HVAC system temperature diagnostics depend more on your skill and understanding than the accuracy of the test instrument you’re using. Let’s take a look at several test methods, tips, and terms to see how you can sharpen your ability to solve your customer’s problems. 

We’ll focus on air temperature measurement methods for now. We’ll save combustion, refrigerant, hydronic, and electrical system temperature diagnostics for a future article.

Hopefully you’ll learn some new way to improve as you read on. I personally re-learned one method I had forgotten and saw two test methods in a new light as I wrote it. Thank goodness we’re still learning.

Equipment Temperature Change

The most common HVAC industry temperature measurement is equipment temperature change. It requires measuring air temperature into and out of the heating or cooling equipment. The two measurements are subtracted from each other to find the temperature change over the equipment.

• Avoid false temperature readings. Place your probe far enough away from the coil, heat exchanger, or motor, so its surface reads only air temperature and is unaffected by radiant heat of the equipment. Some tests may require you to install test ports in the duct system to insert your probe into the most accurate position in the air stream.

• Assure equipment is operating. Equipment cycles on and off to satisfy momentary heating or cooling demands. When testing, verify the equipment is operating at full capacity and the system is stabilized. You may inspect the burners or check compressor amp draw for instantaneous verification. If the heating or cooling cycle has just ended or is just beginning, air temperatures can change up to 50%. Diagnostics are worthless under these conditions.

Delta T

The term, “Delta-T,” (ΔT) represents the difference between two temperature measurements. Delta-T describes the most basic testing and diagnostics. To find the difference between two temperatures, subtract them from each other. For example, air temperature entering the heating equipment is 66.5°, and leaving air temperature is 110.7°.  To find the ΔT between the two, do the math: 110.7° – 66.5° = 44.2°. The equipment ΔT is 44.2°.

• Take related temperatures at the same time. Steady state rarely exists in the field. If the equipment is heating or cooling, the temperatures into and out of the equipment change moment to moment. Take multiple temperatures at the same time to increase accuracy.

• Use calibrated instruments. When you use two different instruments, verify each reads the same as the other for accurate ΔT calculations. Several dual-wand instruments are now on the market that performs this function. Affordable multi-probe sensors that communicate to a data collection point are also available. These packages considerably increase temperature diagnostic accuracy.

Record Temperatures to the Nearest Tenth of a Degree

One temperature test method labeled fanatical by HVAC novices, is to read and record to the nearest tenth (.1) of a degree. Almost all digital instruments now read to the tenth of a degree, so please take the time to record the decimal place.

• Allow the system temperature and instruments to stabilize. When moving a probe from room temperature air into a heating airstream, the difference may be 50°or more. Allow time for the probe to stabilize and settle down before recording readings.

Many technicians watch the tenth of a degree stabilize until it remains unchanged for 10 seconds. As soon as it does, the reading is deemed stable and recorded. 

• See small changes as you work towards a solution. As your experience and accuracy increase and you become accustomed to watching and recording decimals, you’ll learn to “see” through changes in tenths of a degree. Then you’ll be hooked as you jump to the next level of skill and precision.

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