Refrigerant charge quantity for air conditioners & heat pumps: This air conditioning repair article series discusses the the diagnosis and correction of abnormal air conditioner refrigerant line pressures as a means for evaluating the condition of the air conditioner compressor motor, which in turn, is a step in how we evaluate and correct lost or reduced air conditioner cooling capacity.
We explain how overcharging or undercharging of refrigerant in an air conditioner or heat pump is detected and we list the effects of overcharged or undercharged refrigerant. We also explain the various causes of liquid slugging a compressor motor.
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Refrigerant pressure readings measured at the air conditioning compressor/condenser unit and which are found to be too low on the high pressure side (compressor output) or on the low pressure side (compressor input or suction line) can indicate a problem with the compressor's ability to develop normal operating pressure ranges and thus will affect the cooling capacity of the air conditioning system.
Abnormally high compressor output pressures are possible but less likely. (See Two Basic A/C Refrigerant Pressure Diagnostics below).
Article series contents
First let's explain "low-side" and "high-side" air conditioner compressor motor pressures and what they mean.
Air conditioning service manuals and training refer to:
Typical residential air conditioning refrigerant pressures vary depending on the model, compressor motor size and design, and the refrigerant used. The design pressures may be provided on labels attached to the equipment but the actual air conditioner operating pressure will vary in part as a function of the incoming air temperatures.
"Charging Charts" (such as the commercial unit charging chart shown here) are provided in service manuals to determine the target suction vacuum (negative) pressure and output pressure for a given compressor motor.
Use of the charging chart for the specific compressor is the correct way to service it. The following example pressures are based on "rules of thumb" that get you in the right "ballpark" if no charging chart is at hand.
Example actual air conditioner compressor high side output pressure: using R-22 refrigerant and assuming an outside air temperature of 85 degF called for 120 degF. inside the compressor (add 35 degF. to incoming air temperature) and an output high-side compressor pressure of about 260 psi.
Example of actual air conditioner low side input or suction line pressure during operation (low-side pressure) during normal operation of the same compressor model and refrigerant and the same outdoor air temperature of 85 degF called for 45 degF. temperature entering the compressor (subtract 40 degF. from incoming air temperature) which on the service chart indicates that the incoming or suction line pressure would be about 75 psi.
Example of a more theoretical air conditioner or heat pump pressure and temperature at the compressor and at the cap tube or thermostatic expansion valve during normal operation: at an outdoor temperature of 72 degF, liquid refrigerant (R12 for example) leaving the outdoor condensing coil and entering the cap tube or TEV might be at 100 psi and 95 degF.
These numbers vary by changes in ambient temperature, compressor model, and refrigerant gas used. On the low side of the TEV or cap tube (in the cooling coil in the air handler) where the liquid refrigerant is changing state to a gas, it may be cooled down to 10 degF. and by the time the refrigerant leaves the cooing coil (evaporator coil) and gets back to the compressor motor it will be all vapor and may be at just 15 psi. [R12 refrigerant changes from liquid to vapor at 14.6 psi at 10 degF.
Air Conditioner or Heat Pump Refrigerant Equalization Pressure - System-OFF refrigerant pressures
When you measure heat pump or cooling system pressures makes as much difference as where you measure it. When an air conditioning or heat pump system has turned off and been off for some time (30 minutes or more) pressures equalize throughout the system between the high and low sides.
At that point the refrigerant pressure in both the high side and low side of the air conditioner or heat pump system will be in accordance with the ambient air temperature and the properties of the particular refrigerant gas present.
The static or equalized system refrigerant pressure will be defined by the refrigerant gas type (which defines its boiling point and pressure at various temperatures).
For example with that cute old R12 refrigerant, as long as there is just about any refrigerant in the system - enough so that there is some liquid refrigerant, i.e. it's not all just gas) then in equalized condition at 70 psi ambient temperature the refrigerant pressure will be 70 psi.
With a temperature correction chart you can read the static or equalized refrigerant pressure for any refrigerant gas and the actual ambient temperature.
Reminder: this refrigerant gas behavior means that if you use pressure test gauges (GAUGE, REFRIGERATION PRESSURE TEST) to measure the refrigerant pressure in the static or equalized air conditioning or heat pump system, the gauges only tell you the refrigerant pressure, not the quantity of refrigerant that is present in the system.
