Tag: Superheat

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

 

 

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!


Following mostly from two 45-50 minute podcasts from http://hvacrschool.com/checking-charge-without-gauges-podcast/ discussions between Bryan Orr HVACR School.com, expert tech, teacher, & business owner, and Jim 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 or first-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. Air flow restriction with dirt buildup as main cause – dirty outdoor coil, dirty indoor coil, dirty filter,  dirty blower blades/inside housing, Return/Supply duct restrictions, blower motor speed or operation problems, and if 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 splits, 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 did not use 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) thru 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 atmosphere.

 

Data to record during 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 sensible & latent heat content of inside air.  Higher TD for low RH% (Relative Humidity), lower TD for high RH%.  20°F(-6.66°C)  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 handler between 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 the filter.
  6. Pressure Drop  “wc across the Evaporator 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(-6.66°C) DTD split, and 10°f(-12.22°C) SC nameplate, the LLT = 95°f(35°C)  (= 85°f(29.44°C) outdoor + 20°f(-6.66°C) 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 record during 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 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 the filter and re-plug holes (or visually inspect / replace)
  5. Static Pressure Drop “wc across the Evaporator 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. Primary reason is dirt accumulation on air flow components e.g. coil fins, indoor filter, indoor blower blades, outdoor fan blades.  Other reasons includes 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 air flow 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(-12.22°C)  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.   Evaporator is like a boiling pot of water but a sealed system so if the burner heat is turned up, P&T increase. More than addl 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 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 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

I was fresh out of school working as an apprentice at my first real HVAC job and I was listening in on a shop conversation between a few techs.

They were talking about finding so many overcharged systems and one of the techs turns to me and says “I had a unit yesterday that was so overcharged it was running minus five degrees of superheat”. I don’t remember EXACTLY what I said in response to that but it started a miniature argument and set me on a crusade against misinformation that led me here all these years later.

When in doubt check your tools

Before we move on I want to mention something that Jeremy Smith pointed out to me. When working with a zeotropic refrigerant blend that has “glide” the change from liquid to vapor and vapor to liquid occurs over a range of temperatures and not at a single temperature. When calculating superheat we use the “Dew point” and when calculating subcool we use “Bubble Point” the saturation temperature is the range of temperatures between those two points meaning that it could be “interpreted” as negative superheat or subcool when it is actually just in the saturated range. In air conditioning the traditional R22 and R410a refrigerants do not have any significant glide but newer blends do so it is something to watch out for.

Here is a list of things that if you observe them, it will be worth checking your tools to make sure they are set up correctly, connected correctly and properly calibrated BEFORE you start making an exotic diagnosis.

Negative Superheat   

Superheat is the temperature gained in the refrigerant once it is completely boiled into a vapor. When it is still in the process of boiling it will be in a mixed state and will be at saturation temperature for that given pressure. Zero superheat is something you will see often when a system has a flooded coil and liquid still boiling in the suction line. While this generally isn’t a good thing it is something that you will observe from time to time and will usually result in you as the tech taking corrective action.

Negative superheat goes by another name SUBCOOLING and the only way a substance can be in the subcooled range is if it is 100% liquid and has given off additional heat below the saturated (mixed) state. It is impossible in a running air conditioning system for the suction line to be 100% liquid subcooled below saturation, therefore it is impossible to have negative superheat both by definition or in practice.

So what happens when you measure negative superheat you may ask? Good question.

It is one of a few possibilities

  1. You are looking at the the wrong refrigerant PT scale
  2. The refrigerant is mixed (somebody put something in on top of the original refrigerant)
  3. You are dealing with a blended refrigerant with “glide” like many of the new 4 series blends such as R407c
  4. Your suction gauge is reading too high
  5. Your line clamp thermometer is reading too low
  6. You do not have a good connection on the line / schrader core isn’t depressing / king valve isn’t open
  7. A combination of the items listed above

Negative Subcooling 

Just like we mentioned above, negative subcooling is superheat. There is no such thing as negative subcooling.

