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Photograph of a dial thermometer reading a high output temperature at an air conditioning compressor A/C Air Temperature MeasurementProcedures
How to Make Air Conditioning System Temperature Measurements

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This article describes air conditioning temperatures and for measuring the air temperatures in an HVACV system we discuss thermometers, digital thermometers, infra red thermometers, temperature measuring equipment, temperature measuring devices, dial thermometers, sling psychrometers.

How & where to measure temperatures for air conditioners or heat pumps: this article explains exactly how and where to measure input and output air temperatures at air conditioning equipment in order to determine whether or not it is operating properly, as part of checking basic air conditioning system operation and for detection of air conditioning operating defects.



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Procedures for Making Temperature Measurements of Air Conditioning Systems

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.

Refrigeration and Air Conditioning Theory: Refrigeration systems rely on two state changes of the refrigerant: gas to liquid, and liquid back to a gas to move sensible heat from the low side of the air conditioning system to the high side. At the evaporator coil heat (BTUs) is absorbed when the refrigerant evaporates (liquid to gas), and at the condenser coil heat is released during condensation (gas to liquid). R12 refrigerant has a boiling point of -21 degF (change of state from liquid to gas vapor) and R22 has a boiling point of -41 degF.

But if you are measuring air temperature close to the evaporator coil or condenser coil you won't record these two temperatures on your thermometer. Rather the temperature that you can record will be significantly affected by ambient conditions. For example, at the evaporator coil the temperature of indoor air entering the coil, the distance between the coil surface and the thermometer, the air velocity, and other factors will produce a temperature reading that is different from and certainly higher than the boiling point of the refrigerant entering the coil.

While we may form an opinion about just how cool the air should be right at an evaporator coil, or inside of an air handler supply plenum, most diagnostics look for temperature differences between air entering the air handler and air leaving the air handler to evaluate what's going on in the system.

NOTE: an air conditioning technician has more precise tools to evaluate the condition of a system such as gauges to measure the pressures on the high side and low side of the system and an ammeter to measure current draw of the compressor.

How to Examine Air Conditioner Temperatures

Temperature Measurements & Observations at the Room Thermostat

Air conditioner thermostat settings: observe the settings on the wall-mounted room thermostat (assuming you've already established that the switches and controls have turned the system on and that it is in cooling mode and has been operating for half an hour or longer.

Note the set-temperature (the cooling target set on the thermostat) and note the ambient temperature (the actual air temperature close to the thermostat).

Most thermostats will tell you both the set temperature and the actual room temperature. These should be within 2-3 degrees F. of one another if the system has been in operation for several hours and if the air conditioning system is working properly and has adequate capacity and of course if you're using it normally with the building windows and doors closed, supply and return registers open, filters not clogged, etc.

Temperature Measurements at the Evaporator Coil in the Air Handler

Schematic showing where to measure air temperature drop in an air conditioning system (C) Carson Dunlop Associates

How to Measure Air Conditioning Register outlet temperatures using a dial thermometer probe: we simply wedge the probe of our dial thermometer between the vanes of a ceiling supply register, or drop it probe-first through the slots of a cool-air supply floor register where we leave it for at least five minutes. Sketch courtesy of Carson Dunlop Associates.

[Click to enlarge any image]

We select a supply air register closest to the air conditioning equipment if at all possible, and if there is access we confirm visually that the duct is not crimped, blocked, and by simple feel, we confirm that air is flowing out of it.

We use tissue or toilet paper to confirm for skeptical owners that the direction of air flow is "in" at a return register and "out" at a supply register since sometimes this can be confusing to a novice.

Measuring Air Conditioning Duct temperatures using a dial thermometer probe: if there is not an existing duct opening such as a foil-covered hole or a removable plug, we drill a 1/4" diameter hole in the sheet metal of the duct. BE CAREFUL not to drill where you can damage a refrigerant line, coil, wire, etc. After inserting the probe into the hole for measurement, waiting, taking our measurement, we close the hole using a square of adhesive foil tape, duct tape, or snap-in plugs sold for that purpose.

Temperature Measurements at the Condensing Coil & Fan/Compressor Unit

Measuring temperatures at an Air Conditioning Compressor: By holding the thermometer's probe in any air path (and patience) it is trivial to measure ambient air temperature, air temperature flowing into the condenser unit at the condensing coils, and temperature flowing out of the condenser at its fan output (keep the probe out of the blades!)

Measuring Air Conditioning Temperatures Using an Infrared Thermometer: permits measurement of surface temperatures such as the surface of a metal duct (is it insulated?), surfaces of refrigerant suction and high pressure lines (do we know target temperatures?), or surfaces inside a building.

We use infrared temperature scanning of building surfaces to scan for currently wet conditions (this won't tell you if there was previously a leak that has since dried, leaving behind a mold problem), and to scan a building for air infiltration or ex filtration losses, and to scan for the presence or absence of insulation (there must be a good difference, perhaps 20 degrees F, between indoor and outdoor temperatures to make this check accurately) but we do not use infrared scanning on air conditioning equipment except to look at the surface temperature of a heretically sealed compressor or to check for hot electrical connections in the panel or switch box.

Remember that when using an infra-red sensor you may not be reading actual surface temperature unless you're measuring a black surface. Some scanners are provided with a black crayon or marker to make a spot to be used for temperature sensing. These devices are excellent, however, for comparing the temperatures of different surfaces.

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

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.

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