?” or “My _____ is at ______ does that sound right?

Usually, when the conversation is over both the senior and junior techs walk away feeling frustrated because the junior tech just wanted a quick answer and the more experienced tech wants them to take all of the proper readings and actually understand the relationships between the different measurements.

In this series of articles we will explore the, “What should my _______ be?” questions one at time and hopefully learn some things along the way.

So what should the superheat be?

First, what is superheat anyway? It is simply the temperature increase on the refrigerant once it has become fully vapor. In other words, it is the temperature of a vapor above it’s boiling (saturation) temperature at a given pressure.

The air around us is all superheated! Head for the Hills!

How can you tell that the air around us is all superheated? Because the air all around us is made of vapor. If the air around us were a mixture of liquid air and vapor air, first off you would be dead and secondly, the air would be at SATURATION. So the air around us is well above its boiling temperature (-355° F) at atmospheric pressure which means it is fully vapor and SUPERHEATED. In fact, on a 75-degree day, the air around you is running a superheat of 430°

But why do we care?

We measure superheat (generally) on the suction line exiting the evaporator coil and it helps us understand a few things.

#1 – It helps ensure we are not flooding the compressor

First, if we have any reading above 0° of superheat we can be certain (depending on the accuracy and resolution of your measuring tools) that the suction line is full of fully vapor refrigerant and not a mix of vapor and liquid. This is important because it ensures that we are not running liquid refrigerant into the compressor crankcase. This is called FLOODING and results in compressor lubrication issues over time.

Image courtesy of Parker / Sporlan

#2 – It gives us an indication as to how well the evaporator coil is being fed

When the suction superheat is lower it tells us that saturated (boiling) liquid/vapor mixture is feeding FURTHER through the coil. In other words, lower superheat means saturated refrigerant is feeding a higher % of the coil. When the superheat is higher we know that the saturated refrigerant is not feeding as far through the coil. In other words higher superheat means a lower % of the coil is being fed with saturated (boiling) refrigerant.

The higher the % of the coil being fed the higher the capacity of the system and the higher the efficiency of the coil.

This is why on a fixed orifice system we often “set the charge” using superheat once all other parameters are properly set. Adding refrigerant (on a fixed orifice / piston / cap tube) will feed the coil with more refrigerant resulting in a lower superheat. Removing refrigerant will increase the superheat by feeding less of the coil with saturated (mixed liquid and vapor) refrigerant.

This method of “setting the charge” by superheat does not work on TXV / TEV / EEV systems because the valve itself controls the superheat. This does not negate the benefit of checking superheat, it just isn’t used to “set the charge”.

#3 – We can ensure our compressor stays cool by measuring superheat

Most air conditioning compressors are refrigerant cooled. This means that when the suction gas (vapor) travels down the line and enters the compressor crankcase it also cools the motor and internal components of the compressor. In order for the compressor to stay cool, the refrigerant must be of sufficient volume (mass flow) and low temperature. Measuring superheat along with suction pressure gives us the confidence that the compressor will be properly cooled. This is one reason why a properly sized metering device, evaporator coil, and load to system match must be established to result in an appropriate superheat at the compressor.

#4 – Superheat helps us diagnose the operation of an active metering device (TXV / TEV/ EEV)

Most “active” metering devices are designed to output a set superheat (or tight range) at the outlet of the evaporator coil if the valve is provided with a full liquid line of a high enough pressure liquid (often at least 100 PSIG higher than the valve outlet / evaporator pressure). Once we establish that the valve is being fed with a full line of liquid at the appropriate pressure we check the superheat at the outlet of the evaporator to ensure that the valve itself is functioning properly and /or adjusted properly. If the superheat is too low on a TEV system we would say the valve is too far open. If it is too high the valve is too far closed.

#5 – Superheat is an indication of load on the evaporator 

On both TEV / EEV systems and fixed orifice systems (piston / cap tube) you will notice that when the air (or fluid) going over the evaporator coil has less heat, or when there is less air flow (or fluid flow) over the evaporator coil the suction pressure will drop. However, on a TEV / EEV system as the heat load on the coil drops the valve will respond and shut further, keeping the superheat fairly constant. On a fixed orifice system as the load drops so will the superheat. It can drop so much on a fixed orifice system that when the system is run outside of design conditions the superheat can easily be zero resulting in compressor flooding.

When the load on the evaporator coil goes up a TEV / EEV will respond by opening further in an attempt to keep the superheat constant. A fixed metering device cannot adjust, so as the heat load on the coil goes up, so does the superheat.

When charging a fixed orifice A/C system you can use the chart below to figure out the proper superheat to set once all other parameters have been accounted for or you can use our special superheat and delta t calculator HERE

Using this chart requires that you measure indoor (return) wet bulb temperature so that the heat associated with the moisture in the air is also being accounted for as well. This is one of MANY target superheat calculators out there, you can use apps, sliderules etc… Here is ANOTHER ONE

Remember, this chart ONLY applies to fixed orifice systems.

So what should your superheat be in systems with a TEV / EEV? The best answer is… like usual… Whatever the manufacturer says it should be.If you really NEED a general answer you can generally expect

High temp / A/C systems to run 6 – 14 degrees of superheat

Medium Temp  – 5-10 

Low Temp – 4-10

Some ice machines and other specialty refrigeration may be as low as 3 degrees of superheat

When setting superheat on a refrigeration system with any type of metering you often must get the case / space down close to target temperature before you will be able to make fine superheat adjustments due to the huge swing in evaporator load. Once again, refer to manufacturer’s design specs.

— Bryan

P.S. – Trutech has a really great resource on charging best practices

Try out our new, simple superheat calculator for fixed metering A/C systems

Tech Tips

The “Approach” Method of Charging and Condenser Efficiency

August 12, 2018 By No Comments


We have been discussing a lot of methods for checking a refrigerant charge without connecting gauges over the last few months. This got me thinking about the “approach” method of charging that many Lennox systems require.

Approach is simply how many degrees warmer the liquid line leaving the condenser is than the air entering the condenser. The approach method does not require gauges connected to the system but it does require a good temperature reading on the liquid line and suction line (Shown using the Testo 115i clamp and 605i thermo-hygrometer smart probes).

When taking an approach reading make sure to take the air temperature in the shade entering the coil and ensure you have good contact between your other sensor and the liquid line.

The difference in temperature between the liquid line and the outdoor temperature can help illustrate the amount of refrigerant in a system as well as the efficiency of the condenser coil. A coil that rejects more heat will have a leaving temperature that is lower and therefore closer to the outdoor temperature. The liquid line exiting condenser should never be colder than the outdoor air, nor can it be without a refrigerant restriction before the measurement point.

Here is an approach method chart for an older 11 SEER Lennox system showing the designed approach levels.

While most manufacturers don’t publish an approach value, you can estimate the approach by finding the CTOA (Condensing Temperature Over Ambient) for the system you are servicing and subtracting the design subcooling.

