As winter settles in across the country, we’ve had an increasing number of tech support calls asking for a quick and simple way to calculate airflow in forced air heating systems. We take this as an indication that more and more of you are measuring system airflow and understand its critical role in delivering a top-performing system.

There are a number of methods that have been used over the years. But with the introduction of new furnaces, any method would need to consider the type of furnace and respond accordingly. The method we teach to calculate heating airflow differs from one type of furnace to another, so let’s first take a look at the basic formula for gas fired heat.

Gas Fired Furnaces
Start with the rated BTU input of the furnace. Divide the rated BTU by 10,000.

Multiply that figure by the factor form the table below depending on the type of furnace to find required system airflow.

Natural Draft Furnaces 100 CFM per 10,000 BTU of rated BTU input
Induced Draft Furnaces 130 CFM per 10,000 BTU of rated BTU input
Condensing Furnaces 150 CFM per 10,000 BTU of rated BTU input

This is a straightforward, no frills method that is so easily to follow, it can be committed to memory in less than a minute. Let’s take a look at the required steps in a field situation.

Example
Check the nameplate on the furnace you’re servicing. It states the equipment rated BTU input is 80,000. Then you check and find a fan at the base of the flue, so you determine it’s an induced draft furnace.

Divide the BTU input of 80,000 by 10,000 to find a multiplier of eight. Since you have an induced draft furnace, multiply it by 130 to find the required system airflow of 1040 Cubic Feet per Minute. That’s it.

Okay, one more. You’re servicing a condensing furnace, you can tell because the flue is made of PVC Pipe. The nameplate reads the equipment is rated at a BTU input of 120,000.

Divide the input of 120,000 by 10,000 to find a multiplier of 12. Since you have a condensing furnace, from the table above, multiply the 12 by 150 to find the required system airflow of 1800 CFM. It sounds like a lot of airflow for a furnace, but that’s what the furnace requires to operate at maximum efficiency.

This method of calculating heating system airflow works on just about every system I’ve ever applied it to. To be certain for yourself, pull some engineering data from the office on several gas furnaces and run the numbers. You’ll be impressed with how consistent the simple formula applies across the board.

Heat Pumps
It may sound weird if you’ve never checked, but heat pumps require the same airflow in heating mode, as they do in cooling mode. The magic number is 400 CFM per rated nominal ton. 450 Per ton is often discussed, but that’s to assure you’ll get at least the required airflow of 400 CFM per ton. I questioned this number myself until I began to measure the delivered BTU of heat pumps years ago.

As our customers would complain of cool supply air temperatures, our service techs occasionally would reduce fan speed to heat up the temperature. This solved one problem, but created a bigger one a few weeks later when the temperature fell and the building failed to heat. This happened because the required airflow over the coil wasn’t available to remove the heat generated by the heat pump, so total BTUs took a nosedive.

Don’t Mess With It
I often hear misguided technicians that claim to “design for 350 CFM per ton to dehumidify better.” Well, although the intent is good, and based on a correct principle — unless you measure airflow — design is only an indication of what you hope to achieve. The same principle applies to heating airflow.

Design for, test, and verify the required airflow as recommended in this article. Then, and only then, if an element is lacking, you may consider lowering airflow to increase supply discharge temperatures. Once the desired temperature is obtained, then retest the system to verify you haven’t lowered the total heat delivered by the system to a point where the equipment cannot produce the rated BTU. Or you may be left with a far worse predicament.