I recently got a new heat pump installed. I realized that the filter is just slid in without any door or sealing, see here:

enter image description here

I find this a bit odd because everything else (the ductwork incl the return duct!) is so perfectly sealed.

Is this normal? (Note: It passed inspection!)

If not, what can I best do about it? Easiest would be just taping over it but that makes changing the filter hard. I was also thinking of cutting a piece of metal and using magnets (I did not try if magnets would even attach) but then I am concerned that this solution would still not seal nicely.

  • Is there another filter inside the metal cabinet? Is this a sort of pre-filter on the return air? We have a SpaceGard pleated paper filter system 7 inches or so between pleats in well sealed enclosure. This open slot is surprising to me. Jan 22 at 13:02
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    The cover on the commercially manufactured filter housing holds on with a tab and two magnets. It works really well. The forces against such a cover are suction forces, so the cover already wants to stay in place.
    – KMJ
    Jan 22 at 18:28
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    When the fan is on the inside should be at lower pressure than outside so this would pull any loose covering against the slot and seal. Magnets holding a piece of plastic should seal when the fan comes on. Jan 23 at 4:00

1 Answer 1


Yes that's normal. It's a perfect example of how energy codes make code implementers feel good, while leaving situations that make a reasonable person ask "wait - what??"

Yes you could improve the sealing. A shallow 5-sided rectangular box could be mounted to act as a door over the filter with a hinge on one side, latch on the other, and compressible foam weather strip around its perimeter. Or maybe just stuff a strip of soft foam to fill the gap between the sheet metal and the cardboard filter frame.

If this equipment is located inside the conditioned space I wouldn't bother sealing the filter access slot -- a small percentage of leakage around the filter is not a big deal, and leakage from the space that the return duct returns from is not a problem either. If the equipment is in unconditioned space, especially an attic, sealing would be more beneficial.

  • Does it only have impact on temperature or also air quality? It is in basement where it's very dusty and dirty. While unconditioned space, it never saw it going below 50F (even without heating the house at all) and most of the time it's in the upper 50ies in winter, warmer than other places in the house.
    – divB
    Jan 23 at 7:29
  • @divB Because there's some suction around the filter whatever is in the air in that space will be mixed in with the air passing through the system, albeit at a relatively low ratio. When there's nobody moving about in the basement the dust probably settles so that there's little of it suspended in the air.
    – Greg Hill
    Jan 23 at 13:47

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