Refrigerant Leak, short charge: Low head pressure: If the head pressure at the compressor is low we figure that there is a short charge - that is, the system has lost refrigerant.
Condenser coil plugged: High head pressure: if the head pressure at the compressor is abnormally high we figure that the condenser coil is plugged and needs replacement.
Short refrigerant charge: Low pressure on both the High and Low sides of the system typically means that there has been a loss of refrigerant or a short charge.
Frozen TEV: Low side pressure or zero pressure on the low side of the air conditioner or heat pump system may mean that the metering device such as a TEV is frozen or jammed and is not releasing any refrigerant into the cooling coil. In this condition the high side pressure may go up, then down. You can test and temporarily cure this condition by warming the TEV or cap tube.
Plugged or saturated drier: by comparison with the above conditions, a partially clogged drier will form a restriction in the refrigerant line so that the low side pressure drops and the high side pressure increases. You may also notice that the refrigerant line temperature is significantly different on the inlet and outlet sides of the drier.
High side pressures in the air conditioner or heat pump system that are too low
High side pressures in the air conditioner or heat pump system that are too low (100 psi for example) can indicate that the compressor is failing (cannot pump up to pressure) or that the refrigerant metering device is stuck wide open and the system is not developing enough pressure difference between the high and low sides.
High side pressures in the air conditioner or heat pump system that are too high
High side pressures in the air conditioner or heat pump system that are too high can also mean serious trouble: a blocked condensing coil, blocked filter/dryer on the high side, or a refrigerant metering device (TEV) that is stuck closed.
A small refrigerant pressure change on the high pressure side of a refrigeration system will make a big change on the low side. A common field diagnostic step is to quickly look at the system low-side pressure since if that reading is bad you know that there is a problem on the high side.
Low side pressures in the air conditioner or heat pump system at 90 psi or up mean trouble
Low side pressures in the air conditioner or heat pump system reflect the compressor's ability to draw refrigerant through the system and the rate of metering and evaporation of refrigerant in the cooling coil.
While newer higher efficiency air conditioners and heat pumps run at higher suction pressures than older units, a rule of thumb used by many HVAC techs is that the low side pressure should be well under 90 psi. If you are seeing 90-100 psi (or higher) on the low side of the system then either the compressor is damaged (not able to pump down to a low enough pressure) or the refrigerant metering device is stuck wide open and flowing too much refrigerant through the system.
The manufacturer specifies the quantity of refrigerant that should be placed into any system: air conditioner, heat pump, refrigerator, freezer.
Especially on residential systems installing the proper total refrigerant charge, which has to take into account not just refrigerant liquid volume but also ambient temperatures, is critical. [On many commercial refrigerant systems there is a receiver that holds a larger buffer quantity of refrigerant, so you'll notice the effects of refrigerant loss only after quite a bit has leaked out.]
Changes in air conditioner or heat pump operating pressure can be effected by adding or removing refrigerant from the system. Changing the amount of refrigerant will cause a pressure change at the point where the refrigerant changes state. Normally an HVAC technician will charge the system to its recommended pressure and we won't vary the total refrigerant charge away from what the system manufacturer recommends.
How to Use the Equipment Data Tag Information to Charge Air Conditioners, Heat Pumps, or other Refrigeration Equipment
Technical detail: refrigerators and some other equipment have a data tag that give a test pressure. Ignore this number when charging the system. This is a leak test pressure.
The data tag also gives the type of refrigerant that should be used in the system (no you cannot substitute). And the data tag will give the proper refrigerant charge quantity, typically in ounces. For a small residential equipment (a refrigerator) this may be 5-11 ounces.
Remember that the location of the frost line (on the cooling coil and refrigerant piping) can indicate evidence of overcharging.
Bottom line about overcharging refrigerants: even if you don't destroy the compressor motor by overcharging the system will be operating at a higher temperature and thus will be operating less efficiently. For residential equipment such as air conditioners, heat pumps, refrigerators, freezers, to work properly you must have exactly the correct charge in the system.
Effects of Overcharging the Refrigerant Level in an Air Conditioner, Heat Pump, or other refrigeration equipment
The effect of too much refrigerant in the system - overcharging, over-metering, or other high refrigerant pressure situations are detailed at EFFECTS of OVER CHARGING of REFRIGERANT
Watch out: over-charging refrigerant can damage or even destroy the compressor unit in some HVACR systems.
