# Tag: refrigerant

## What does “Saturated State” mean for Techs?

Like we often do in these tech tips, we will start with the common and more practical explanation of saturation and then move to the more technical and nerdy explanation later.

When we say “at saturation” or “saturated” in the HVAC/R trade we are generally referring to refrigerant that is in the process of changing from liquid to vapor (boiling) in the evaporator or vapor to liquid (condensing) in the condenser.

We generally look at a set of gauges or find the temperature on a PT (Pressure – Temperature) chart that matches a particular refrigerant and pressure and we call that the saturation temperature.

So when a tech connects gauges to the liquid line (high side) of a system and they look at the needle they will refer to the pressure in PSI and the temperature for the particular refrigerant as saturation temperature. On the gauge above the refrigerant in the system is R22 (green scale) they would say that the pressure is 200 PSI and the saturation temperature is 102°F.

To be even more specific, a tech might say that the condensing temperature of this system is 102°F because the saturated state is occurring during the process of condensing in this particular case.

From a practical standpoint in a refrigeration circuit when we say saturation we are referring to –

the pressure and temperature a refrigerant will be if both liquid and vapor are present at the same time and place

One of the most common cases where we will see refrigerant at saturation is inside systems that are off as well as inside a refrigerant tank. If you were to connect a gauge to tank (like this Testo 550 shown) the refrigerant pressure inside the tank will be equal to the pressure that correlates to the saturation temperature of the tank (I know that’s a mouth full but it’s really pretty simple).

In the case of the refrigerant shown above the room temperature is 71.9°F and the refrigerant is R-422D. All I had to do was connect the Testo 550 and select R422D and the saturation temperature (show above the psi on the right) is EXACTLY 71.9°F. In this case, we can say the saturation PRESSURE of R422D is 136.8 PSI at 71.9°F or that the saturation TEMPERATURE is 71.9°F at 136.8 PSI.

Either way, what are saying is that there is both liquid and vapor present inside the tank so it is at SATURATION or in the saturated state if you would rather. So as techs we see refrigerant at saturation pressure and temperature when the system is off, inside a tank and when it is in the midst of boiling in the evaporator or condensing in the condenser.

Now for the more in-depth explanation

I will warn you that this is a bit of a beating around bush explanation, but I’m writing the explanation I wish I had been given early on… so be patient young grasshopper.

The state or process that occurs when no more of something can be absorbed, combined with, or added.

So when something is “full” and can hold no more of something it is said to be saturated, like a sponge saturated with water, or air saturated with water vapor or a in this case, a liquid saturated with kinetic energy.

Many (including Wikipedia) will define saturation as the boiling point of a liquid. This definition is correct but can lead to a misunderstanding. Just because a liquid is at its boiling point doesn’t mean it is actively boiling. The refrigerant in an air conditioner is technically at the boiling point when the system is completely off. Refrigerant in a tank is at saturation (so long as it has some liquid in the tank) even though the refrigerant is static (nor flowing).

In nature, gasses (vapor) and liquids are free to move around and interact with one another with the predominant pressure being atmospheric pressure (14.7 psia at sea level).

You may have wondered why water exposed to the air will evaporate even though it has not reached the boiling temperature? This is because the temperature of a substance is the AVERAGE kinetic energy of the molecules in a substance not the specific kinetic energy of every single molecule. While there may not be enough energy for the entire substance to boil, there is enough energy in a few of the molecules to break free from the surface.

This is why when sweat evaporates off of your skin your skin cools. The highest energy molecules are leaving and taking themselves and their high energy ways with them!

Translation – Some molecules have more energy than others and are able to escape the liquid form out in nature and we call this evaporation. This evaporation can be measured but it happens below the boiling point and when a substance is uncontained it results in less and liquid remaining.

Translation of the Translation – If you leave water out in a pan it will eventually disappear even when it isn’t boiling

Now if you put a liquid in a jar and screw the lid on, some of the molecules will escape the liquid bonds and fill the void in the jar until pretty soon the jar will be at equilibrium (static) pressure with an equal number of molecules condensing back into the liquid as those that are escaping. The more active the molecules in the jar the more pressure there will be in the jar. Since the definition of temperature is the average kinetic energy of the molecules you can translate that as “The hotter the jar the higher the pressure” or “The higher the pressure the hotter the jar”.

