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Community and Q&A

Inline air filter boxes

jonny_h | Posted in Mechanicals on

I’ve built in an 8″ intake duct for an ERV + kitchen ventilation makeup air, and I’d like to include decent filtration on it — the kitchen makeup air path won’t have its own filters, and ERV filters tend to be small and proprietary, so I feel it’s better to have something more standard upstream.  I’ve come across a few options:
1: — there’s a version using two 10x10x2 filters for 8″ ducts.  Relatively compact package.
2: The 8″ duct version of this uses 18x18x4″ filters, which is a significantly larger filter area than the first one.  Also a larger box, but the filter just slides in through a small door.
3: uses a 14x20x1 filter, which seems to be a more widely available size, but likely less filter area due to the reduced depth.

I also know that the rating of the filter itself is a bit meaningless without good sealing around the filter, and this is something that I’m not getting a good idea of from photos alone.  Has anyone used any of these, or similar products, and can provide feedback on how well they avoid leaks?  From an initial look, option 2 looks like it provides the most filter area, but I don’t see how it could achieve good sealing around the edges — the construction of options 1 and 3 at least look like some kind of compression seal could be present, or at least retrofitted in.  I feel like the best option, which I’ve been unable to find an example of, would look like option 3 turned on its side, with the filter accessed from the face and fitting into a frame where the edges could be taped, like Allison Bailes recommends for return grilles (  Should I “just” try to fabricate something on my own?

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  1. DennisWood | | #1

    I'm using the CFB series 6" version, which uses one 14x14x4 filter. None of these boxes are going to be 100% for sealing, but if you look at the HEPA versions, you can see that the prefilter is not sealed but the HEPA portion is fully gasketed with weather seal around the edges.

    With a single 14x14x4 MERV13 filter in place and 6" duct, resistance is not too bad..less than .1" at 100 CFM. I checked pressures after about 3 months with the same filter, and the difference was less than .05" (dirty filter vs new). If I was looking for a larger box and low resistance, I'd for sure look at the dual filter setup.

    If you're using any of these boxes (uninsulated), they need to be on the warm side if you're in a cold climate. The HRV/ERV filters are basically prefilters for the core and inline filter box with that setup.

  2. Expert Member
    Akos | | #2

    I would go with the build your own route, much cheaper if you don't mind the work. Get a 4" filter frame from an HVAC supplier that takes a standard furnace filter (16x25 is common). Cover the two sides with sheet metal and install a 8" top takeoff on either side. If depending on the direction you angle the takeoffs, this can be a pretty compact layout plus you can adjust them to make filter access easy. I guess you can even add a 2nd takeoff to feed your MUA. The opening on the takeoff is not big enough to cover the whole filter area but the filter is already way oversized, so it doesn't matter much. Wrap it in 1" rigid and call it a day.

    1. jonny_h | | #5

      Sheet metal work isn't something I'm particularly good at, but I can probably handle it -- leaning towards this route I think.

      1. Expert Member
        Akos | | #6

        A good pair of tin snips make most of the work a breeze, I like the offset ones for most work (ie Milwaukee 48-22-4523). A hand full of self tapping metal screws and a sharpie and you can do a lot of duct work.

        Wood can also work, I made my HEPA filter box out of MDF because it had to fit into a very specific location. This is inside conditioned space but could see making something similar and lining it with rigid on the inside if it needs to be on the cold side intake. I still used a standard takeoff fitting for connecting it to the ERV.

        1. Expert Member
          DCcontrarian | | #10

          Is wood legal for concealed ductwork? I thought it had to be fireproof.

      2. Expert Member
        BILL WICHERS | | #8

        You could find a mechanical (HVAC) contractor with a duct making shop and have them do the work for you. It's usually not all that expensive to build a basic box with some kind of simple door. A "filter box" is probably something those contractors will already be familiar with.

        If you want to do the work yourself, you really need at least a bending brake. Simple bending brakes are available from Harbor Freight for under $100, and they are good enough for simple things like this. You'll need some tin snips (sheet metal shears are much better, but also much more expensive), a good basic metal carpenter's square, and some time. BE VERY CAREFUL though -- the edges of the material, especially after cutting with tin snips, can be really sharp. There is also a tendency for the tin snips to leave little "jaggies" that will stab you. Cut resistant gloves can help protect you, but they also make it a lot harder to hold onto the material, and the jaggies can stab you through the gloves. You may find it's cheaper to have an HVAC contractor just build the box for you, and they'll probably be able to do a better job since they'll have all the tooling to do it, and experience using that tooling.

        Another option might be to find some premade duct sections that are close to what you need for your filter box, then adapt those sections to make them work as a filter box. This might save some work for you, since some of the bends and joins would already be done.


        1. Expert Member
          DCcontrarian | | #9

          For just one I think you're overthinking it. You can make a crude box out of sheet metal, sheet metal screws and foil sealing tape. Tools would be tin snips, pliers and a screw gun. Buy a piece of rectangular duct from Home Depot and use it as the starting point.

  3. DennisWood | | #3

    Are you planning to use the ERV for makeup air? If balanced, it won't work that way...

    1. jonny_h | | #4

      I wish there were a reasonable ERV that could also provide makeup air -- the CERV2 can, but that's a beast of a different sort. Anyway, I'm planning on a separate makeup air path, just sharing the same intake port / prefilter. I've oversized the intake with respect to what the ERV requires to provide for this extra airflow to the makeup air path.

  4. DennisWood | | #7

    I’ve become a huge fan of induction after installing a cooktop in our kitchen reno last year…just saying :-)

    If you plan on a 450 CFM makeup fan inline on the duct after the inline filter, just be aware that Y’ing off will potentially cause a significant balance issue due to low pressure at the ERV intake, while the MUA is running. In cold temps, on an exhaust only defrost cycle ERV, you may also run into issues with inside air being pulled back through the ERV if that is an easier path. At the least you’ll need to install one or two back draft dampers in each supply. Hopefully a code expert will chime in here…

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