Furnace Filters and Air Flow Rates
I posted some observations about thick vs. thin filters after reading an article by Alison Bailes on the subject. His article is here:
And my post is here:
I was at a friend’s place on the weekend and noticed he’d purchased a case of four 16 x 25 x 4″ thick MERV 8 furnace filters, and rose at the opportunity to ask him to cut one open and measure the length of the filter fabric. He did it tonight and sent me a picture showing the filter material is 7 feet long when stretched out.
I was trying out a Kirkland MPR 2200 filter from Costco, also in a 16 x 25 size, but only 1″ thick. It was time to change it so I cut it apart tonight as well and measured the filter fabric to be only 52″ long, so the 4″ thick filter my friend is using it has about 60% more filter area.
I replaced the MPR 2200 Kirkland filter with a 3M 1500 MPR filter, the type I was using before, and counted 48 pleats. The Kirlkand only has 31 pleats! All else equal, I would expect the 3M to have about 48/31=55% more filter material when stretched out, darn close to what my friend’s 4″ filter has.
I also tested the pressure drop of these two filters when new on my 800 CFM furnace, and it’s no surprise now why the Kirkland has 50% more pressure drop across the filter when new, 0.31″ H2O vs 0.20″ for the 3M. After three months of use the Kirkland was up to 0.4″, where the 3M was only up to 0.3″, about where the Kirkland is when new.
The 3M filter actually has a table of airflow vs restriction printed on the bottom of the filter. I’ve seen this on very few filters, and not all 3M filters even have them. This should be a mandatory item on a filter as it’s just about the only way to judge a filter’s performance. What good is a filter that filters great but chokes your airflow?
Reading reviews on the Kirkland filters before buying, there were several people reporting their furnaces stopped working due to low airflow. I upgraded my 800 CFM furnace from a 14 x 25 to a 16 x 25 filter box so I wasn’t too concerned about low airflow (and Costco will allow you to return just about anything if you’re not satisfied), but with no flow rating on the filter people don’t know that they’re potentially reducing their furnace and AC efficiency.
Are flow ratings something that’s being pushed for with furnace filters? I’m a pretty technical guy and I had to do some legwork (and buy a manometer) to make sense of this stuff.
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