IAQ Basics: A Contractor’s Viewpoint

April 30, 2019
By Tom Johnson

With the theme of this month’s magazine being indoor air quality, I wanted to talk some basic IAQ. If you have read this column in previous Contractor editions, you know that I am a huge proponent of training our technicians on proper air flow, combustion setting and duct design.

A basic formula that all who work in HVAC should know is that maximum face velocity across a typical air filter is 300 feet per minute. Let’s do a basic example with a 16 inches x 25 inches typical air filter. To figure how much air we can move across this filter and have it function as designed, we first need to convert its size to square feet.

16 in. x 25 in. = 400 sq. ft. / 144 =2.78 sq. ft.
Then, multiply it by our 300 feet velocity.
2.78 ft. x 300 ft. = 833 CFM

Our maximum air flow for this very common filter size is 833 CFM. So, the maximum size heating/cooling system that is OK to use this filter on is a 55,000 Btu furnace and a 2-ton air conditioner.

So how often in our industry is a 16 inches x 25 inches filter on a 3-ton, 4-ton, or 5-ton system? Most of us see it every day. If we don’t know enough to design our duct system properly so that the basic of all IAQ products, the basic air filter, can work properly, should we be adding other IAQ accessories we likely don’t understand or know how to size or apply to our systems?

Before we start talking about all the great add-on products that we can “bolt on” to our HVAC systems, let’s start teaching basic design principals to our industry. PHCC should be leading the way at this. We have a 150-year heritage of being the leading plumbing and HVAC companies in our industry. I believe it is time we show our industry why it is important to align with other companies that care about quality and design. Let us, as an organization, be the leaders in education and change.

Real continuing education is the only way to this end. Just doing the minimum no longer cuts it. Invest in your people, invest your company, and invest in our industry by investing in real and meaningful education for your technicians and installers. It truly benefits us all!


Tom Johnson, president of TM Johnson Bros., Inc., Cambridge, Minnesota, is a master plumber and certified IAQ instructor for the industry.

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