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Photograph of a dial thermometer reading a high output temperature at an air conditioning compressor How & Where to Measure Central Air Conditioning Temperatures

  • AIR CONDITIONER TEMPERATURES - CONTENTS: typical operating temperatures for Air conditioning systems & heat pumps. 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.
  • POST a QUESTION or READ FAQs about how, where, & when to measure central air conditioning or heat pump system temperatures
  • REFERENCES
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Air conditioner or heat pump operating temperatures:

An introduction to air conditioner temperatures and some rules of thumb that are quick and easy to apply in diagnosing air conditioner problems.

This article answers: what are the normal operating temperatures of air conditioning equipment and what temperatures can be expected when measured at different locations, information that we look at as part of checking basic air conditioning system operation and for detection of air conditioning operating defects.



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OPERATING TEMPERATURES - Air Conditioning System Operating Temperatures

Photograph of an air conditioning system suction line temperature chart

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.

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:
AIR CONDITIONER TEMPERATURE MEASUREMENT
.

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?]

AC Compressor (C) D FriedmanTemperatures 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 temperature 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.

Wall mount air conditionre split system (C) D FriedmanTemperature 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" (we're measuring low pressure on the "low side" of the cooling system) is critical for proper refrigeration equipment operation assessment.

Typical Suction Side or Low Side Pressure & Temperature

Normally suction line pressure on air conditioning equipment is about

These low side suction line temperature and pressure measurements will vary as a function of refrigerant, ambient temperature and other factors.

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.

In normal air conditioner or heat pump or other refrigeration equipment operation we see positive pressure on both the HIGH side (high pressures) and on the LOW side (low pressures) of an HVACR system. The low side is also referred to as the vacuum line or suction line as it's the compressor input side. Only during system installation or servicing would we actually pull a true vacuum on the system as part of evacuating it prior to installing a refrigerant charge. IN that case too it's "pressure" but negative pressure or vacuum.

Thanks to reader Charles for clarifying by comment that the suction line is under low pressure in HVACR systems during normal operation, 14 Aug 2015.

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

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.

If your air conditioning or heat pump 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.

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Or see AIR CONDITIONER TEMPERATURE FAQs for diagnostic questions & answers regarding AC or heat pump temperatures

Or see HVAC TEMPERATURE INSTRUMENTS

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