Air Temperature Measurement Basics

A claim was made in a regulatory meeting I attended in August in California that shocked me. A prominent building scientist claimed that supply air temperature could not be effectively measured at the equipment. Since that time I have met with hundreds of HVAC contractors and technicians that accurately measure equipment and system temperatures and effectively measure the live operating performance of HVAC systems. Let’s look at some basic temperature testing principles so we can know for sure the performance of the HVAC systems we work on every day and overcome the old idea that live system performance cannot be measured.

Test Instruments

Test instruments have advanced by leaps and bounds in the past 10 years. If you’re still using a $10 analog pocket thermometer, consider upgrading your temperature measurement tools.

Several accurate and quick digital hygrometers are available for less than $150. For an average service technician, the cost for this jump in accuracy costs about 5 cents per test over a year and will save over 40 hours in testing time each year because of the increased speed over analog instruments.

Hygrometers measure wet bulb, dry bulb and relative humidity digitally in less than a minute and will display the reading down to the nearest tenth of a degree. This allows for increased accuracy in diagnostics and troubleshooting. The main benefit is the confidence you can have in your decisions that will allow you to act on the information you gather with easy and certainty.

Testing Near Equipment

When measuring temperature near equipment several principles must be followed. The first is to know that radiant heat is not your friend when measuring on the supply side. Heat exchangers heat the air, and air temperature is what we are measuring. Heat exchangers also emit radiant heat, which is additional heat that will increase the air temperature reading and destroy the accuracy of your reading.

Drill test holes in ductwork in a location where the probe will not “see” the heat exchanger. Ideally, the test site is in a full air stream, within 5-ft. of the discharge of the equipment. This can be in a branch duct or after a transition in the main duct. Several test instruments provide a shield for the sensor that protects it when not in use. Place the hygrometer in the airstream so it is protected from the radiant heat from the heat exchanger. The same principle applies to a cooling coil. You may measure as close as three-ft. from the heat exchanger with this type of tool.

On the return side of the equipment, avoid measuring air temperature near the blower motor or near any opening in the return duct where hot or cold attic air from unconditioned space may affect your temperature reading.

One valuable exercise you should try is to take a series of temperature readings all around the equipment and create a temperature map of readings close to and far from the equipment, as well as from side to side in the duct. This series of tests will enable you to see temperature differences around the equipment and know the strengths and weaknesses of testing in flawed locations. You might want to try this at home, or on a system in the office. Few of your clients will appreciate the “Swiss cheese” effect of a couple dozen-test holes in and around their equipment.

Testing at the Grilles and Registers

There are often significant differences between equipment temperature changes and system temperature changes. This winter you may find an equipment delta T of 60F and the system delta T of only 40F. This happens due to the loss of heat through the duct system as it passes through cold attics or crawl spaces. In this scenario, one third of the equipment generated BTUs are being lost through the duct system. Or a 90 AFUE system was just reduced to a 60 AFUE system. This is a common reality of system performance.

Equipment manufacturers have taught us to measure temperatures at the equipment. This has been done for generations, because their interest is in the performance of the equipment, and rightfully so. Our interest must be focused on the performance of the system. This represents one of the greatest shifts in system performance thinking in 50 years.

When measuring temperature at supply registers, be certain the probe is measuring 100% supply air temperature. This is done by inserting the probe into the grille before the airstream mixes with the room temperature. Also be sure not to get the probe too close to the metal in the boot where temperature may be affected by a 40F or a 140F attic air temperature on the opposite side of un-insulated sheet metal fitting.


Measuring temperature at the right time is also critical. Be certain the equipment has been operating for at least five minutes before measuring air temperatures and that temperatures in the system have been able to stabilize. If the fan is variable speed be certain it has ramped up completely, or there will be little value in the temperature reading. Measure supply and return air temperature at the same time with two calibrated hygrometers to avoid the change in system temperatures that constantly occurs while a system is operating.

The main point is to move past simply taking an air temperature reading and assuming a single point reading is sufficient. Hunting and searching for true system air temperatures is the name of the game. Regularly question your readings and expand your ability to measure and troubleshoot your systems.

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company with technical and business level membership organizations. If you're an HVAC contractor or technician interested in a free HVAC System Temperature Measurement Procedure, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at for free information, articles and downloads.

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