6 – 10 SEER Equipment (Older than 1991) = 30°F(-1.11°C) CTOA

10 -12 SEER Equipment (1992 – 2005) = 25°F(-3.88°C) CTOA

13 – 15 SEER Equipment (2006 – Present) = 20°F(-6.66°C) CTOA

16 SEER+ Equipment (2006 – Present) = 15°F(-9.44°C) CTOA

I did this test on a Carrier 14 SEER system at my office so the CTOA would be approximately 20°

Then Find the design subcooling. in this case, it is 13°F(7.15°K)

Subtract 13°F(7.15°K) from 20°F(11°K) and my estimated approach is 7°F(3.85°K) +/- 3°F(1.65°K). I used the Testo 115i to take the liquid line temperature and the 605i to take the outdoor temperature using the Testo Smart Probes app and I got an approach of 4.1°F(2.25°K) as shown below.

More than anything else, the approach method can be used in conjunction with other readings to show the effectiveness of the condenser at rejecting heat.

If the system superheat and subcooling are in range but the approach is high (liquid line temperature high in relation to the outdoor air), it is an indication that the condenser should be looked at for condition, cleanliness, condenser fan size and operation and fan blade positioning. If the approach is low it can be an indication of refrigerant restriction when combined with low suction, high superheat and normal to high subcooling.

If the approach value is low with normal to low superheat and normal to high suction pressure and high subcooling it is an indication of overcharge.

The approach method is only highly useful by itself (without gauges) on a system that has been previously benchmarked or commissioned and the CTOA and subcooling or the approach previously marked, or on systems (like Lennox) that provide a target approach specific to the model.

— Bryan

Podcast Episodes, Tech Tips

Low Ambient, Condenser Flooding and Headmaster Valves Podcast Companion

April 19, 2018 By 2 Comments

Download the podcast Directly HERE

As always if you have an iPhone subscribe HERE and if you have an Android phone subscribe HERE

Condenser Flooding / Motormaster Podcast Companion

This article and podcast is courtesy of Jeremy Smith, one of the most knowledgeable and helpful refrigeration techs I know.

It’s my feeling that, no matter how well explained, this topic really requires a treatment that is more in depth and one that can be absorbed slowly with the ability to continually return and re-read certain sections to allow for best understanding of the subject matter.

As discussed in the podcast, as the outdoor temperature drops, the capacity of the condenser increases dramatically causing it to be, essentially, oversized for normal operation.   To counteract that, we use a valve (headmaster) or valves (ORI/ORD) to fill the condenser with liquid to effectively reduce the amount of coil that is actively rejecting heat and condensing refrigerant.   This also maintains a high enough liquid pressure feeding our TEV.   This prevents wild swings in TEV control because it is a pressure operated mechanical device.

First things first, let’s open up Sporlan’s 90-30-1 … seriously go ahead and click it , it will open in another tab so you can go back and forth.

This is a document I reference all the time when dealing with condenser flooding problems.  If you’re tech savvy, save it on your mobile device.  If you’re more of a low-tech guy, listening to a podcast and reading an internet publication on your flip phone or whatever, go ahead and print this out, laminate it and keep it in your clipboard.   Heck, even if you are a high tech guy, sometimes nothing beats a hard copy of this the first few times you work through it.

If ,after the podcast, you haven’t read through this to familiarize yourself with it, take the time to do so.   It seems like a really complicated procedure to work through, and the first few times that you do it on your own, it can be.  With practice, however, you’ll get used to it.

We’ll work through a condenser flooding calculation here in slow time, outlining all the different calculations taken into account.

First lets lay out the basic info we need.  The measurements and counts will vary, of course, depending on the equipment that you have.

If we have an R22 unit, 44 condenser passes ⅜” in diameter each are 38 ¾” long with 42 return bends.   Our evaporator temperature is 20°F, current temp is 35°F and the lowest expected ambient is -20°F.

Now, that seems like a lot of information, but we’ll break it all down.

First, we need to figure the total length of the condenser tubing in feet.   So, we take 44 x 38 ¾ and get 1705” of tubing.   1705 ÷ 12” per foot gives us 142.083 feet of tubing.   Now, that’s just the straight tubing.   We’ve got return bends to account for.

Refer to our Sporlan document.   In TABLE 1, you’ll find an equivalent foot length per return bend.   In the case of a ⅜” return bend, it’s. 2 feet per bend, so 42 x .2 gives us 8.4 feet more.

Add those together for total length of 150.483.  Back to TABLE 1 look in the R22 section under ⅜” tubing and follow the line for -20°F across.   You’ll find a density factor of 0.055.   This number is how many pounds of liquid refrigerant is needed to fill one foot of tubing at that temperature.   So, 150.483 x 0.055.  This gives us 8.28 pounds.  This is the amount required to fill the entire condenser with liquid, but we don’t really need to fill the WHOLE coil….

Back to the document..TABLE 2 this time.

Across the top, find 20° evaporating temp, now follow that down to the -20°F row.   This gives us a percentage.   82%  so, this unit at -20% will have 82% of its condenser filled with liquid.   So let’s take 8.28 x 0.82 to get our flooding charge.

6.78 pounds.

Now, what does this number really mean.   This is the amount of refrigerant we need to add to a system that we’ve JUST cleared the sightglass on when the ambient temperature is 70°F or higher.  If our ambient temperature were 70 degrees or warmer, we could add just that amount past a clear sight glass and walk away, satisfied in knowing that the unit will run properly no matter what the weather throws at it.

Remember, though, that our current ambient is 35°F.   So, now what?

Time to stop.  Get your Sharpie out and WRITE THIS NUMBER DOWN!   Record it on the unit somewhere.  Somewhere easy to see but somewhere that the sun doesn’t degrade the ink over time.   That way, you only have to go through this one time.  If you’re doing a new installation and startup, do the next guy a favor and write both this AND the total system charge down somewhere so that I don’t have to guesstimate the charge when it all leaks out.

Now, let’s go back to TABLE 2 and look at the 35°F row.   We find that at 35°, we need to have 63% flooded.   Well, we’ve got a clear sight glass and it’s 35° ambient so, we’re already 63% flooded.

Since the most we need is 82% flooded, 82%-63% gives 19% so, we take our total, 8.28 x 0.19 to get 1.57 pounds.  At our current conditions, that’s all the flooding charge that we need to add because we’ve already got some flooding going on to have a clear sightglass because we’re under the 70 degree mark and the low ambient controls are in play and doing their job.

Some techs claim that just spraying water on the coil will flood the condenser enough to allow the use of that as a charging technique.    Let’s think about it for a minute.   What variables come into play with a method like that?  Variables that we can’t control…  for starters, what is the wet bulb temperature of the air entering the condenser?  How well is the condenser wetted? With the stakes being what they are, I’m not excited about the prospect of using this because I’m probably going to be the guy who winds up on the roof when it’s -20 and the wind is howling and this unit is low on gas because someone tried to use this method to figure a flooding charge, didn’t get enough gas in the unit and now it’s short.    I’ve still got to my due diligence as a service tech, do a full leak check, not find anything, and walk away wondering if I missed a leak somewhere all because someone else didn’t take a couple minutes to do a little work to do the job properly.  This is a totally preventable service call.