Is it possible for the liquid line to contain superheated vapor? It is THEORETICALLY possible but not practical. For example if someone short circuit nearly the entire condensing coil and connected to the liquid line you could see superheated vapor…. but let’s be realistic.

When techs measure a negative subcooling (superheat) at the liquid line it could be

  1. You are looking at the the wrong refrigerant PT scale
  2. The refrigerant is mixed (somebody put something in on top of the original refrigerant)
  3. You are dealing with a blended refrigerant with “glide” like many of the new 4 series blends such as R407c
  4. Your high side gauge is reading too low
  5. Your line clamp thermometer is reading too high
  6. You do not have a good connection on the line / schrader core isn’t depressing / king valve isn’t open
  7. A combination of the items listed above

Liquid Line Cooler than the Outdoor Air 

There are two cases where the liquid line can be cooler than the outdoor air when measured at the condenser outlet

  1. A Wet Coil
  2. A restriction inside the condenser cabinet in the liquid line, usually in a factory installed filter drier

Because the liquid line temperature will often be VERY close to the outdoor temperature on new, high efficiency system this is often a point where you will measure a liquid line as colder than the outdoor air when that may not really be the case.

Often you may SEE a liquid line colder than outdoor ambient and it may be simply be

  1. Miscalibration of the line clamp or the ambient air thermometer
  2. Measurement of the ambient air in sunlight where the probe can be effected by sunlight
  3. The coil is still damp after cleaning or a rain (evaporative cooling)

It is always a good practice to have a back up set of thermometers and gauges so you can double check the calibration of your tools against one another. Whenever possible, test them under the conditions that you are using them.

If you have two clamps, place them on the same line right next to one another, when testing two air probes, stick them both in the same return air stream side by side. For temperature measurement you may also test in an ice bath just make sure that the water is pure and that the water and ice are fully mixed and circulating when you test for 32°F(0°C) degrees.

Also keep in mind that every measurement device has “uncertainty” in the measurement of +/- a certain amount depending on the tool. Don’t expect your tools to provide a greater accuracy than what is published in their specifications.

— Bryan

P.S. – If you don’t have a “backup” set of temperature and pressure measurement instruments I will suggest the Testo refrigeration smart probes kit for line temperatures and pressures and the Testo 605i for air temperature and humidity. They are a FANTASTIC price and you will get an additional 8% off by using the offer code getschooled at Trutechtools.com

First off, if you’ve never heard the term “beer can cold” you are either not in the trade, or you have been living a pretty sheltered existence. I started as a tech apprentice when I was 17 years old and on my first day in the truck my trainer grabbed the suction line of a running split system and said “She’s running good! beer can cold”. Now before you freak out, my trainers were primarily a couple of guys named Jimmy Wells and Dave Barefoot and these old school techs would JOKE about beer can cold and then they would proceed to connect their gauges and properly check superheat and subcooling.

There are two things to know about old sayings like “beer can cold” or listening to your vacuum pump or feeling the air velocity out of a register.

#1 – They Can Be Useful Tools of An Experienced Tech

When I was in trade school my instructor taught me to “feel my way” through the refrigerant circuit to identify the liquid line, suction line and discharge line by touch. This resulted in some minor burns and a perspective on the “qualitative” or intuitive understanding of the refrigerant circuit.

Using your senses to hear, feel and smell the system are really important tools an efficient and effective tech builds over time to alert themselves of slipping belts, a vacuum pump that isn’t operating properly, a burned board or transformer, a bad bearing or even… an underfeeding or overfeeding evaporator. This is where “Beer can cold” (grabbing the suction line to get an approximate temperature) isn’t always a bad thing… but only when used as an initial qualitative test.

