I looked under the HVAC furnace today and noticed a hole near the filter installation. It seems like it should have been sealed up because air can be taken in bypassing the filter. Is this typical and won't cause problems or should this be sealed?

See images for additional detail. With the filter out, the blower fan can be seen.


enter image description here

  • I would talk to the ones who installed that furnace. It looks like that hole was snipped open, so there might be a reason for it or they forgot about sealing it after.
    – crip659
    Apr 25, 2023 at 17:01
  • 1
    No idea who installed it. This was like that when I moved in.
    – jpQuick
    Apr 25, 2023 at 17:05
  • 3
    Given that it seems to have been cut after manufacture, I think we can say it isn't "normal". Whether it matters or not depends on what's happening beyond that hole -- whether it actually connects to the air intake, what it's intended to provide access to, etc. Can't tell that from this photo.
    – keshlam
    Apr 25, 2023 at 17:10
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    If it were my house, I would seal that.
    – negacao
    Apr 25, 2023 at 17:36
  • I added another photo with the filter out. A blower fan is right near the hole.
    – jpQuick
    Apr 25, 2023 at 17:54

1 Answer 1


Since we have no choice but to do a little guessing here, I'll do that and answer:

My guess is there was a regular filter there and it was replaced by the electric one in the picture. The new one was a little smaller, and either the installer or some subsequent repair person failed to close the gap below it.

You should seal it up. Buy a small strip of sheet metal at the hardware store and tape it on with HVAC tape.

You should expect this area of the house to become less "conditioned" from this repair, EG if that's heating this room/floor will become colder, etc. because there will be less air flow through this room into the system.

  • 1
    The house has two furnaces and both have the same set up so it probably wasn't replaced. The second unit does not have the hole. I'd guess the installer made a hole too large and didn't seal it up and the previous owners never checked that low. Wanted to make sure this is not a common form of installation. Thanks for you input.
    – jpQuick
    Apr 25, 2023 at 18:27

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