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How & Where to Measure Central Air Conditioning Temperatures InspectAPedia®  -     Air conditioning system operating temperatures to measure Rules of thumb for temperatures you should find at various points in an air conditioning system How & where to measure air conditioner temperatures & what they mean Air conditioning suction line typical vacuum pressures and temperatures

This chapter explains what are the normal operating temperatures of air conditioning equipment and what temperatures can be expected when measured at different locations, as part of checking basic air conditioning system operation and for detection of air conditioning operating defects. Air conditioner temperatures that are too high or too low can indicate specific operating problems on central and portable or window air conditioners. Simple measurements of air temperatures, if made at the right place, can assist in diagnosing what may be wrong and what repairs may be needed for the air conditioner. This document is a portion of our website which describes the inspection of residential air conditioning systems (A/C systems) to inform home buyers, owners, and home inspectors of common cooling system defects.

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. 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.

OPERATING TEMPERATURES - Air Conditioning System Operating Temperatures

The following Rules of Thumb for air conditioning system temperatures are used for performing a rough check for proper operating temperature of an air conditioning system include several procedures and describe the points at which you can take your air conditioning system's temperature as a step in evaluating its health. Details about exactly where and how to make air conditioning system temperature measurements are at our next and more detailed article: Procedures for Making Temperature Measurements. Temperatures at the AC refrigerant lines: some inspectors and air conditioner service technicians make a quick check of cooling system operation by looking-at and feeling the refrigerant lines. When the air conditioning system has been operating for at least 20 minutes we expect the (large-diameter) refrigerant suction line (returning refrigerant to the compressor) to feel cool or close to outdoor air temperatures at the compressor. (If the same condensate return line or suction line is covered with heavy frost right at the evaporator coil in the air handler, the refrigerant level in the system may be too low.) At the same time, also at the compressor/condenser unit, the high-pressure refrigerant line (refrigerant leaving the compressor to return to the indoor air handler and evaporator coil) will be hot. The little chart shown in the sketch here shows how the refrigerant suction line temperature varies as a function of the outdoor air temperature at the compressor. We don't suggest actually measuring these temperatures, but a quick look-at and careful touch-of the refrigerant lines can tell a lot about how the air conditioning system is working (or not). [Remember when medical doctors used to actually touch their patients?]

Temperatures outside at the air conditioner compressor/condenser: if the air conditioning system has reached steady state and normal operation, the temperature of air blowing out of the outside compressor/condenser unit will feel [subjectively] warmer than the ambient outdoor air temperature. This is because a properly working system is transferring heat from the compressed refrigerant (perhaps 150 deg .F. inside the condenser coil) to the outside air (perhaps 85 to 90 degF ambient outdoor temperature).

Temperature differences inside at the air conditioning system air handler's evaporator coil: if the A/C system is has reached steady state operation and normal operation, the temperature of air should be reduced as it moves across the evaporator coil. Typically this drop is 15 to 20 deg .f. [Carson Dunlop adds it may be as little as 14 degF. or as much as 22 degF.]. Look for this difference between the indoor air in the building as it reaches the coil and the air leaving the coil.

Low side temperature example: A home refrigerator evaporator coil will operate at an internal temperature range of -38 degF. to 45 degF. so air blown across that coil will approach that temperature (IF you could measure air temperature right at the coil surface). .

Temperature leaving the air handler's evaporator coil: similar to above, measure the supply air temperature at a hole or crack in the supply plenum above (or "downstream from) the evaporator coil. It should be around 55 degF. Some experts argue that this is the only accurate way to examine the cooling system and that measuring temperatures across the evaporator coil is unreliable.

Temperature of conditioned air at a supply register: If a measurement opening is not available at the evaporator coil at the air handler, measure air temperatures leaving the nearest supply register. There too, unless the supply duct is long, uninsulated, running in a hot space (such as an attic), the air temperature at the supply register should be around 55 degrees. As long as the building has not reached its thermostat's set point. the system should be blowing air that is noticeably cooler than the building's air.

Too Low air conditioning output temperatures may indicate that there is a problem with air movement in the system. If air moves too slowly across the coil its temperature may be lower than the numbers specified above. Look for an evaporator coil that is partly blocked by dirt or is icing up. Too low a temperature can also be caused by improper metering of refrigerant into the evaporator coil (a bad TEV or thermal expansion valve), or by improper charging (too little refrigerant in the system).