What about TABLE 3, you ask?  Very astute and that tells me that you’re reading ahead. Excellent.  I have never had to use it.

It gives a different flooding percentage for units with an unloader and low ambient controls where they’ll be running in low ambient conditions.  With the unloader, remember that we’re really moving less heat, changing the condenser dynamic and making it even MORE oversized than it would be if there weren’t an unloader, so more refrigerant needs to be added to properly flood the condenser.

— Jeremy Smith

P.S. – You can checkout the Testo 770-3 multimeter we mentioned in the middle by going here

Tech Tips

Refrigerant Charging Basics For Air Conditioning & Heat Pump Systems

March 8, 2018 By 4 Comments

Before I start on this one… At HVAC School we focus on a wide range of topics, many of them are very basic. My experience as a trainer for over 16 years has taught me that no matter what I assume others SHOULD know, it doesn’t change that fact that they often do not. This write up is very basic but you may find that some of the content will be useful for you to give apprentices or junior techs or it may give you a new idea of how to explain it to them… or maybe not. Either way, I feel an obligation to cover even the most basic concepts in the trade to help ensure that nothing gets missed. 

Thanks for understanding.

Before reading this you need to understand some of the terms surrounding air conditioning charging and diagnosis, specifically the term saturation

Next, you need to know something of the basic refrigerant circuit, I suggest that you understand these words and concepts before you ever dive into attempting to charge an air conditioner.  Many who start here may ask “what should my pressures be”, this is NOT how you charge a system so if you are reading this to try and find that answer just be aware, it isn’t that simple.

In order to set a proper charge on an A/C system, you must first know the type of metering device.  The piston / fixed orifice type system primarily uses the superheat method and the TXV / EEV primarily uses the subcooling method.  When setting a charge, it is always preferable to set the charge in cool mode.  Whether you set the charge in heat or cool mode, you should always follow the manufacturers recommended charging specifications.  In this section, we will discuss manufacturer recommended charging and some indicators that you have set a proper charge in heat mode.

But first, There are some things that Trump these guidelines and should make you stop and do more diagnosis

A properly running A/C system with indoor and outdoor temperatures above 68 degrees will have a suction saturation above 32 degrees (freezing), don’t leave a system with a below 32˚ saturation suction without doing more diagnosis even if the superheat/subcool looks correct.

If you see a liquid line pressure that is more than 30 degrees saturation above outdoor temperature (like a 440 psi liquid pressure on an R410a system on a 90 degree day), do not proceed until you have further addressed the possible causes of high head, regardless of what the superheat or subcool might be reading.

Always purge your hoses to prevent introducing air into the system and never mix gauges when using low loss fittings and different types of refrigerants.

Charge in the liquid phase (tank upside down) and add the refrigerant slowly and carefully to ensure you do not flood/slug the compressor with liquid refrigerant. You can do this by watching your manifold sight glass or using a special liquid preventing adapter such as the Imperial 535-C Kwik Charge.

These precautions will prevent causing system damage.

Note: This is only a basic guide for charging. There are innumerable conditions that can alter refrigerant pressures, superheat, subcool and saturation that are not related to the refrigerant charge. This is not intended to cover the complete diagnosis of the refrigerant circuit. 

Superheat Charging

To charge a system using superheat, you will need to monitor the actual temperature of the low-pressure suction line, the saturation temperature of the low side suction gauge and the indoor and outdoor temperatures entering the unit(s).

Most if not all manufacturers have a charging chart available with their respective units.  With the information you have gathered on indoor and outdoor temperatures, you can calculate the recommended superheat or in a pinch, you can use a superheat calculator such as the Trane superheat calculator or a free app like our superheat calculator or even better the MeasureQuick app. A good calculator will require that you determine the wet bulb temperature in the return air stream.  Without a sling or digital psychrometer or hygrometer, you will not be able to determine wet bulb temperature.

Once you know the target superheat you can adjust the system charge to hit it. Let’s say, the recommended superheat was 18 degrees, you would add/remove refrigerant to the system until the actual temperature of the suction line was 18 degrees above the indicated saturation temperature from your low-pressure gauge. Adding charge will decrease the superheat and recovering refrigerant will increase the superheat. 

Subcool Charging

To charge a system using subcool, you will need to monitor the actual temperature of the liquid line and indicated saturation temperature on the high-pressure gauge.  Information on the entering temperatures is not necessary to charge the unit by the subcooling method.

Most manufacturers have recommended subcool charging information with the units if it is designed for a TXV (TEV).  If for some reason, there is no information with the unit, or if it has worn off, you can set a typical residential air conditioner charge to 10 to 12 degrees of subcooling which is a relatively safe range to use.

Let’s say for example the manufacturers recommended subcool is 14, you would add enough refrigerant to the system so the actual temperature of the liquid line was 14 degrees less than the saturation temperature, as indicated on the high-pressure gauge for that particular refrigerant. Adding more refrigerant will increase the subcool reading and recovering refrigerant will decrease the subcool reading. 

Approach Method

 Lennox factory information asks that we charge by the approach method on TXV systems. I suggest charging to at least a 6˚ subcool before even attempting to calculate the approach method. 

The approach method is a calculation based on the relationship of liquid line temperature to outdoor temperature.  To calculate approach, subtract outdoor ambient from actual liquid line temperature.  The outdoor temperature used to calculate approach should always be taken in the shade and away from the hot condenser discharge air. To increase the approach differential you would remove refrigerant to decrease it you would add refrigerant. 

Some Lennox heat pump systems come with a subcool chart next to the approach chart. This subcool chart is for < 65˚.  This means the subcool chart is only valid when the outdoor temperature is below 65˚.  Follow the instructions on the unit carefully when charging in subcooling in <65˚ temperatures.  The method requires that you block sections of the coil to achieve higher head pressures before setting by subcooling.

Heat Mode Charging for Heat Pumps

In most, if not all, cases you will charge a unit in heat mode according to the manufacturer’s recommendations.  In those cases where no information is available, there are other indicators that you may use to set a proper charge in heat mode.

First, make sure you switch your hoses so the suction gauge is reading off of the “common suction” port that taps in between the compressor and reversing valve. You may put your high side gauge on either the discharge or liquid (on most systems) depending on what you are checking.

Before doing any heat mode charging use common sense, if installing a new system the best bet is to calculate line distance and weigh in any additional charge before moving on to the detailed testing phase.

The first one is the 100˚ over ambient discharge temperature rule.  The general rule to this is that a properly charged unit will have a discharge line temperature of 100˚ above the outdoor ambient temperature.  If the discharge line is too hot. you would add refrigerant which would lower the discharge temperature.  Alternately, if the discharge line were too cool, you would remove refrigerant to raise the discharge temperature.  This rule is to be used only as an indicator and, in some instances, may not be accurate given some other factors such as dirty coils, excessive superheated refrigerant entering the compressor, etc.  

