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 HVAC 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.
To 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.
• 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,” (DT ) 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 66.5ᵒ and leaving air temperature is 110.7ᵒ. To find the DT between the two, do the math: 110.7ᵒ - 66.5ᵒ = 44.2ᵒ. The equipment DT 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 DT calculations. Several dual-wand instruments are now on the market that perform this function. Affordable multi-probe sensors which communicate to a data collection point are also available. These instrument 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.
When Airflow Changes, Temperatures Change
In a cooling system, when airflow decreases, temperature change through the equipment increases. And when airflow increases, temperature change through the equipment decreases. Temperature change over the equipment increases as less pounds of air move across the coil; less pounds of air means less heat transfer, so temperature changes goes up.
Temperature change over the equipment increases as less pounds of air move across the coil; less pounds of air means less heat transfer, so temperature changes goes up.
• Without measuring airflow, you assume airflow. Unless you verify airflow is delivered at equipment specifications, your temperature diagnostics will be wrong in direct proportion to your airflow diagnostics.
When technicians measure a 20° DT over cooling equipment, many assume the system performs well. But what if system airflow is at 250 cfm per ton? Is 20°the right temperature change? No. It would be higher.
• Plot fan airflow. A quick and easy way to determine fan airflow is to plot it from the manufacturer’s fan tables. Measure total external static pressure and determine the fan speed setting. These two data points can plot fan airflow on the manufacturer’s fan tables in minutes.
Once you know airflow, your diagnostic accuracy will skyrocket.
System Temperature Measurement Methods
Move your thermometer beyond the equipment to open an entire new world where you can discover system performance opportunities. Duct heat losses and gains are missed if you only measure equipment temperatures.
There are times when equipment in cooling mode was diagnosed to perform perfectly. But at the same time temperatures at the grilles and registers indicated the system was adding heat to the building. The culprit is duct temperature gain from a 140° attic.
• Measure register temperatures behind the register. Many supply registers are designed to mix room air by throwing air streams up to the ceiling, along the wall, or along the floor. Air is mixed as the register airstream pulls room air to its face.
If air temperature is measured at the register face and not behind it, supply air temperature readings can be off by up to 40%. Use a probe that can go between the register vanes to measure supply air readings behind it.
Carefully evaluate your temperature test methods and practice the knowledge and tips offered here. Odds are you’ll enjoy a simple, but valuable break-through as you improve your troubleshooting skills.
Rob “Doc” Falke serves the industry as president of National Comfort Institute, Inc., an HVAC-based training company and membership organization. If you're an HVAC contractor or technician interested in a free test procedure to measure average register airflow, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles, and downloads.