Effects of Undercharging the Refrigerant Level in an Air Conditioner, Heat Pump, or other refrigeration equipment
Effect of too little refrigerant in the system are described in detail at EFFECTS of UNDER CHARGING of REFRIGERANT
Separately at FROST BUILD-UP on AIR CONDITIONER COILS we also explain that in a properly tuned and adjusted refrigeration system there will be liquid refrigerant found all the way to just at the end of the evaporator coil .
Technical Background on Air Conditioner Pressure Gauge Readings
Gauge pressure" can read either the pressure inside the condenser unit (the "high side" of the system) or the pressure inside the evaporator (cooling coil) or "low side" of the system.
The gauge set accepts three connecting hoses:
Note that gauge pressure is an absolute pressure reading before any correction for ambient temperatures around the unit. When comparing measured or gauge pressure with recommended refrigerant pressures it is necessary to correct gauge pressure for ambient temperature variations. A/C equipment, gauges, refrigerant charging manuals etc. include pressure charts to aid in this correction.
Low-side pressure calculation example
Using now-obsolete R-12 refrigerant gas as an example, looking at the low-side refrigerant pressure (the low side is the side at which the refrigerant liquid has boiled into a gaseous state), we can calculate the temperature at which the refrigerant should boil for given ambient conditions:
(38-45 degF example) - (18 degF temperature difference between inside the condenser and ambient) = 20 degF = the temperature at which the refrigerant must boil, i.e. the state change from liquid to vapor. Looking at 20 degF. in the table for R12 state changes shows us that we should see 21 pounds of pressure.
High-side pressure calculation example
At 80 to 100 psi pressure on the high side, if ambient temperature is 72 degF, heat will be transferred successfully to outdoor air at temperatures of 84 degF to 117 degF at the condenser coil.
More Notes About Residential Air Conditioner Compressor Pressures
Put another way, high temperature or high pressure on the air conditioner low side is a sign of a problem. That is, as pressure on the high side goes way up, low side pressure will increase as well, and we may exceed the operating temperature of the system. The Low side temperature must be low enough to get transfer of heat from the indoor air into the evaporator coil. The High Side temperature must be high enough to get transfer of heat into the outdoor air.
If your air conditioning or heat pump system has lost its cooling capacity or won't start see REPAIR GUIDE for AIR CONDITIONERS.
Continue reading at REFRIGERANT SIGHT GLASS or select a topic from the More Reading links shown below.
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Frequently Asked Questions (FAQs)
(Feb 19, 2014) tommy said:
Does the volume of the Freon charge within a heat pump system need to be increased when it is heating a house vs. cooling the house. My HVAC contractor has told me that the system needs a larger volume of Freon when heating the house. He recently added two additional pounds over what was deemed a normal operating refrigerant charge condition during the summer months(cooling the house). This seems a bit fishy to me. I would appreciate some feedback on this tTommy,
Nice question. The answer is no. The refrigerant charge in an HVAC system does not change (unless there is a leak). In heating mode rather than cooling mode basically the same system is operating with the same components and refrigerant, but it runs backwards: moving heat from outdoors to indoors when in heating mode, and moving heat from indoors to outdoors in cooling mode. The same refrigerant and refrigerant quantities are used.
Your contractor may have been thinking of something else, or may be arm-waving, or we may have misunderstood. For example, s/he MIGHT have been trying to say that the total capacity of your heat pump was adequate for cooling but not for heating in your climate.
But by NO means would that problem be fixed by changing the refrigerant charge in the system. Instead you' need a different (greater capacity) heat pump system installed.
Technical note: unlike water whose volume changes only a small amount as a function of temperature, most refrigerant liquids change more significantly in volume. That's why on an old refrigerant charging cylinder you'd see that volume gauge printed right on the cylinder side giving different charge volumes at different temperatures.
To understand the relationships between pressure and volume of gases, such as refrigerant gas, take a look aty our discussion of Boyle's Law (P1V1 = P2V2) at WATER TANK PRESSURE CALCULATIONS and also at OIL TANK PRESSURE
(Mar 8, 2014) maryjo said:
hi, my trane air conditioner has pressure readings of 75 and 135 respectively. the a/c people that came out said that the unit was failing and should be replaced. Are these pressures ok?, and will it fail?. 1 month prior, a service guy came out to do the annual and said the unit was fine?....any suggestions?