Another common rule of thumb is suction pressure will be close to the outdoor temperature in an R-22 system, this is totally a fluke and has no scientific basis other than it just generally tends to work out that way. this means that on a properly functioning R22 system if it is running in heat mode and its 40 degrees outside the suction pressure tends to be around 40psig. This guideline obviously doesn’t work on an R-410A system or any other refrigerant.

A more applicable guideline is 20˚- 25˚ suction saturation below outdoor ambient temperature. This means if it is 50˚ outside the suction saturation temperature would generally be between 25˚and 30˚on a functioning system.

Remember that in heat mode the colder it gets outside, the lower the suction pressure and the hotter it gets inside, the higher the head pressure.  Since the roles of the coil are reversed in heat mode, if you notice an abnormally high head pressure it may be due to a dirty air filter or evaporator coil.  A dirty condenser coil would cause the suction pressure to drop below normal and also cause superheat problems.

Once heat mode a charge is set, whether by manufacturer specification or an alternative method, you can still verify the subcool and superheat on the unit in some cases.  Do not confuse the superheat or subcool methods recommended by the manufacturer though when running in heat mode.  These are only used for setting the charge in cooling mode and not in heat. Look for heat mode specific or low ambient guidelines. 

 

Finally and most importantly is ALWAYS TEST EVERYTHING. Airflow, Delta T, Superheat, Subcool, Suction Pressure, Head pressure, Amps, Incoming voltage, Filter etc…

Read manufacturers specs, understand the units the units you are working on, only then will guidelines and rules of thumb help instead of hinder you.

— Bryan

 

 

Tech Tips

The Case for Checking the Charge Without Using Gauges

January 26, 2018 By 5 Comments

Jim Bergmann and I recorded a podcast for HVAC School that covered when and how to check the refrigerant circuit without connecting gauges. Listener Joe Reinhard listened several times and wrote up this summary of what he gained from the episode. I edited it lightly but most of this is his work. Thank you so much Joe!

Keep in mind that when we make Fahrenheit to Celsius conversions we use K (Kelvin) to show temperature difference like splits and DTD and we use C (Celsius) to show measured temperatures.

Following mostly from two 45-50 minute podcasts from https://hvacrschool.com/checking-charge-without-gauges-podcast/ discussions between Bryan Orr HVACR School.com, expert tech, teacher, & business owner, andJim Bergmann, renowned HVAC-R expert & teacher, from Redfish instruments and the MeasureQuick app, providing a detailed explanation of why techs should not connect gauges & hoses to system just to check refrigerant charge (in many cases).

Why Not Connect?

The benefits of NOT connecting gauges during every visit for HVAC-R business owners, technicians, and clients include:

  1.  Non-invasive measurements with only temperature data taken. Exact same way one checks if a typical refrigerator was operating properly which has no ports to attach hoses and gauges.  
  2. Just measuring DTDs (Design Temperature Differences) and line set piping temperatures are non-invasive, involve less liability both for the system and technician safety, and demonstrates technical knowledge and best practices.   
  3. Better for the refrigeration system and the environment (“green”) since it saves R22 and R410A released to atmosphere.
  4. Time savings at site so techs can concentrate on better and more preventative maintenance (PM) of the air flow system (including condensate drainage) and PM checking electrical characteristics of various control components (capacitors, contactors, sequencers, etc.,.)
  5. Eliminate more call backs and potential premature system cooling (and heating for heat pumps) performance problems and failures due to cross contamination, moisture contamination and lost refrigerant.
  6. Saves the customer money on refrigerant added due to connection losses.


Term Definitions 

  • Evaporator DTD (Design Temperature Difference) is the designed difference between the evaporator coil saturation/boiling temperature as measured on the suction gauge and the return air temperature. 35°f (1.66°C)of difference is considered normal for a typical system set at 400 CFM(679.6 m3/h) per ton airflow. Oversized evaporator coils and increased airflow above 400 CFM(679.6 m3/h) per ton will result in lower DTD and lower airflow with smaller coils will result in higher DTD.
  • Condenser CTOA (Condensing Temperature Over Ambient) is the temperature difference between the condensing coil saturation / condensing temperature as measured on the liquid line high side gauge and the outdoor temperature. This difference will vary depending on the efficiency of the system/efficiency of the condenser coil.

6 – 9 SEER Equipment (Very Old) = 30° CTOA

10 -12 SEER Equipment = 25° CTOA

13 – 14 SEER Equipment = 20° CTOA

15 SEER+ Equipment = 15° CTOA

  • Delta T (Evaporator Split) is the temperature difference between the return and supply air. Delta T will vary quite a bit depending on airflow and indoor relative humidity. This chart shown below is designed for a 400 CFM(679.6 m3/h) per ton system. Lower airflow will result in a higher delta t and higher airflow will result in a lower delta t. This is why Jim Bergmann does not prefer Delta T as a firm diagnostic or commissioning tool but rather as an approximation of airflow.
  • Target Superheat on a TXV system is dictated by the design of the TXV. Usually target superheat on a TXV system will be 5°f- 15°f (2.75°K – 8.25°K) at the outlet of the evaporator where the TXV bulb is located. On a piston system the target superheat is calculated using a superheat chart and measuring and plotting the outdoor dry bulb temperature and the indoor wet bulb temperature.
  • Target Subcooling on a TXV system will be listed by the manufacturer but is generally between 8° – 14°(4.4°K – 7.7°K)subcool. Subcooling will vary quite a bit on fixed orifice systems but 5°-20°(2.75°K – 11°K) is a common range.

DTDs (Design Temperature Difference) of the coils, after a system is newly commissioned orfirst-time assessed with gauges, should not change over the life of the sealed refrigeration system once a system has been charged correctly unless one or more of the following has developed:  

  1. Airflow restriction with dirt buildup as main cause – dirty outdoor coil, dirty indoor coil, dirty filter,  dirty blower blades/inside the housing, Return/Supply duct restrictions, blower motor speed or operation problems, and if the homeowner installs a so-called high efficiency, nothing-gets-thru-including-air filter.
  2. Critical component failure.
  3. Refrigerant flow restriction.  

So after the first-time visit performance assessment or a new system is commissioned, subsequent system checkups or maintenance visits should be performed without connecting gauges.    

The following risks, problems, and liabilities occur and eventually develop when technicians attach gauge hoses every time to check the system refrigerant characteristics versus just using measured system temperatures and knowledge of Return/Supply air TD, Evaporator/Condenser split, and refrigerant P/Ts.   Not attaching hoses & gauges to systems without good reason is actually correct practice and the following could be avoided or greatly minimized.

