Measuring Airflow on Fossil Fuel Systems

February 12, 2024

Motor Minute^™ Technical Tip – Measuring Airflow on Fossil Fuel Systems.

Hi, Chris from Regal Rexnord here with your Motor Minute technical tip. Helping you become an HVAC motor pro one minute at a time.

Today’s Motor Minute^™ topic is measuring airflow on fossil fuel systems.

The value of measuring airflow cannot be overstated, in my opinion. Fundamentally there are three pieces of information and/or knowledge areas required.

1. The method and tools required to measure airflow.
2. The required airflow value for a particular HVAC system.
3. How to change the airflow if it is not correct.

Let's apply these fundamentals to measuring airflow on a fossil fuel furnace.

The Method and Tools Required to Measure Airflow

This method is called measuring the temperature rise across the furnace. There are other methods; however, I have found this to be the preferred method by furnace manufacturers according to their installation manuals.

The tools can be as simple and inexpensive as a pocket thermometer with a minimum range of 0-200⁰F. Or they could be as versatile and accurate as a digital dual probe temperature measuring tool.

Insert the thermometer(s) or probes into the return ductwork (as close to the furnace as possible) and into the supply ductwork (as close to the unit as possible but out of line of site of the heat exchanger).

The process of measuring airflow begins with operating the furnace until it reaches steady state conditions, which usually takes 5-15 minutes. This is the point at which the supply temperature of the furnace is stable or no longer increasing.

The Required Airflow Value for a Particular HVAC System

To find the required temperature rise for the furnace, look at the unit rating plate. If that is not available, reference the installation manual.

Now it’s simply math. The supply temperature minus the return temperature equals the actual operating temperature rise.

Compare the measured temperature rise to the rated temperature rise and decide if it needs to be adjusted.

The rated temperature rise is a range. Technically any value in that range is acceptable. The generally accepted practice is to try to achieve a temperature rise that is in the middle of the required range. However, that topic alone could be an entire podcast.

If the temperature rise needs to be increased, the airflow needs to be decreased.
If the temperature rise needs to be decreased, the airflow needs to be increased.

How to Change the Airflow if It Is Not Correct

Airflow can be increased or decreased by:

• changing the speed selection of the indoor blower motor
• adjusting the airflow selection using the DIP switches, jumper pins or segmented display found on the furnace circuit board
• adjusting the menu selections on the user interface formerly known as the thermostat

Time does not allow me to cover all of the what ifs, nuances and differences between equipment and installation applications. Please reference the HVAC equipment manual for more information.

For more information related to adjusting airflow selections, please visit regalmmu.com and click on the Motor Minute^™ icon to find our previous tech tip title:

• DIP Switches (November 13, 2023)

And that’s a wrap on this segment of Motor Minute technical tips. Remember that we provide motor training and product information in multiple formats including videos, literature, podcasts, articles, of course Motor Minute technical tips and classroom education both online and face to face. All of this industry leading training is available at no charge to HVAC professionals at regalmmu.com. Thank you for taking the time to build your motor knowledge.