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Community and Q&A

Changing Large Return-Air Filters

utkdoc | Posted in General Questions on

My house has a unique setup – the engineer who lived in the house previously designed the air return to use these massive high MERV filters with deep pleats. The filter I have been using is MERV13 that measures 24x24x29.

I have been changing them out a little after a year or so of use. My question is, is that too often for such a large filter? These seem oriented for 24/7 commercial use.

My house has two hvac units and this filter is in each. We’re around 4000 sq ft. I do heat the house in the winter with a high efficiency EPA wood stove and hvac is rarely on during those 4-4.5 months- usually only if a bit of smoke leaks out when I feed the stove but that is not a frequent occurrence. PM2.5 usually 50 or less, usually on lower end at < 20 throughout the year. Cooking with the gas stove raises it a bit but it falls down on its own usually.

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  1. Expert Member
    BILL WICHERS | | #1

    You really need to watch the pressure drop across the filter to know for sure when it's really time to change it out. A dual-port differential manometer is the way to do this, with one of the two ports connected to the ductwork on either side of the filter. Assuming both units run about the same number of hours each year, a measurement on one of the two is probably close enough to tell you the change interval for both units.

    Without a measurement, it's hard to know if a one year replacement schedule is too long, too short, or just right. I have some large filters too (Aprilaire's filters, which aren't nearly as deep as yours, but are still much deeper than most), and one year is a little early for mine. I can easily go out to about two years under normal circumstances, maybe even a little longer if I push it.

    Note that filters actually get better in terms of filtering as the clog up with gunk. That means filters don't "wear out" and starting letting more stuff through, they actually work the opposite of that and filter even better. What does happen is that as they gradually clog up with gunk, then reduce the air flow more and more, which eventually means they restrict the airflow enough to be a problem for your equipment. That's what you really need to watch out for.


  2. utkdoc | | #2

    Thanks Bill - That would make sense- I don't see how one could eye ball when a filter change is needed (unless you are way overdue!)

    Would a cheap model like this suffice?

    How much of a pressure differential would you look for?
    Spec sheet for this filter lists a few resistance differentials but not sure which to go by. I know ideally I would be using a new filter in my setup as the baseline - I will have to measure that on my next change. I'm just not sure of the significance of 0.05, 0.1 or 0.5 wc -- i.e is a 0.1 gradient "a lot" or is 1.0 considered a lot?

    IIRC, my units are 3.5 and 3 ton units (single stage) so I am probably looking for upper 0.2's for a new filter differential.

    Initial Resistance @ 500 fpm
    0.30 in wc

    Initial Resistance @ 300 fpm
    0.18 in wc

    1. Expert Member
      BILL WICHERS | | #3

      That unit has a really high range. You want something in the small inches of water range, something like this:

      No need for fancy digital stuff, use an older style analog gauge and you can install it and leave it and not worry about battery changes. An idea range is probably 0 to 1/2" water, but I'm not sure if they make one in that range. a 0-1"WC range is OK too, but won't have as good resolution for smaller differentials. Remember that you're only measureing across the filter, not the entire duct system. The differential gauges measure the difference between their two sensing ports, so this lets you measure the filter alone. Typical pressure differentials for an "old" filter that needs replacement are probably ~0.25 to 0.5" WC or so, but it's really difficult to say for sure. What I would do is measure a new filter, then watch it for a while and determine when a good place to change it is. Once you've done that, you can use the gauge as a "when to change the filter" indicator.


  3. pdgessler | | #4

    Allison Bailes has an article with recommendations much aligned with the responses you've received here so far:

    I haven't put this all together myself yet, but it's been on my mind and I plan to do it soon. Would like to get a better idea of when to replace my MERV 13 filters than just my gut feeling. :-)

    1. user-5946022 | | #6

      That link to the EV blog is a really interesting article. Unfortunately the comments on it are closed.
      Does anyone know if it is possible to make these measurements even if there is NOT ductwork on either side of the filter? My return air frames are installed on the wall and are, in effect, the "start" of the ductwork. I'd love to figure out a way to measure the pressure drop across the filter.

      1. Expert Member
        BILL WICHERS | | #7

        All you need to do is get the sensor tubes into the duct and perpindicular to the airflow. You can put the sensors into the filter housing if that's what's accessible, they just have to be on either side of the filter and perpindicular to the air flow (sticking out the of duct or filter housing at a right angle to the airflow path). The exact placement of the tubes is not critical.


  4. greenright | | #5

    Generally speaking the drop across an air filter is linear in time. At some point though it becomes suddenly non- linear and pressure starts to drop much quicker- this is when you want replace/ clean media as this is the point when it starts to get clogged. Every filter is different. Get a low range manometer and start plotting the drop on paper. Once the line becomes a curve- it is time.

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