  1. Techs are inducing system contamination if, prior to connecting the hoses the techs didnotuse dry nitrogen to purge air, moisture, and/or old refrigerant out of their hoses & manifold from the prior system the gauges were attached.  Perhaps the prior system had a different refrigerant that may/may not have been contaminated with non-condensables and other refrigerant(s)
  2. Were the hoses on the gauges left open to the atmosphere in the back of the truck used for the prior R410A system?  If so, the coating of POE (polyester) refrigerant oil (highly hygroscopic) would have absorbed moisture which, if not correctly purged with dry nitrogen, would contaminate systems by inputting moisture which will cause TXV and liquid filter-drier freeze ups (blockages), cause contaminated refrigerant (making R22 recycle subject to high fees and fines), and cause acids which will attack and corrode compressor surfaces (copper plating), valves, and windings.  Hoses should always be tightly connected to the manifold parking ports to prevent moisture contamination.
  3. Are techs properly & carefully disconnecting gauge hoses while the system is running?   If not, perhaps a service call back will shortly occur since, every time hoses are connected and disconnected, some refrigerant is lost.  If the liquid hose is not charged back through the manifold and Suction hose, several ounces or more in the liquid hose are lost if techs inadvertently or on purpose blow or dump refrigerant by not properly disconnecting gauge hoses while the system is running.  This occurs if techs are inexperienced or decide not to take the time or are not equipped with low-loss-ball-valve hose end fittings to slowly, carefully, after purging hoses if needed, charge from the liquid hose (holds 7x the R410A as the vapor or suction line; 10X for R22) through the gauge manifold into the vapor or suction hose back into the running system.  If this procedure is not done correctly, air and moisture can enter the system.   After one, two, or three years of visits, techs can be chasing “leak(s)” created by multiple connects/disconnects.  
  4. Caps no longer inadvertently left off on Schrader valve ports leading to leaks.
  5. Reduced safety issues for techs since less chance of refrigerant in eyes and frozen-fingers and loss refrigerant to the atmosphere.

Data to recordduring first-time system performance assessments and new system commissioning using refrigerant gauges so that benchmarks exist to compare to future checkup visits but without attaching gauge hoses if no observed or reported system problem reasons.  

  1. TD or Temperature Difference between the Return air dry-bulb (DB) and Supply air DB.   TD level depends on the sensible & latent heat content of the inside air.  Higher TD for low RH% (Relative Humidity), lower TD for high RH%.  20°F (11°K)  TD is good if system operating properly at 75°F(23.88°C), 50% RH and set for 400 CFM/(679.6 m3/h)ton.  If reduce CFM/( m3/h) ton, TD increases, but if RH% increases, the TD decreases back-and-forth so the TD can range 16°f – 24°f(8.8°K – 13.2°K) (or more in extreme cases, see the Delta T chart)    
  2. Evaporator DTD (Design Temperature Difference), also called “Split”, is temp difference between the Return air dry-bulb (DB) temp and the refrigerant saturation temp of the coil – either 35°F(1.66°C) at 400 CFM/(679.6 m3/h) ton to 525 CFM/(891.98 m3/h)ton or 40°F(4.44°C) at 350 CFM/(594.65 m3/h) ton.  
  3. Evaporator outlet SLT (Suction Line Temp) and SH (SuperHeat) On a TXV system the superheat range 5°f(2.75°K) to 15°f(8.25°K) depending on factory setting +/– 5°F(2.75°K) of 10°f(5.5°K).   Fixed-bore or piston reading depends on inside heat load, Return air WB, and outside air DB temp
  4. TESP (Total External Static Pressure) inches WC of the air handlerbetween non-turbulent point in Return plenum before a clean filter and in the Supply plenum non-turbulent area.  With caution, drill 3/8”-1/2” holes to cover when done with vinyl or plastic professional looking plugs. On a furnace drill above the filter for the return reading and between the furnace and the coil for the supply reading. Note if the coil was wet or dry since TESP changes. 
  5. Pressure Drop  “wc across thefilter.
  6. Pressure Drop  “wc across theEvaporator coil, note if wet or dry coil, and plug holes.
  7. Indoor Blower motor (IBM) running load amps (RLAs) compared to nameplate Rated or Full Load Amps (FLA) with the panels on.
  8. SLT and SH at the Condenser (Compressor inlet).  SH within +/– 5°f(2.75°K)  is acceptable. For a TXV, superheat average 10°f(5.5°K) plus additional 1-3°F (.55°K – 1.65°K)of SH the Suction/Vapor line absorbs (as measured).  For a fixed-bore or piston Metering Device at the indoor coil, a total “target SH” is determined by outdoor DB and indoor WB temps.  
  9. Condenser DTD or Split is temp difference between the refrigerant saturation temp and the DB temp of air at entering middle of the coil. As SEER increases, condenser surface areas are larger but are limited by diminishing heat transfer capability as the temperature difference between the outdoor air and the coil temperature decrease.  
  10. LLT (Liquid Line Temp) and SC (SubCooling) at the Condenser outlet.   SC within +/– 3 °f(1.65°K)  is acceptable. Ex. for 85°f(29.4°C) ambient, 13 SEER with a 20°f(11°k) DTD split, and 10° (5.5°k) Subcool nameplate, the Liquid Line temp = 95°f(35°C)  = 85°f(29.44°C) outdoor + 20°f(11°k) CTOA  – 10°f(5.5°K) Subcooling ).
  11. Compressor and OFM running load amps (RLA) compared to nameplate Rated and Full Load Amps (FLA), respectively.  
  12. Measured suction temperature differential between the suction line leaving the evaporator and entering the compressor in °f. So if the suction line is 50°f (10°C) inside and 53°f (11.66°C) outside there would be a 3°f (1.65°K) temperature rise.
  13. Measured liquid temperature differential between the liquid line leaving the condenser and entering the metering device in °f. So if the liquid line is 95°f (35°C) outside and 92°f (33.33°C) inside there would be a 3°f (1.65°K) temperature drop.

Again, benchmarked DTDs, SHs, SC, and ESPs should not change during the life of the system unless one or more of the following has developed:  

  1. Air flow restriction
  2. Component failure
  3. Refrigerant flow restriction  

Data to recordduring follow-up seasonal checkup visits and compare to benchmark data.  See if problems have or are developing and show improvement after any services are performed which offers value to clients/customers paying for the service call or membership fee.  Service could be simple as a filter change, coil cleaning, and blower maintenance but, since have more time for PM, also identify potential electrical parts failures and inform clients to choose to fix now or later.