#2 – Senses Should Lead to Measurement

A good diagnostic technician finds THE problem first, whatever is primarily causing the problem is the first order of business. Once that primary problem is identified THEN a good tech moves on to inspecting the entire system and making more measurements as possible to identify additional issues.  Once the initial set of know issues have been rectified then a good technician will always verify proper system performance using real measurements that PROVE that the system is operating properly.

So let’s be 100% clear

You cannot charge a system by “Beer Can Cold“. It is nothing more than a long running inside joke that refers to grabbing the suction line and it feeling cold like a beer on a functioning A/C system.

But….. (Warning, I’m about to take this WAY TOO FAR) 

Depending on the type of beer and the preference of the drinker, beer can be anywhere from 36°F(2.22°C) for a good old can of American Lager all the way to about 55°F(12.77°C) for a British stout kept at cellar temperature. Craft Beer enthusiasts will tell you that about 45°F(7.22°C) is a good compromise between flavor and temperature.

On average your evaporator temperature will have a 35°F(19.25°K)DTD (Design Temperature Difference) which means the coil temperature will be about 35°F(19.25°K)) colder than the return air DB temperature. This means if it’s 75°F(23.88°C) in the return the evaporator will be at about 40°F(4.44°C). We then need to add in superheat which will vary quite a bit on a fixed orifice system. On a TXV or EEV system it will be between 5°F and 15°F(2.75°K – 8.25°K) on a properly functioning system. This means that the suction line indoors could range from 45°F to 55°F(24.75°K – 30.25°K) by the time you account for the TXV superheat range, the uncertainty of the temperature measurement and the variability in DTD. If you are grabbing the suction line outside you will also need to account for anywhere from a 1°F to a 8°F(.55°K –  4.4°K) rise in temperature on the suction line by the time it get’s from the coil to the outdoor unit where “Beer Can Cold” is taken. Now the range is all the way from a acceptable beer temp of 46°F(7.77°C) all the way up to a putrid 63°F(17.22°C) that even the British would find unacceptably warm.

All of this, just at 75°F(23.88°C) return temperature WITH a TXV

I don’t know about you, but my hand is only calibrated to within +/- 4°F(2.2°K), when you add that to the mix I find that using my hand to feel the suction line gives me only the roughest estimation of what is going on and if 50°F(10°C) is the average… that is too warm for my taste in beer anyway.

Beer can cold, like most “rules of thumb” is far too inaccurate to be useful (at the risk of overstating the obvious).

What I do recommend, is becoming fully familiar with….

  • The Design CFM of the System and the sensible / latent requirements of your area
  • The efficiency of the equipment you are working on (to help anticipate condensing temperature)
  • Type of metering device (To understand target superheat)
  • Evaporator Coil Design Temperature Difference (DTD)
  • Condensing Temperature Over Ambient  (CTOA)
  • Superheat
  • Subcooling
  • Delta T
  • Static pressure

Add a good understanding of all of these readings and when and where to take then in the mix and THEN and ONLY THEN have you earned the right to make jokes about beer can cold. If you have not yet understood the concepts I would advise starting by reading THIS and then listening to THIS

— Bryan

P.S. – Just for fun I created some “Beer Can Cold” T-shirts that you can buy for the next 5 days by going HERE but ONLY if you solemnly swear not to actually charge a system that way.

 

 

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

As we talked about in an earlier podcast, a TXV is designed to maintain a specified and constant superheat at the outlet of the evaporator coil. It does this through a balance of forces between the bulb pressure (opening force), Equalizer pressure (Closing force) and spring (Closing force). It is the spring pressure that can be adjusted on some valves, but why and when would this be done?

For the quick, cut to the chase version, turning the adjustment on the bottom of an adjustable valve clockwise = higher superheat and counterclockwise = lower superheat. However, before you start messing with the adjustment, I suggest you read on.

First, the valve must be an adjustable type, many valves on small equipment are not adjustable and have no hex cap at the base.