Maryjo I'm sorry but I'm not sure how to interpret pressures without knowing where they were measured and what was the status of the equipment at the time. In general, if in an HVAC system that has the proper refrigerant charge quantity the compressor is unable to reach the proper high and low side pressures that something is wrong. The problem could be a costly - one: need a new compressor motor, or a less costly one such as a failing refrigerant metering valve (TEV or "thermal expansion valve"). I would listen to the tech's description of what she or he checked and found to get an idea of the scope of investigation that was made.
Luis Castro owner of castro mechanical said:
Re posted without prohibited link. - editor
5/13/14 Anonymous said
Where must the low pressure (green-hose) and the high pressure (red hose) on the refrigerator be connected because the yellow hose must be connected on the refrigerant ?
Anon, although the HVAC system you are looking at may not show the high and low side connector ports or just one of them, this sounds as if you ought to be asking for an on-site trained HVAC technician.
(May 15, 2014) Chris said:
When I connected the refrigerant gauge to the low port the reading was in a good range (green on the gauge). When I pulled the trigger the gauge goes to zero. I put one can in and the air did get much cooler, but the reading really did not change. Is this a indication the compressor is bad if the pressure is fine, but no cold air.
Chris I can't say, not knowing the system type, but it sounds as if you might want to check to see what is the proper charge for the system in the first place.
(May 17, 2014) zee said:
When I connected the gauge to my ac unit the low side read 10psig and high side read 100psig. What wrong with the unit? low freon, TEV block, air in the system, or bad compressor?
Sounds like a bad compressor in that you're not seeing much high side pressure, but a very low total refrigerant charge could also explain the problem so I'd start by evacuating the system and installing a proper charge quantity.
(May 23, 2014) Anonymous said:
shouldn't there be considerable pressure on a unit, even if it hasn't been run for a while?
No Anon. When the compressor is off the system PRESURE on typical residential cooling systems or heat pumps drops to the normal pressure for a refrigerant at that temperature. When the compressor motor has been off for 15-30 minutes or longer, the high side and low side pressures on most equipment will be the same. It's the compressor that creates the high pressures when it's running.
(May 27, 2014) Anonymous said:
yes, the system was running the low side read 10psig and high side read 100psig.
(June 7, 2014) Tim said:
System has capillary tubes and scroll compressor. AC cooling by -10 degrees (dry supply - dry return) and no cooling when measuring: (dry supply - wet return = 0).
What could be the problem. My understanding is that neither capillary tubes nor scroll compressor can partially malfunction. They either run or breakdown completely.
Appreciate your feedback and thanks for the great post.
Tim that's a rather high pressure on the suction line, suggesting a refrigerant metering problem.
(June 9, 2014) Priyam said:
(The system is in a laboratory, and is used for testing)
On a different note, great information on the website! Very useful.
Priyam, check the adjustment of the TEV itself.
(June 30, 2014) Rich English said:
Heat pump will not cool at high outside temps. I read your site (very nice job guys) and I will list what the problem isn't
Problem when out side temp hits 90F the system starts to cool less. Below 90 degrees I see a differential inside temp of about 18-22 degrees. at 90F the differential drops to about 15 degrees. at 95F outside we go down to about 12 degrees and at 98 degrees it drops to about 8-9 degrees making the system runs all day in the Texas summer. Unit is 20 years old and I'm wondering what part or parts could cause this strange problem. Please help as the ac guys have all failed to figure this out. I call them gas men as they seem to think playing with the refrigerant level will solve it.
A few more comments...
A couple of things to check first:
1. check the air temperature drop across the cooling coil.
2. check for partial frosting at the AC coil - a small refrigerant leak drops pressure and can cause that problem.
(July 18, 2014) Larry said:
I purchased a slightly used 2.5 ton R-22 heat pump to replace my 10 yr old a/c condenser unit. The "new" unit is pressurized but I don't know if the R-22 was pumped down into the condenser or was reclaimed and filled with inert gas. Is there a way to determine without losing the R-22? Would a non operating condenser filled with some amount of R-22 show about 120 psi at 78 degrees F? I have tried locating the original installer but have had no success. Worst case, I will recycle whatever is in it and refill with virgin R-22.
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