  1. TD between the Return air dry-bulb (DB) and Supply air DB.  Should be in 16-24°F(8.8°K – 13.2°K) range depending upon sensible & latent heat content of inside air (see chart).  
  2. Evaporator outlet SLT.  If a TXV, should be within +/– 5°f(2.75°K)  of benchmark reading.   Fixed-bore or piston reading depends on inside heat load, Return air WB, and outside air DB temp.  More practical SLT determine at outdoor coil Suction/Vapor line.
  3. TESP (Total External Static Pressure) “wc of the air handler and note if wet or dry coil.
  4. Static Pressure Drop “wc across thefilter and re-plug holes (or visually inspect / replace)
  5. Static Pressure Drop “wc across theEvaporator coil, note if wet or dry coil, and re-plug holes.
  6. SLT at the Condenser (Compressor inlet). For an indoor TXV, should be within +/– 5°f(2.75°K)  of benchmark reading.  For fixed-bore or piston indoor coil Metering Device, determine total “target SH” from outdoor DB and indoor WB temps.  
  7. LLT (Liquid Line Temp) and SC (SubCooling) at the Condenser outlet.   LLT using SEER-rating split, should be within +/– 3°f(1.65°K)  of benchmark reading. Outdoor air temperature + CTOA based on system efficiency – subcooling = target liquid line temperature

Other notes:

Always use pre-tested, calibrated (as possible) digital thermometers to measure air temps and line set pipe temps or insulated temp sensors.  Do not depend upon the space thermostat to accurately represent inside air temps since could be Return duct leakage, bypass ducts not dampered correctly, and air handler cabinet leaks e.g. holes/gaps at indoor coil line set inlet affecting the Return air temp.  

Air flow through/across Evaporator and Condenser coils will only decrease and not “magically” increase. The primary reason is dirt accumulation on air flow components e.g. coil fins, indoor filter, indoor blower blades, outdoor fan blades.  Other reasons include leaky air handler cabinets from gaps at Return & Supply duct connections, holes at line set inlet to Evaporator cabinet, and a bypass duct with no damper to close off air flow between Supply and Return in Cooling mode.

Systems should not be benchmarked with a wet Condenser coil or if the LLT is at or below the outdoor ambient air DB temperature.

Use a battery or cord operated leaf blower to dry out the coil in 5-10 minutes.  

The only action that increases airflow is increasing the fan or blower RPM or speed.   If Suction line supposed to be 54°F(12.22°C) (40°F(4.44°C) coil + 10°f(5.5k)  SH if TXV + say 2F SH addl to Vapor line length) but is 47-48°F(25.85°K – 26.4°K), look for indoor air flow restriction issues.   The evaporator is like a boiling pot of water but a sealed system so if the burner heat is turned up, pressures and temperatures increase. More than additional 2–4°f(1.1°K – 2.2°K)  Superheat at the Compressor inlet, probably better insulate the Vapor line.

Maximum inlet temperature Suction line at Compressor inlet should be below 65°f(18.33°C) .   If not, the Compressor will have the potential overheat and oil breakdown can occur do to excessive discharge superheat / temperature.

TXV designed to maintain 5-15°f(2.75°K – 8.25°K) superheat (10°f(5.5°K) given +/- 5°f(2.75°K) range) but only at the Evaporator outlet or where the sensing bulb is located on the suction line.  Some SH is added to the suction line before gets to the Compressor inlet.   However, if the line set is located in a 145°F(62.77°C) attic and Vapor line not well insulated, significant SH gain will be seen at the Compressor inlet. Vapor line needs good insulation (also for Heath Pumps in Heating mode) e.g. with thicker tubing insulation and/or using a foil-bubble wrap or “Reflectix” attached with foil tape since reflects IR heat.

Summary of the Jim Bergmann / Bryan Orr Podcast on checking the charge without using gauges by Joe Reinhard

P.S. – As mentioned in the podcast the Testo 605i and the 115i make a great pairing to check a system in the way described above

You can now do ALL of these calculations easily with the MeasureQuick app at MeasureQuick.com/downloadnow

Tech Tips

Flash Gas

January 14, 2018 By No Comments

When we say that there is “flash gas” at a particular point in the system it can either be a bad thing or a good thing depending on where it is occurring.

Flash gas is just another term for boiling.

It is perfectly normal (and required) that refrigerant “flashes” or begins boiling directly after the metering device and as it moves through the evaporator coil. In order for us to transfer heat from the air into the refrigerant in large quantities we leverage the “latent heat transfer of vaporization”. In other words we transfer heat into the boiling refrigerant, or “flash gas”.

In a boiling pot of water we create flash gas by increasing the temperature of the water until it hits the boiling temperature at atmospheric pressure.

Inside of a refrigeration circuit we get flash gas when the pressure on the liquid refrigerant drops below the temperature / pressure saturation point or if the temperature of the refrigerant increases above the same point.

This “flashing” can occur in the liquid line when the liquid line is long or too small and also in cases with line kinks and clogged filter/driers. All of these instances result in a pressure drop and a drop in the saturation temperature.

It can also occur in the liquid line if it is run uninsulated through a space that is hotter than the liquid saturation temperature like on a hot roof or in an unconditioned attic. This is more rare and will generally only cause flashing in conjunction with another issue.

This flashing can be prevented by keeping line lengths and tight bends to a minimum, insulating the liquid line where it runs through very hot spaces and keeping the refrigerant dry and clean with one properly sized filter/drier.

It can also be prevented in most cases by maintaining the proper levels of subcooling. A typical system that has 10°+ of subcooling will not experience flashing in the liquid line under normal conditions.

When you walk up to a liquid line near the evaporator and you hear that hissing/surging noise or when you look in a sight glass and see bubbles you are seeing refrigerant that is at saturation, meaning it is a mix of vapor and liquid. This doesn’t necessarily mean it is “flash gas”, it could very well be that the refrigerant was never fully condensed to liquid in the condenser in the first place. This can be due to low refrigerant charge and in these cases the subcool will be at 0° Even when taken at the condenser.

True liquid line “flash gas” issues are cases where you have measurable subcooling at the condenser coil outlet but still see, hear or measure boiling/flashing refrigerant in the liquid line before the metering device.

— Bryan

Tech Tips

Checking the Charge on a Heat Pump in the Winter

December 1, 2017 By 5 Comments

When you ask many people nowadays how to check the charge on a heat pump during low outdoor temps they will say that you need to “weigh in and weigh out” the charge. While this may be an effective method it isn’t always practical.

Now… If you are making a refrigerant circuit repair, weighing out and weighing in makes perfect sense, especially since microchannel condensers and scroll compressors make pumping down less viable anyway. But there are many cases where you just need to check the charge to make sure the system is working properly and in these cases, weighing in and out is just plain silly.

I originally wrote this guideline back in 2003 and truthfully, not much has changed since then in regards to checking a heat mode charge on a heat pump.

Step #1 – If there is any frost on the outside unit get it completely defrosted first.

Step #2 – Check all the obvious things first, filter, coils, blower wheel etc… If the unit isn’t clean it will be really hard to check.

When charging in heat mode Read manufacturer specifications first. Lennox gives specific instructions for charging their units in below 65˚ outdoor ambient conditions. It involves blocking off the condenser coil with cardboard (or even better using a charging jacket) while continuing to run the system in cool mode. Lennox gives specific instruction for how high to raise the head pressure, and what level of subcooling you should expect.

Remember that in heat mode on a heat pump the evaporator is outside, and the condenser is inside. This is important because in cool mode a dirty air filter caused low airflow on the evaporator. This would typically cause a low suction pressure, and a low superheat. In heat mode, a dirty air filter causes low airflow across the condenser. This can cause Extremely high head pressure. In heat mode, a dirty outdoor coil can cause a low suction pressure.

