Photograph of a
commercial air conditioning compressor charging gauge set (C) Air Conditioner Compressor & Refrigerant Pressure Readings
Typical HVAC refrigerant pressures & quantities

  • REFRIGERANT PRESSURE READINGS - CONTENTS: What are the Typical Air Conditioner Compressor & Refrigerant Pressure Readings ? A/C Refrigerant Pressure. How to diagnose refrigerant pressure problems; how to determine the proper refrigerant charge quantity. Definitions of HVACR "high side" & "low side" pressures; typical refrigerant pressures; effects of overcharged or under-charged refrigerant
  • POST a QUESTION or READ FAQs about refrigerant pressure readings in air conditioners, heat pumps, & other refrigeration equipment

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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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What are the Typical Air Conditioner or Heat Pump System Pressures During Normal Operation

Photograph of a
commercial air conditioning compressor charging chartRefrigerant 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

Definition of Air Conditioner High Side and Low Side Pressures

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:

  • Low-side air conditioner compressor motor pressure: this is the pressure in the air conditioner's refrigerant suction line (low side pressure during compressor operation) and this will be a relatively low number, often less than 100 psi.

    During operation refrigerant is returning to the compressor from the cooling (evaporator) coil in this line. If we connected the suction line directly to a sealed vacuum test gauge we'd actually find that the compressor could pull an actual vacuum on the line. (We used to use an old Frigidaire™ rotary-design compressor motor as our vacuum pump when we needed to get the air out of a refrigeration system prior to charging it with new refrigerant.)

    The low-side of an air conditioning system is always located inside of the space to be cooled, or inside of an air handler which moves air through the space to be cooled.

    By lowering the pressure in the cooling coil located on the "low side" of the air conditioning system, the compressor permits liquid refrigerant to be discharged into the cooling coil where the change of refrigerant state from a liquid to a gas absorbs heat and brings the cooling coil to the proper operating temperature.

    The LOW SIDE of a refrigeration system is the low-pressure and low temperature half of the system. Normally this is the indoor air handler - located inside the space that is to be cooled by bringing indoor air to operating temperature. (For a refrigerator this is typically 38 to 45 degF.)

  • High-side air conditioner compressor pressure: output (high side pressure during operation) is the pressure of the compressed refrigerant gas as it leaves the compressor motor. In other words, refrigerant gas returns to the compressor through the suction line from the cooling coil (which is cooling building air).

    The low-pressure refrigerant gas is compressed to a high-pressure refrigerant gas inside the compressor motor. This high temperature refrigerant gas is then cooled down to condense into a refrigerant liquid before it is returned indoors to the air handler and evaporator coil to be used to cool building air. (Thus we get the name the name "condensing coil" and "condensing unit" or "condenser" for the outside half of an air conditioning system.)

    The high side components of an air conditioning system, such as the compressor, condensing coil, and fan unit used to cool the condensing coil are located outside of the conditioned or refrigerated space, and will be immersed in air at ambient outdoor temperature, say 72 degF.

    Here is where the magic of air conditioning occurs. As long as the compressor can produce a temperature in the outdoor condensing coil which is above ambient outdoor air temperature, heat will flow from the condensing coil into outdoor air (for example outdoor air blown across the condensing coil by a fan).

    If you studied thermodynamics in high school you learned that heat always flows from the warmer to the cooler material. The effect is to transfer heat gathered in the indoor or conditioned space into outdoor air.

    The HIGH SIDE of a refrigeration system is at high temperature and higher (refrigerant) pressure and will always be above ambient temperature. So in a cooling system it will be located outside in order to transfer heat to the outdoor air. A heat pump designed to pump heat into a building will, of course, reverse these roles when in heating mode.

Typical HVAC Refrigerant Pressures

Photograph of a
commercial air conditioning compressor charging chart

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.

A/C or Heat Pump Refrigerant Pressure Problem Diagnostic Checklist

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.

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

Photograph of a
commercial air conditioning compressor charging gauge set (C) InspectAPedia.comIn 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:

  1. Low pressure side (blue hose on my gauges)
  2. High pressure side (red hose)
  3. 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.

  • See SEER RATINGS & OTHER DEFINITIONS for additional definitions of the high side and low side of an air conditioning system.

  • See COMPRESSOR CONDENSER for an explanation of how and why high side and low side pressures in the cooling system enable an air conditioner to move heat from indoors to outdoors.

  • See OPERATING TEMPERATURES HVAC for a discussion of the typical temperatures at which various types of cooling systems operate.

If your air conditioning or heat pump system has lost its cooling capacity or won't start see REPAIR GUIDE for AIR CONDITIONERS.


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