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Furnace Filters and Air Flow Rates

lance_p | Posted in Mechanicals on

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:

Allison Bailes – Thick vs. 1 inch furnace filters, pressure drop and high MERV

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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  1. Expert Member
    BILL WICHERS | | #1

    You sometimes see published flow ratings, but not always. You would actually need two parameters to specify a filter this way: the back pressure AND the flow rate through the filter when that backpressure was measured. You end up with a set of numbers something like "0.2 inwc @ 800 CFM", which would mean that the filter would show a 0.2 inches of water pressure differential across it when you were blowing 800 CFM worth of air through it.

    The typical datacenter air handlers I deal with at work take four 24x24x4" filters. We monitor them for back pressure (using those same magnehelic gauges Allison mentioned in his article), and we change them when the back pressure gets higher than we want. For our application, with 24x7 service (the blowers run continously), we see about a 3-6 month filter replacement cycle. You're likely to see the filters last longer in a residential application with less blower runtime unless you have a particularly dusty enviornment. For comparison, my Aprilaire filter (which is about a 6" deep heavily pleated filter) lasts for well over a year between changes.

    You can usually stretch out filter changes without any downsides by measureing the backpressure across them to gauge when they need to be replaced instead of just replacing them on a schedule. This means fewer filters used over the course of a year, which is less money spent, less waste, and more green. Win-win.


    1. lance_p | | #2

      Excellent info, Bill. That’s my eventual approach too; to oversize the filters on a new air handler and change as needed, not as scheduled.

      Absolutely, the flow rate vs restriction is exactly what’s listed on this 3M filter, along with filtration efficiency at different particle sizes (see attached photo). Again, not all 3M filters have this, and I haven’t seen it on any other brand I find in retail stores.

      My 800 CFM furnace tests at just around 0.2” on this filter, so the chart is right in line with expectations.

      1. bfw577 | | #7

        I have the same size 1" filter and run a manometer as well. I found the 3m 1900 Merv 13 had the lowest initial pressure drop new and can filter for an incredibly long time before it drops significantly. There is a YouTube video out there comparing and it beat out even many of the thicker 4 inch filters. The guy rolled out the filter material and it was one of the longest.

        1. lance_p | | #8

          Do you have a link perhaps? I'd like to see that!

          1. bfw577 | | #9


            I also attached a screen shot of the sqft of filter material. As you can see the 1900 had one of the highest sqft.

          2. lance_p | | #13

            Great video, thanks!

    2. brian_wiley | | #3

      Hi Bill,

      Do you have any advice on when you change them? That is, what type of pressure increase do you look for before swapping them out?

      1. Expert Member
        BILL WICHERS | | #4

        I don't want to give a hard "at this much, change it out" pressure drop number, because those numbers will vary with the airflow of the system in question. My recomendation would be to know what your system can handle, and change the filter before you exceed the allowable pressure drop for your system. That can be a bit tricky though, since the furnace will be specified for a SYSTEM pressure, which includes the filter AND all the ductwork -- but you only want to measure the filter!

        The rule of thumb is to replace the filter when it's reached about double the pressure drop it showed when new at a given amount of airflow. I would add to that to make sure that whatever "double" is is still within the limits for the rest of your HVAC system though.


        1. brian_wiley | | #5

          Thanks, Bill; I think that all makes sense, and totally get that it is system-dependent.

          As an example, I would need to first measure my total external static pressure of the system (lets just say it's .3), and know that .4 is the maximum my system can handle as per the manual. Then I'd need to individually measure my filter when it's new (say .15 at 1200 cfm). In this case I can't let my filter's pressure go beyond .25 pressure drop (a .1 increase above the existing new condition) so as not to exceed the .4 TESP limit of my system, correct?

          1. Expert Member
            BILL WICHERS | | #6

            Basically yes, but your example leaves you no room. If the system's max allowable pressure is 0.4"wc, and the ductwork, etc., is already 0.3"wc, then your filter's new-condition 0.1"wc already puts you at the max.

            Let's make a slightly better example, keeping all your numbers but assuming your system's max is 0.6"wc. Now you can have up to 0.3"wc in the filter (0.6"wc max - 0.3"wc ductwork). Since the filter is 0.1"wc when new, your system runs at 0.4"wc -- well below the 0.6"wc limit.

            Since you generally try not to let the filter get up past double it's initial value, you'd replace it at 0.2"wc, which would be when the system is running at 0.5"wc (0.3"wc ductwork + 0.2"wc for the dirty filter). That still leaves 0.1"wc margin. You could push things and change the filter even later, as long as it stays under 0.3"wc as a max, but you'd need to see how your system performs with the extra dirty filter.

            Note that I do do things like this at work, since the filter changes are thousands of dollars and it's an operating expense. We measure system performance over time and empirically determine when we should replace the filters. Sometimes this means less than double pressure, sometimes more, depending on conditions. We keep records of everything and write maintenance manuals based on our data. This is how you arrive at the optimate cost/benefit for this kind of thing. In a house, you can still do this, but it's a lot of effort to save only a little money if you have one one filter :-) In this case, the rule of thumb is probably fine.

            BTW, it's worth pointing out that filters actually filter better as the get dirty. The dirt plugs the big holes first, resulting in the filter being able to trap smaller and smaller particles as it gets dirtier. The downside is the restriction to air flow -- backpressure -- increases as this process continues. At some point the filter can get so dirty that it will break due to excessive backpressure, then it dumps a bunch of stored up gunk into your system -- a Bad Thing. Just don't worry about a slightly dirty filter not filtering out dirt as well -- that is a common misconception about how filters work.


        2. brian_wiley | | #11

          Thanks for the explanation below, Bill. That clears it up.

          For some reason I was under the impression that the pressure drop of the filter was included in the TESP, but after doing a bit more reading last night it seems that you exclude it (assuming the manufacturer doesn't specifically include it) from that total. That of course makes more sense as there are so many different types of filters that you'd want a reading that excluded it.

  2. JC72 | | #10

    Am I missing something? The 3M filter is less efficient vs the Kirkland product. Either are probably more than adequate for a furnace which is designed to filter out large particles rather than provide "filterd" air like a HRV/ERV.

    1. lance_p | | #12

      The Kirkland claims to filter more effectively, but it presents a much higher restriction to airflow. My house does not have a dedicated ventilation system, so the only filtered air we get is through the furnace.

      1. JC72 | | #14

        As it should. The composition and amount of filter media both influence filter performance and lifespan.

        IMO the issue isn't the amount of media but the volume of air being filtered. One way to reduce the resistance to the volume of air moving through the filter is to increase the number of filters. An example would be adding a filter at each return and remove the filter at the furnace.

        1. lance_p | | #15

          I increased my filter size from 14x25 to 16x25 for lower restriction, as well as using a much more commonly available filter size. In my experience all filters of a certain type cost the same regardless of size. If I could have fit a 20x25 filter box on my furnace I would have.

          The return air ductwork on my furnace is an absolute joke as far as sealing goes, with stud and joist bays being used as return air channels (a common practice in production housing as far as I know). I've sealed up what I can get access to, but based on what I could see there's no way I can assume any sort of airtightness in the ductwork. Moving filtration to the return air grilles would certainly allow for more filter area, but would also introduce large amounts of unfiltered air to my furnace.

          That could be a good approach when designing and installing a new system, as long as the ductwork is well sealed.

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