As an example, Trane includes a pressure curve chart with many heat pump condensing units. Be sure to use the scale all the way to the right that says heat mode. Indoor and outdoor dry bulb temperatures are necessary to use the Trane pressure curve. Carrier supplies many heat pump condensing units with a pressure guideline chart. Carrier only wants the heat mode pressure chart used as a guideline, not as a charging tool. Always reference manufacturer guidelines before setting any charge.

100˚ Over Ambient Rule of Thumb

Even though manufacturer specifications should be followed, there are some basic guidelines that will aid in charging and diagnosis in a pinch. The most widely quoted rule of thumb is the 100˚ – 110˚ over ambient discharge rule. This guideline states that a properly charged unit will have a discharge line temperature of 100˚ – 110˚ above the outdoor temperature. If the discharge line is too hot add refrigerant (If the charge is the issue and not another problem). If the discharge line is too cool remove refrigerant (again only if the charge is diagnosed as the issue).

First off, the photo above was taken in 2003 so give me some slack on my gauges. Nowadays I would be using my Testo 550’s.

To give a simple example using the 100˚ – 110˚ over ambient rule. If it were 60˚ outside you could say by the 100˚ – 110˚over ambient rule, the charge is about correct. If it were 30˚ outside the 100˚ – 110˚ over ambient rule would show undercharge (or other conditions that can cause high discharge line temp see this article) . If for example the discharge temperature were 210˚ with a 150 P.S.I. head pressure and a 10 P.S.I. suction with a 50˚ outdoor temperature; this would show an extreme undercharge. Subcool and superheat can still be checked in heat mode, the problem is since there are rarely any set guidelines it is difficult to tell when the charge is set correctly by simply checking subcool or superheat alone. Generally, you will see normal superheat (8-14) on a system with  heat mode TXV and the subcooling will generally be a bit higher than usual, especially when measured outside.

Suction Pressure / EVAP DTD Rule of Thumb 

Another common old school rule of thumb is suction pressure should be close to the outdoor temperature in a R22 system. However, this rule of thumb (obviously) does not work on an R-410A system. A more applicable guideline is 20˚-25˚ suction saturation below outdoor ambient. This means if it is 50˚ outside the suction saturation temperature should be between 25˚and 30˚ (on most systems).

Head Pressure / CTOA Rule of Thumb

Because the evaporator coil is substantially smaller than the condenser you will usually see higher head pressure (condensing temperature) in relationship to the condensing air, in this case, the indoor air. This can vary a lot depending on the age / SEER of the unit, the size of the coil and how the indoor airflow is setup but generally will be 30˚ – 40˚ condensing temperature over the indoor dry bulb.

Checking Without Gauges 

Here are some quick tests you can do on a heat pump to confirm it is operating close to specs without using gauges when the coil is frost free and the outdoor temps are 65˚ – 15˚.

  • Check the discharge (vapor) line, it should be 100˚ – 110˚ over outdoor ambient temperature
  • Suction line Temp should be 5˚ – 15˚ cooler than the outdoor temperature
  • Liquid Line should be 3˚ – 15˚ warmer than the indoor temperature
  •  Delta T indoors will vary greatly depending on outdoor temperature.

If anything looks off, go ahead and connect gauges to verify further…. and like I said several times already, follow manufacturers guidelines.

— Bryan

Tech Tips

The “5 Pillars” of Residential A/C Refrigerant Circuit Diagnosis

May 23, 2017 By 2 Comments

Suction pressure, head pressure, subcooling, superheat, Delta T

Taking all five of these calculations into account on every service call is critical. Even if further diagnostic tests must be done to pinpoint the problem, these five factors are the groundwork before more effective diagnosis can be done. I would also add static pressure as an important reading that should be checked regularly (Keep TESP between .3″wc and .7″ wc on most systems) but I would still place it slightly below these five as far as fundamental HVAC technician measurements.

Some of these are “rules of thumb” and obviously are for reference only. Refer to manufacturer recommendations when setting a charge.

Suction Pressure / Low Side
Suction pressure tells us several things. The first thing it tells us is what the boiling temperature of the refrigerant in the evaporator is. If the suction pressure is below 32° saturation temperature, the evaporator coil will eventually freeze.

As a general rule, the higher the temperature of the air passing over the evaporator, the higher your suction pressure will be. A good rule of thumb for suction pressure is 35°  saturation below indoor ambient +/- 5° (Return temperature measured at the evaporator coil). This temperature differential is often called an evaporator split or design temperature difference (DTD). When calculating DTD a “Higher” DTD means lower suction pressure in comparison to the return temperature, a lower DTD means higher suction pressure.

This means that when the temperature of the air passing over the evaporator is 80°, the low side saturation temperature should be 45° when the system is set for 400 CFM per ton output. Remember the temperature scale next to the pressure scale on the gauge represents saturation or if you don’t have the correct sale on (or in your gauge if you have a Digital manifold) you would need to use a PT chart.

This 35° rule only works at 400 CFM per ton, when a system is designed for 350 CFM per ton the DTD will be closer to 38° – 40° +/- 5° 

Make sure you know the actual CFM output of the system before you calculate DTD. It can vary significantly based on the setup of the particular blower. Also keep in mind that oversized evaporator coils that some manufacturers specify for efficiency can also result in slightly lower DTD (higher suction). If you don’t know all the details it is my experience that using 35° is the best bet.

Head Pressure / High Side
When used in conjunction with liquid line temperature, we can know what state the refrigerant in the liquid line and that the compressor is pumping / operating in the required compression ratio. We can also know something about the state of the metering device as to whether or not refrigerant is “backing up” against the metering device. A good rule of thumb for head pressure is a 15° – 20° saturation above outdoor ambient +/- 3° for most modern systems. These saturation / ambient calculations are only indicators; they are not set in stone. Keep in mind, when I say ambient; I am talking about the air entering the evaporator for suction pressure and the condenser for head pressure.

Jim Bergmann points out that different equipment efficiencies will have different target Condensing Temperature Over Ambient (CTOA) readings. Keep in mind that these date ranges don’t guarantee the SEER but rather give the date ranges that these efficiencies will be most likely. The larger the condenser coil in relationship to the volume of refrigerant being moved the  lower the CTOA will be.

6 – 10 SEER Equipment (Older than 1991) = 30° CTOA

10 -12 SEER Equipment (1992 – 2005) = 25° CTOA

13 – 15 SEER Equipment (2006 – Present) = 20° CTOA

16 SEER+ Equipment (2006 – Present) = 15° CTOA

Superheat
Superheat is important for two reasons. It tells us whether or not we could be damaging the compressor and whether we are fully feeding the evaporator with boiling, flashing refrigerant. If the system has a 0° superheat, a mixture of liquid and vapor is entering the compressor. This is called liquid slugging and it can damage a compressor. A superheat that is higher than the manufacturer’s specification can both starve the evaporator, causing capacity loss, as well as cause the compressor to overheat. So how do we know what superheat we should have? First, we must find out what type of metering device the system is using. If it is using a piston or other fixed metering device, you must refer to the manufacturers superheat requirements or a superheat chart like the one below.