Different liquids have a more or less tendency to escape the liquid form (evaporate), liquids that have a very high tendency to escape will evaporate more quickly and have a higher “vapor pressure” and are also said to be more “volatile”. Alchohol or gasoline are liquids that are more volatile and have a higher vapor pressure at atmospheric pressure than water and disappear quickly even when the ambient temperature is below their boiling point.

Some Liquids (like vacuum pump oil for example) have a very low tendency to evaporate and are said to have very low volatility and a low vapor pressure.

Liquids with low boiling temperatures (like most refrigerants) are very volatile and have a higher vapor pressure than liquids that remain a liquid at atmospheric pressure. We know that refrigerant does more than evaporate at atmospheric pressure and normal atmospheric temperature, it literally BOILS.

A liquid boils when the vapor pressure of the liquid matches the atmospheric pressure. At that point the liquid molecules begin to break free rapidly and if they are uncontained they will simply fly away like water vapor out of an open pot.

If the molecules are boiling and contained they will begin increasing the pressure as they boil until the temperature of the liquid no longer increases and it hits equilibrium between the vapor pressure of the liquid and the pressure inside the vessel (tank, pressure cooker etc..).

Once the vessel is allowed to reach a state of perfect equilibrium it may no longer be boiling but it can still be at the boiling point, that exact POINT of equilibrium between vapor pressure and temperature is the SATURATION POINT.

So long as the pressure remains constant on a boiling or static vapor/liquid mixed substance we can say that it is at saturation temperature because it remains at the same temperature until either

1. The pressure changes
2. The substance is fully boiled

But it is important to remember that it is the vapor pressure of a liquid substance being equal to the pressure around it that results in saturation and then boiling or in the opposite direction, condensing.

Also… Evaporators should be called boilerators but I’m doing being nerdy for now.

— Bryan

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

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.

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)
• Reversing valve bypassing
• Discharge line restriction
• 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
• Reversing valve bypassing (heat pump units)
• Kinked suction line
• Restricted discharge line
• Severe Liquid Line Restriction
• 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
• 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

## Does Refrigerant Get Old or Wear Out?

Over the years I have heard technicians say that refrigerant can wear out or “lose it’s blend” by sitting in a tank.

This does not happen… at least not like that

What can and does happen is called “Fractionation”. Refrigerant blends that are composed of a mix of refrigerants with different vapor and liquid PT characteristics known as Non-azeotropic, Zeotropic or in some cases near-azeotropic. All fancy words to mean that these refrigerant blends must be added or removed completely or in the liquid state to prevent more / less of one refrigerants in the mix to be added or removed than the other.  If the refrigerant is allowed to fractionate and some of it is added in the vapor only state both the refrigerant left in the tank, and the refrigerant added to the system will no longer have the designed properties of the listed refrigerant.

If one of the refrigerants in the blend leaks out faster, what you have left isn’t the same refrigerant

While all blends should all be charged in the liquid state, some refrigerants are more likely to be impacted by fractionation than others.

For example, R-410a  (50% R-32 & 50% R-125) has very little “glide” between liquid and vapor and so while it is a blend, it is less likely to fractionate severely when charged in the vapor phase (which you still shouldn’t do).

A refrigerant like R-407c ( a mixture of R32/125/134a) will fractionate much more easily resulting in far greater pressure / temperature swings and poor performance when it occurs.
Fractionation will often happen for three reasons

1. A technician charged the system in vapor phase (tank upright) instead of in liquid phase (upside down)
2. The tank had a small leak while stored upright
3. The system has a significant leak.

The particular case of fractionation being caused by a system leak depends on many factors including what part of the system the leak occurs, the physical location of the leak and how much refrigerant leaked out. There was a study done at Purdue that shows that fractionation after leakage can be a factor in high glide systems like R407c.

The ramifications of this depend on the specific situation, but in some cases, the only viable option will be to completely recover and recharge with a virgin charge. This is not because refrigerant has “lost its mix” from sitting, but rather because some of the”mix” has left the tank or system at a different rate, leaving an improper mix behind.

— Bryan

## Weighing Refrigerant In and Out

If you don’t use a scale every time you add or remove refrigerant I would suggest you begin doing so immediately if not sooner. Weighing in while adding is fairly obvious and is useful so you can keep track of what you are using and how much to charge a customer.

