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Air Conditioner Compressor & Refrigerant Pressure Readings AirCondAPedia © What are the Typical Air Conditioner Compressor & Refrigerant Pressure Readings ? How to diagnose & repair loss of air conditioning cooling capacity

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.

Also see OPERATING TEMPERATURES for a discussion of what temperatures to expect at different points in the air conditioning system, and see COOLING CAPACITY, RATED of air conditioning equipment if the system seems to be working but is inadequate to cool your building. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

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. More Reading

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

If your air conditioning system has lost its cooling capacity or won't start see REPAIR GUIDE for AIR CONDITIONERS. See How to determine the cooling capacity of air conditioning equipment if the system seems to be working but is inadequate to cool your building. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left.

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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

• Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects" section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and home inspection educator. Thanks to Jon Bolton, an ASHI, FABI, and otherwise certified Florida home inspector who provided photos of failing Goodman gray flex duct in a hot attic.
• 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.).
• "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]

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Technical Reviewers & References

Search InspectAPedia

• Daniel Friedman - principal author/editor of the InspectAPedia^® Website
• InspectAPedia Bookstore lists recommended books, organized by topic & available for purchase. Most of our articles also include a list of recommended books for the specific article topic as well as other references, and information sources.
• Critique, contributions wanted: Contact Us to suggest corrections or additions to articles at this website, and if you wish, to receive online listing and credit as a contributor. Particular thanks are due to the many experts and also consumers who read and critique technical articles at InspectAPedia.com.
• Additional technical contributors & reference sources for this article are listed below.

Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

AIR CONDITIONING & HEAT PUMP SYSTEMS
AGE of AIR CONDITIONERS & HEAT PUMPS
AIR CONDITIONER BTU CHART
AIR CONDITIONER COMPONENT PARTS
AIR CONDITIONER TYPES, ENERGY SOURCES
AIR CONDITIONER NOT WORKING
AIR FILTERS for HVAC SYSTEMS
AIR HANDLER UNIT
BACKUP HEAT for HEAT PUMPS
BLOWER DOORS & AIR INFILTRATION
BOOKSTORE - Air Conditioning "How To" Books
CLEANING & Legionella BACTERIA
COMPRESSOR & CONDENSING COIL
  BURNED-OUT COMPRESSOR
  CAPACITORS for HARD STARTING MOTORS
  CONDENSING COIL FINS, SUPPORT PADS
  HARD STARTING COMPRESSOR MOTORS
  INSTALLATION ERRORS, COMPRESSORS
  MOTOR OVERLOAD RESET SWITCH
  NOISY COMPRESSOR UNITS
  PRESSURE READINGS, COMPRESSOR
  REPLACING A COMPRESSOR
  TIGHT or SEIZED AC COMPRESSORS
CONDENSATE HANDLING
CONTROLS & SWITCHES
COOL OFF HEAT Thermostat Switch
COOLING CAPACITY, RATED
COOLING COIL or EVAPORATOR COIL
DATA TAGS on AIR CONDITIONERS
COMBUSTION GASES & PARTICLE HAZARDS
COMBUSTION PRODUCTS & IAQ
DEFINITION of Heating & Cooling Terms
DEW POINT TABLE - CONDENSATION POINT GUIDE
DUCT SYSTEMS
DUCTS - Asbestos
DUCT SYSTEM DEFECTS
DUCT INSULATION, Asbestos Paper
DUCT INSULATION for SOUNDPROOFING
DUCTS, Asbestos Transite Pipe
DUST FROM HVAC?
FAN AUTO ON Thermostat Switch
HEAT LOSS (or GAIN) in BUILDINGS
HUMIDITY LEVEL TARGET
INSPECTION CHECKLIST - OUTDOOR UNIT
INSPECTION LIMITATIONS
LOST COOLING CAPACITY
  What to check first
  Compressor failure diagnosis
  Duct & Air Handler diagnosis
MOTOR OVERLOAD RESET SWITCH
OPERATING COST
OPERATING DEFECTS
OPERATING TEMPERATURES
  Air Conditioning System Temperatures
  Instruments Used to Measure A/C Temperatures
  Procedures for Making Temperature Measurements
REPAIR GUIDE for AIR CONDITIONERS
REPAIR & DIAGNOSTIC FAQs for A/C
REFRIGERANTS
  REFRIGERANT LEAK DETECTION
  REFRIGERANT LEAK REPAIR
SEER RATINGS & OTHER DEFINITIONS
SYSTEM OPERATION
  OPERATING CONTROLS
  SAFETY CONTROLS
THERMOSTATS
THERMOSTATIC EXPANSION VALVES
CRITICAL DEFECTS
Air Conditioning "How To" Books

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