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.
Some Basic A/C Refrigerant Pressure Diagnostics
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.
How Much Refrigerant is in the Proper Charge?
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.
Effects of Overcharging the Refrigerant Level in an Air Conditioner, Heat Pump, or other refrigeration equipment
Effect of too much refrigerant in the system - overcharging, over-metering, or other high refrigerantpressure situations: normally we want the low side pressure to be as low as possible for refrigeration systems. Excessive refrigerant in the system raises the system operating pressure and temperature and actually reduces the cooling ability of the system.
Overcharging of the refrigerant in a closed refrigerant system such as a typical residential air conditioner, heat pump, or refrigerator, where no liquid refrigerant receiver is included, has the following effects:
- Improper operating pressures - too hight: Refrigerant pressure will be increased on both the high side and low side of the system
- More expensive operation: The system may still be able to change state, but at a higher temperature than intended, thus in an air conditioner, it won't cool as effectively nor as efficiently as with the proper charge
- Causes of Liquid Slugging the compressor motor:if liquid refrigerant enters moving parts of the compressor motor, the motor is likely to be destroyed. The compressor motor's moving parts (piston & cylinder, scroll spirals, or rotary vane pump) expect to be compressing a gas and can't handle liquids. HVAC technicians refer to these conditions as liquid slugging.
Several causes of liquid slugging, (also cited at AIR CONDITIONING & HEAT PUMP NOISES) include:
- An overcharge of refrigerant in the system can result in liquid refrigerant entering the moving parts of the compressor motor where only a gas is expected .
- A crankcase heater that is burned out or that is not being turned on when it should by the system controls (which can also destroy a heat pump compressor that is trying to operate in cold temperatures)
- A bad therostatic expansion valve (or one that is improperly adjusted): Liquid slugging might also occur if a refrigerant metering device is not properly restricting refrigerant flow from the high side to the low side of the refrigerant piping system. Liquid refrigerant may not all turn to gas on the low side of the system - if liquid refrigerant enters the compressor motor it is likely to destroy it
Air Conditioner or Heat Pump Compressor Motor or other Electric Motor Runs Backwards?
An electric motor may start running backwards due to a failed start capacitor, a reaction to high refrigerant pressure (for refrigeration motors), or other electrical events.
High refrigerant pressure in an A/C unit or heat pump or some other refrigeration motors can cause the equipment to run backwards. Cooling compressors, heat pump compressors and some other electrical motors can run backwards too: well pumps, fans, even an A/C or heat pump compressor.
If the compressor motor is a scroll-type design and in some cases if the motor is a rotary vane type design high refrigerant pressure combined with a loss of electricity can, on return of power, start the motor backwards. Scroll type compressor motors use an anti-restart control (basically a timer) to prevent the compressor from starting to run backwards.
A scroll-type compressor (and possibly a rotary vane A/C compressor motor) can start to run backwards if the motor is suddenly stopped (due to brief power loss or because you foolishly switched its electrical power "off" and back "on" quickly. In that condition, high refrigerant pressure on the outlet side of the compressor motor gives it a backwards push. When power is restored that backwards push gets the motor running backwards.
And as we discuss at CAPACITORS for HARD STARTING MOTORS, a bad start capacitor can let a motor run backwards. We've also found cases of water well pump motors running backwards after a lightning strike. And at Questions & Answers about compressor/condenser unit fans we include a field report of a backwards running condenser unit fan.
Effects of Undercharging the Refrigerant Level in an Air Conditioner, Heat Pump, or other refrigeration equipment
Effect of too little refrigerant in the system
When we undercharge an air conditioner, heat pump, or other refrigeration equipment
- Improper operating refrigerant pressures, too low: surprisingly to the novice, too little refrigerant in the system can actually drop the temperature in the cooling coil below its normal operating range; that's why we mention at FROST BUILD-UP on AIR CONDITIONER COILS that a cause of coil frosting in the air handler might be an early sign of a refrigerant leak.
Loss of cooling capability: eventually when enough refrigerant leaks out of the system temperatures rise again because we no longer have any heat exchange between the condenser coil and the outdoor air nor between the then empty cooling coil and the indoor air.
- More expensive operation: There is not enough refrigerant in the system, for example to properly fill the cooling coil - then we remove less heat (per unit of time operation of the equipment) so we are decreasing the operating efficiency of the system.
- Cooling Coil Frosting: Too-low refrigerant levels in some systems also can cause frosting and freezing at the cooling coil.
- Compressor motor damage: Too little refrigerant in an air conditioner, heat pump, refrigerator, or even a dehumidifier is likely to cause overheating of the compressor motor. That is because in a properly-charged refrigeration system, the refrigerant is cooling the electric motor that is sealed inside the compressor unit. An overheated A/C motor may trip a circuit breaker, may have trouble starting, and eventually will fail to run at all.
Watch out: when an air conditioner or heat pump motor "burns out" the result is a costly contamination of the entire refrigeration system as burned lubricants and even shellac or coatings on motor windings will have been circulated through the refrigerant piping system. That's why an experienced HVAC repair person will insist on cleaing the entire system and installing one or more "burnout" refrigerant filter/driers (REFRIGERANT DRIERS & FILTERS) on the refrigerant lines when the compressor is replaced.
Bottom line about undercharging refrigerants: For residential equipment such as air conditioners, heat pumps, refrigerators, freezers, to work properly you must have exactly the correct charge in the system.
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 - this gives us maximum cooling efficiency of the equipment.
Guide to Using the Data Tag Information to Charge 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.
Technical Background on Air Conditioner Pressure Gauge Readings
 In our illustration of air conditioner service equipment pressure test gauges at page top and at left, you notice that there are two gauges and two sets of connectors and control valves. "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:
- Low pressure side (blue hose on my gauges)
- High pressure side (red hose)
- Refrigerant gas source (center fitting)
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.
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