When you have a system that has just been repaired it is a good practice to weigh in the charge to factory specs plus or minus adjustments for lineset if it is a split system. This is all pretty evident, but why would you weigh a charge out? There are many reasons but I watched a video by Stephen Rardon today that re-ignited the importance of weighing refrigerant out in my mind. Whenever you have a failed compressor, weighing out the charge can help indicate whether possible undercharge or overcharge may have contributed to the failure. With any significant failure on an older system, weighing out the refrigerant can indicate whether a leak is likely. Stephen went so far as to weigh out the refrigerant on a failed shorted at the time of diagnosis… BRILLIANT!

Using refrigerant recovery as a means to find possible cause or even diagnose leaks on non-functional systems is next level diagnosis in my book. Use your scale.

Well done Stephen.

— Bryan

## Electronic Leak Detection

Electronic leak detection is a critical part of any HVAC technicians common practice. Unfortunately, it is also one of the most common sources of misdiagnosis. Here are my tips to make your leak detection more successful.

Before starting to use your detector STOP! look for signs of leaks and corrosion throughout the entire system. I see so many techs who use an electronic leak detector with a very large leak when they would have been better served pressurizing and pinpointing the leak with soap bubbles.

Get a Good One

Use a good quality leak detector. Hint: If it costs less than \$300 it probably isn’t great. I am fan of the H10G and the H10Pro although we are testing the Tifzx-1 as a possible option on the recommendation of a few good techs I trust.

Check your detector and make sure it actually works EVERY TIME. The H10G has a reference bottle for testing.. USE IT

Let it Warm Up

Many leak detectors require a warm up time for the sensor. With the H10G I allow it to run for at least 5 minutes before I start to use it.

Start at The Top

Most refrigerants are heavier than air, starting at the top and working your way down will help keep you from picking up a leak below the actual point of origin.

Don’t Rush

Move really slow and when you do get a hit, remove the wand, let it clear and go back to the same point a few times before calling it a leak. Once you think you found a leak, attempt to use bubbles to fully confirm.

Use Common Sense

No matter what leak detector manufacturers tell you.. there ARE other substances that can trigger your detector and refrigerant can move from one place to another due to drafts. I have seen several cases where chemicals in a garage are triggering the detector or where a tech has misdiagnosed an evaporator coil because of a chase leak where the refrigerant is being pulled from beneath the unit into the return. Look around and make sure there is nothing causing interference.

Be Sure

Before you condemn that coil BE SURE. Use all of your resources to positively confirm the exact location of the leak. A little patience goes a long way.

— Bryan

## Refrigerant Charging Basics For Air Conditioning & Heat Pump Systems

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.

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 subcool 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 with a below 32 degree 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 all manufacturers have a charging chart available with their respective units.  With the information your 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 the Emerson HVAC check & charge app .  Some manufacturers require that you determine the wet bulb temperature.  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.  If, lets say, the recommended superheat was 18 degrees  you would add 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 subcool method.

All manufacturers have recommended subcool charging information with the units.  If, for some reason, there is no information with the unit, or if it has worn off, you can set a charge to 10 to 12 degrees of subcool 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 degree 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.  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 outdoor temperature is below 65˚.  Follow the instructions on the unit carefully when charging in subcool in <65o temperatures.  The method requires that you block sections of the coil to achieve higher head pressures before setting by subcool.

### 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 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 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 40 PSI . 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

## The 5 Readings Every Tech Must Know Well

In this episode of HVAC School Bryan cover the “5 pillars of refrigerant circuit diagnosis” and why they matter. They are:

• Superheat
• Subcool
• Suction Pressure
• Air temp split (delta T)

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

## Oil and Refrigerant

We all see a lot of questions about, dyes, leak stoppers, lubricants, inhibitors and snake oil.. all designed to go in the refrigerant circuit and “improve” something.

I just go back to what Ray Johnson always said (Ray is one of my early heroes in the biz and taught me a lot at Carrier classes)

“Oil and refrigerant”

If it isn’t oil and refrigerant, it doesn’t belong in the system.

Now I know there are rare exceptions… Heck, Carrier has a tech bulletin from 2014 dictating that Zerol Ice should be added to certain systems due to TXV sticking issues.

But when in doubt.. Just oil and refrigerant.

— Bryan

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