Air Conditioner Compressor & Refrigerant Pressure Readings InspectAPedia® -
What are the Typical Air Conditioner Compressor & Refrigerant Pressure Readings ?
How to diagnose & repair loss of air conditioning cooling capacity
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This chapter of "How to Inspect the Central Air Conditioning or Cooling System" 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.
Typical air conditioner compressor pressures during normal operation
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. 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(TM) 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.
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.
Examples of Residential Air Conditioner Compressor Pressures
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 air conditioner 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 air conditioner 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.
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 for a discussion of the typical temperatures at which various types of cooling systems operate.
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Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
Carson Dunlop, Associates, Toronto, have provided us with (and we recommend)
Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
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