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 fewer pounds of air move across the coil; fewer pounds of air means less heat transfer, so temperature changes go 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° ΔT 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 airstreams 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 robf@ncihvac.com or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles, and downloads.