Here are some other items you need to consider first-

Proper Subcool

Before an expansion valve can function properly and do its job, it must have a full line of properly subcooled liquid refrigerant all the way to the inlet. On a split system checking the subcooling at the condensing unit is a good start but you also need to make sure there isn’t a significant temperature drop all the way up the expansion valve inlet. Keep in mind that some valves have a screen right at the valve inlet, so a restriction even at that point will cause operational issues.

Required Pressure Drop 

For an expansion valve to function there needs to be a significant pressure differential between the evaporator design pressure and the liquid pressure entering the expansion valve (In many cases 100 PSIG +). During cooler times of the year the outdoor condensing pressure / temperature may drop to the point that the required difference in pressure may not exist and in these cases, the valve may no longer be able to maintain the target superheat. While low ambient controls may be employed to rectify the issue is some cases, in many cases, you must simply be aware that the valve will not function as expected.

Improper Bulb Placement 

Ensure that the bulb is mounted on the suction line flat and tight with a proper strap. It is never a bad idea to insulate the bulb, and anytime it is exposed to ambient air it is a necessity.

When to Consider Adjustment 

Now you are at the point where you can consider whether that valve could use some adjustment. First, read your superheat right at the evaporator outlet in the same general location as the TXV bulb and equalizer in most cases the superheat at that point should be 5-8 degrees but refer to manufacturers specs when in doubt. In some cases, you will not have a pressure port at the evaporator so you must rely on a pressure reading outside. Use common sense when assessing the situation and realize that there may be some pressure drop on a 100′ line set and there should be very little in a 10′ line set. Make some allowance according to the situation.

If the system is running VERY low or VERY high suction pressure and /or superheat readings that are way out of range, it is very unlikely that adjusting the valve will remedy it. Usually, valve adjustments are only for small superheat changes up or down.

The Forces at Play

The bulb pressure is the opening force of the valve, so when the bulb is warmer it exerts more opening force resulting in a more “open” orifice, and when it’s cooler it exerts less opening force resulting in a more “closed” orifice.

The equalizer is a closing force so the higher the suction line pressure, the more the valve is forced closed and the lower the suction line pressure the more the valve is forced open.

The spring is also a closing force and on an adjustable valve increasing the spring tension/force results in lower flow and higher superheat, decreasing the spring tension/force results in more flow and lower superheat. In short, counterclockwise = lower superheat, clockwise = higher superheat.

Making an Adjustment

Before you adjust anything, the system must have been running for a good long while, and you have observed that the superheat has stabilized. You then must check the entire system and surmise that everything else is functional, the valve is being provided with a fully liquid, properly subcooled, high enough pressure feed of refrigerant. If at that point you find it is out of range then you can make adjustments.

  • CAREFULLY remove the hex cap from the base of the valve with a properly sized wrench and a backing wrench exposing the adjustment screw.
  • Turn 1/2 turn at a time clockwise to increase superheat or counter-clockwise to decrease superheat.
  • After a 1/2 turn adjustment, replace the panels and allow the system to run and stabilize.
  • Recheck the superheat and not the change.
  • Repeat as needed until the maximum setting is reached. NEVER force the adjustment screw too far, it should require minimal force to turn other than possibly initially to “unstick” the screw.

Adjusting a TXV / TEV is an advanced skill for a technician who has a good grasp on their readings and the forces at play. Tread carefully.

— Bryan

P.S. – Here is a great resource from Parker / Sporlan and I am also in the process of uploading a video onto the YouTube channel that will be up later tonight.

refrigeration_for_AC_Techs

In this episode of HVAC School Bryan talks with Jeremy Smith and they discuss

  • Reznor startup
  • Being on call in the refrigeration world
  • differences and similarities between rack refrigeration and A/C
  • Hot gas and electric defrost
  • Glycol refrigeration systems
  • Subcool and Superheat
  • Refrigeration TXV settings
  • EPR valves and their settings
  • Rack manifold pressure

And Much more…

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

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