If it is a TXV type metering device, the TXV will generally attempt to maintain between a 5° to 15° superheat on the suction line exiting the evaporator coil (10° +/- 5°) 

TXV target superheat setting may vary slightly based on equipment type.

Subcooling
Subcooling tells us whether or not the liquid line is full of liquid. A 0° subcool reading tells us that the refrigerant in the liquid line is part liquid and part vapor. An abnormally high subcool reading tells us that the refrigerant is moving through the condenser too slowly, causing it to give up a large amount of sensible heat past saturation temperature. A high subcool is often accompanied by high head pressure and, conversely, a low subcool by low head pressure. Subcool is always a very important calculation to take because it lets you know whether or not the metering device is receiving a full line of liquid. Typical ranges for subcooling are between 8 and 14 degrees on a TXV system, but always check the manufacturer’s information to confirm. in general on a TXV system using 10° +/- 3° at the condenser outlet is the best “rule of thumb” in the absence of manufacturer’s data.

On a fixed orifice / piston system the subcooling will vary even more based on load conditions  and you will see a range of 5° to 23° making subcooling less valuable on a fixed orifice system. In my experience during normal operating conditions  the subcooling on a fixed orifice system will still usually be in the 10° +/- 3° range.

Evaporator Air Temperature Split (Delta T)
The evaporator air temperature split (Delta T) is a nice calculation because it gives you a good look at system performance and airflow. A typical air temperature split will be between 16 and 22 degrees difference from return to supply. Keep in mind, when you are doing a new system start up, high humidity will cause your air temperature split to be on the low side. Refer to the air temperature split and comfort considerations sheets for further information.

For systems that are set to 400 CFM per ton, you can use a target Delta T sheet like the one shown below

 

If the leaving temperature/delta T split is high it is an indication of low airflow. If it is low it is an indication of poor system performance / capacity.

Again, this only applies to 400 CFM ton. 350 CFM per ton or less are more common today than ever and in those cases the above chart won’t apply.

Diagnosing With The Five Pillars
The way this list must be utilized is by taking all five calculations and matching up the potential problems until you find the most likely ones. A very critical thing to remember is that a TXV system will maintain a constant superheat, and a fairly constant suction pressure. The exceptions to this rule are when the TXV fails, is not receiving a full line of liquid or does not have the required liquid pressure/pressure drop to operate. This situation would show 0° subcooling and in this case, will no longer be able to maintain the correct superheat. Before using this list, you must also know what type of metering device is being utilized, then adjust thinking accordingly. Also remember, in heat mode, the condenser is inside and the evaporator is outside.

Low Suction Pressure
• Low on charge
• Low air flow /load – dirty filter, dirty evaporator, kinked return, return too small, not enough supply ducts, blower wheel dirty, blower not running correct speed, insulation pulling up against the blower, etc.
• Metering device restricting flow too much – piston too small, piston or TXV restricted, TXV failing closed
• Liquid line restriction – clogged filter/drier, clogged screen, kinked copper
• Low ambient (Low evaporator load)
• Extremely Kinked suction line (after the kink)
• Internal evaporator restriction
High Suction Pressure
• Overcharge
• High return temperature (Evaporator Load)
• Metering device allowing too much refrigerant flow – piston too large, TXV failing open, piston seating improperly
• Too much airflow over the evaporator (Blower tapped or set too high)
• Compressor not pumping properly – bad suction valve, bad discharge valve, bad or broken crank
• Reversing valve bypassing
• Discharge line restriction
Low Head Pressure
• Low on charge
• Low ambient temperature / low load
• Metering device allowing too much refrigerant flow – piston too large, TXV failing open, piston seating improperly
• Wet condenser coil
• Compressor not pumping properly – bad suction valve, bad discharge valve, bad or broken crank
• Reversing valve bypassing (heat pump units)
• Kinked suction line
• Restricted discharge line
• Severe Liquid Line Restriction
High Head Pressure
• Overcharge
• Low condenser airflow – condensing fan not operating, dirty condenser, fins bent on condenser, bushes too close to condenser, wrong blade, wrong motor, blade set wrong
• High outdoor ambient temperature
• Mixed / incorrect refrigerant / retrofit without proper markings
• Non-condensables in the system
• Liquid line restriction + overcharge (someone added charge when they saw low suction) – piston too small, piston or TXV restricted, TXV failing closed, restricted line drier
Low Superheat
• Overcharge
• Low air flow / load – dirty filter, dirty evaporator, kinked return, return too small, not enough supply ducts, blower wheel dirty, blower not running correct speed, insulation pulling up against the blower etc.
• Metering device allowing too much refrigerant flow – piston too large, TXV failing open, piston seating improperly
• Low return air temperature
• Abnormally low humidity
• Internal evaporator restriction
• Very Poor Compression (Compressor, reversing Valve Issues) but will also be combined with VERY HIGH suction
High Superheat
• Low on charge
• Metering device restricting flow / underfeeding / overmetering – piston too small, piston or TXV restricted, TXV failing closed
• High return air temperature
• Liquid line restriction – clogged filter/drier, clogged screen, kinked copper

Low Subcooling
• Low on charge
• Metering device allowing too much refrigerant flow – piston too large, TXV failing open, piston seating improperly
• Compressor not pumping properly – leaking suction valve, leaking discharge valve, bad or broken crank
• Reversing valve bypassing
• Discharge Line Restriction
• Compressor not pumping
High Subcooling
• Overcharge
• Metering device restricting too much flow – piston too small, piston or TXV restricted, TXV failing closed
• Liquid line restriction – clogged filter/drier, clogged screen, kinked copper
• Wet condenser coil
• Dirty Condenser Coil on New High Efficiency Condensers (Increased Condensing Temp Can Actually Result in Higher Subcooling)
• Having an H.R.U. in the discharge line (old school I know)
• Internal evaporator restriction
High Evaporator Air Temperature Split
• Low air flow – dirty filter, dirty evaporator, kinked return, return too small, not enough supply ducts, blower wheel dirty, blower not running correct speed, insulation pulling up against the blower etc.
• Abnormally low humidity (WB Temp)
• Blower not running the correct speed or running backward
Low Evaporator Air Temperature Split
• Undercharge
• Severe Overcharge with fixed orifice metering device – because saturation temperature is increased with overcharge
• Metering device not functioning properly – restricting too much flow or allowing too much flow
• Too much airflow through the evaporator – blower not running correct speed
• Heat strips running with air
• Abnormally high humidity
• Liquid line restriction
• Compressor not pumping properly – bad suction valve, bad discharge valve, bad or broken crank
• Reversing valve bypassing
• Discharge line restriction

 

This is an incomplete list designed to help you. Always keep your eyes and ears open for other possibilities. Diagnosis is an art as well as a science.

— Bryan

BROUGHT TO YOU BY

https://www.refrigtech.com

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