NOTE about air conditioning refrigerant leaks: so a refrigerant leak in an air conditioning system may show up first as abnormally low system output air temperature, followed by rising air temperatures, followed by just plain old warm air coming out of the system, as the amount of refrigerant that has been lost increases. Refrigerant leaks should be found and repaired. At air conditioning school we scoffed at repairmen who developed an air conditioning refrigerant delivery route - regularly adding refrigerant to what is supposed to be a closed, non-leaking system. It may be easy and profitable, and sometimes a refrigerant leak can be hard to find, but a proper repair is to find an fix the refrigerant leak, not just to keep adding refrigerant. See  REFRIGERANT LEAK DETECTION.

Too high air conditioning output temperatures may indicate that the system is not working properly: perhaps the coil has become blocked solid with ice. Alternatively, high A/C outlet temperatures may be occurring, or even no cooling at all, due to other problems such as a loss of refrigerant. (Don't just add refrigerant, find and fix the leak. Otherwise you're just making the refrigerant delivery man rich.)

Operating Temperature Ranges of Types of Refrigeration Equipment

High temperature refrigeration equipment refers to cooling equipment that operates typically in the 25 degF to 45 degF, such as air conditioning systems.

Medium temperature refrigeration equipment refers to cooling equipment that operates typically in the range of 0 degF to 25 degF, such as a food freezer or refrigerator.

Low temperature refrigeration equipment refers to cooling equipment that operates typically at temperatures below zero, such as commercial freezing equipment.

All of these refrigeration systems operate on the same principles and in general they use similar equipment, though the choice of refrigerant liquid/gas, and the operating controls will vary.

Air Conditioner Suction Line Pressure and Temperature - Typical Data and What It Means

Determining proper suction "pressure" (really we're measuring a vacuum, not a pressure) is critical for proper refrigeration equipment operation assessment. If the air conditioner system suction pressure is too low (and that probably means the temperature in the line also is "low") it could indicate

• An incorrect setting of the TEV (thermostatic expansion valve) that meters delivery of liquid refrigerant into the evaporator coil (cooling coil)
• An oil slugged (oil logged) compressor, so refrigerant flow is blocked at the evaporator - oil slugging can lead to a compressor body failure.

Note that if we're looking at oil-slugging, the compressor head temperatures will also be abnormally high - which is how
that condition ruins an air conditioning compressor

Normally suction line pressure on air conditioning equipment is about 45 degF with a suction pressure of 42 psi for Freon 12 and about 76 psi for Freon 22. The data points for the new refrigerants will be a bit different, but this example makes clear that the air conditioning system pressure measurement numbers (both suction vacuum and compressor outlet side pressures and temperatures) will be different for each refrigerant.

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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
BOOKSTORE - Air Conditioning "How To" Books
CLEANING & Legionella BACTERIA
COMPRESSOR & CONDENSING COIL
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 CALCULATION for WALLS
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?
ENERGY SAVINGS in BUILDINGS
EVAPORATIVE COOLING SYSTEMS
FAN AUTO ON Thermostat Switch
HEAT LOSS (or GAIN) in BUILDINGS
INDOOR AIR QUALITY & HOUSE TIGHTNESS
INDOOR AIR QUALITY IMPROVEMENT GUIDE
INSPECTION LIMITATIONS
LOST COOLING CAPACITY
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
SWAMP COOLERS
THERMOSTATS
THERMOSTATIC EXPANSION VALVES
CRITICAL DEFECTS on A/C SYSTEMS
FURNACES WARM AIR HEATING SYSTEMS
INDOOR AIR QUALITY IMPROVEMENT GUIDE

• "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]
• "Pre Season Start/Check/Test Procedure" [for central air conditioning systems], Fuel Oil & Oil Heat & Solar Systems Magazine, May 1985 Issue, p. 56-71, source of the temperature chart used here.

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.
• Complete List of Air Conditioning & Heat Pump Design, Inspection, Repair Books at the InspectAPedia Bookstore.
• Modern Refrigeration and Air Conditioning, A. D. Althouse, C.H. Turnquist, A. Bracciano, Goodheart-Willcox Co., 1982
• Principles of Refrigeration, R. Warren Marsh, C. Thomas Olivo, Delmar Publishers, 1979
• "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]
• Refrigeration and Air Conditioning Technology, 5th Ed., William C. Whitman, William M. Johnson, John Tomczyk, Cengage Learning, 2005, ISBN 1401837654, 9781401837655 1324 pages
• 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.).
• See COMPRESSOR PRESSURE READINGS for a discussion of air conditioner compressor operating pressure readings, and for an explanation of high side and low side pressures on an air conditioner.
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