By D. Brian Baker, Custom Vac Limited January 2009 (reposted from NAFA)

Of course there is no single answer to this question because it depends on many factors such as, what is the fan CFM on high, what is the fan CFM on low, is continuous fan being utilized, what are the heating and cooling CFM’s, how many hours is the fan running in heating speed, cooling speed, continuous speed, etc However, it’s essential that we and the clients understand the sheer volume of air that has to pass through an air filter annually. It is equally important that they know where the indoor pollutants come from.

Let’s use an example in the specification sheet:

The furnace is electric 18 KW with a 24,000 BTU air conditioner

Lets assume single stage heat only factory setting at 1400 CFM; cooling single stage at 840 CFM; continuous blower at 434 CFM.

There are 8760 hours in a year (24 x 365)

Heating accounts for 2000 hours a year

Cooling accounts for 400 hours a year

Therefore continuous blower accounts for 6360 hours (8760 – 2400)

Note: These hours may be challengeable but reflects good educated numbers.

HR               MIN

2000 hrs x 60 min x 1400 cu. ft. =      168,000,000 cu. ft.

400 hrs x 60 min/hr x 840 cfm =        20,160,000 cu. ft.

6360 hrs x 60 min/hr x 434 cfm =      165,614,400 cu. ft.

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Total cubic feet circulated in a year   353,774,400 cu. ft.

If this were a 1700 sq. ft. bungalow with a full basement the total volume would be 27,200 cu. ft. (1700 x 2 x 8)

Total annual air changes circulated by the furnace fan within this home would be 13,006   (353,774,400 cu. ft. / 27,200 cu. ft.)

Indoor pollutants can be compared to Charles Schultz’s character Pig Pen (“Pig Pen” effect, Fugler CMHC)

  • Cotton fibers (lint from clothing and fabrics)
  • quartz (found in sands, gravels, tracked in on footwear)
  • carpet fibers (break down of the fibers)
  • cellulose fibers (paper towels, tissue, toilet paper, newsprint, attic insulation)
  • skin flakes (our bodies)
  • pet dander (our pets bodies)
  • insect feces, etc

Don’t forget the impact that large rooms can have on particulate removal. Low-velocity air allows airborne particulate to fall out of suspension before they can be returned to the air filter.

***Repost from NAFA***