GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Best Solution for Fitting a Filter Cabinet Larger Than the Air Handler

Nic Smith | Posted in General Questions on

Hi all,

A while back I hired a company to replace my old split AC & gas furnace system with a heat pump. From reading up I knew that a larger (thicker) air filter was a good idea for better indoor air quality and overall system performance, and so I asked the company to install one with the new air handler. The tech was annoyed and warned me that the larger filters are more expensive, but complied and installed a 16 x 25 x 4 Honeywell filter cabinet.

I’ve recently noticed that the filter cabinet is about 2″ larger than the air handler. As you can see from the attached photo, the tech just used foil tape to seal the gap in between.

My concern is that a thin layer of foil tape used to prevent unfiltered air from entering the coil and supply plenum seems risky. If the tape comes loose or gets damaged, the coil may become dirty and cause performance and longevity issues, and the indoor air quality would suffer.

What is the correct solution to this issue? Are filter cabinets supposed to be exactly matched (or undersized) to the size of the air handler cabinet, or is a site-made solution (sheet metal?) supposed to be applied?

Thanks for reading 🙂

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. John Clark | | #1

    Foil tape or mastic is normal. Puddy is for penetrations (which as you can see is what they used). Btw..there's always going to be some leakage around the access door to the filter. It's why you have them come out 1 or 2x a year to clean the evap coil.

    As for the size, furnaces have a minimum sizing for the intake I've never heard of a maximum. In any case technicians are often faced with space limitations for these kind of jobs. I'm not sure what I'm seeing in the picture because I just see what appears to be a filter rack with no return on the opposite side of the filter rack.

  2. Charlie Sullivan | | #2

    It's true that a custom sheet metal adapter would be nicer than foil tape, but the foil tape is an OK solution, and custom sheet metal can be expensive. As John noted, we'd need some more pictures to know what the overall configuration is and what the constraints were.

    Since you'll be replacing the filter regularly, that's a good time to check that the tape is still intact.

  3. Nic Smith | | #3

    Thanks for the replies, John and Charlie.

    I've attached another photo. This is a central return system, so there is no return plenum.

    Good to know that foil tape is not a "hack" - although it definitely seems less than ideal to me. I suppose I could bend my own piece of sheet metal to cover the space and then screw/foil tape it in place.

    The original installation did not seal the penetrations, and the tech actually melted one rubber gasket and just left it hanging loose. It was obvious that dust was entering through these penetrations (dust buildup was visible), so I added the duct seal (putty) myself to the seal up the penetrations.

  4. jrpritchard | | #4

    It looks like you have a unique situation there with a wild return. If I am seeing it correctly it looks like your air handler sitd on a stand, and your filter rack is sandwiched in-between the stand and the air handler. You have very few options in your situation. The Honeywell racks like you have are designed to fit on the side or the bottom of a furnace, which have different dimensions than most air handlers. We typically build those racks into the RA drops with transitions or offsets when we use them on air handlers but if you dont have any return ducting and your air handler only has the option of return air from the bottom then you get what you get in this case. Most of time we size filters based on available static and pressure drops and I would guess even with the area blanked off you wont have any issues with this set up since there would be very little static on the return side of your system.

    1. Nic Smith | | #6

      Yes, you are seeing it correctly. This is a small central return closet in a 1960s home that was not originally designed for AC. The air handler does have the option for return air on the sides, but there is not room in the closet for that.

      In terms of return static, I just figured out that the existing return grilles were apparently sized for the original 1960s furnace (and no AC). I enlarged the wall opening on one of the grilles so that I now have ~2.5 sq ft of free area between the two grilles, which I believe is adequate for the 2-stage, 2-ton heat pump.

      I would like to actually test the system's static pressure to make sure all is well because the original installation company did not do this.

  5. Expert Member
    Zephyr7 | | #5

    That looks like a pretty clean tape job (nice and straight, no big wrinkles or bubbles, etc.), so it's probably fine. I agree with the other comment about checking the tape at filter change time. What you want to do to check the tape is look for any areas that are lifting at the edges, and press along the tape line to see if there are any loose/lifted parts in the middle area where you wouldn't otherwise be able to see. If you find any issues, clean the area and apply new tape.

    Duct seal will harden over time too, but it takes years. I check around spring cleaning time on my system. Duct seal will harden over time, crack, and loose adhesion (fancy way to say it will seperate from and sometimes even fall off of whatever it is supposed to be sealing).

    You'll probably get more leaks with a custom adapter unles you seal it with tape or mastic.


    1. Nic Smith | | #7

      Thanks - that is my tape job also :) The original was a single strip of non-rated tape and was pretty sad. I am sure I am overthinking this, but if used a 90-degree piece of sheet metal and then taped it, then I wouldn't have to worry about punctures/tears/rips. I know that impact damage shouldn't happen in a return closet, but just in case.

      Thanks again for your thoughtful replies, everyone!

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |