When I bought my home, my furnace had a 16x20x1 nominal-size filter installed, and that's what I've been buying for replacements. This size doesn't fit the opening very well; there are large gaps around the sides. I measured the opening at 16"x20"x1-1/4".

I assume that the gaps are a problem, as they will allow unfiltered air into the furnace. What can I do to eliminate them?

filter with gaps around the sides

filter cavity without filter installed

Here's a picture of the filter's position when the furnace is on. I can still see the gaps around the edges: filter position with the furnace on and ingesting air

This picture shows the entire slot -- note that there is no "door" or "panel" to close wide shot of the slot

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    take another picture when the blower is running – jsotola Sep 30 '20 at 1:55
  • @jsotola Added a picture. I don't see any difference, myself. – Jack Sep 30 '20 at 13:48
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    I wonder if the previous owner lost the cover for the filter slot (accidentally or otherwise). I would think that you would want this covered to help pull air from throughout the house's cold air return system instead of just the basement or the furnace closet or wherever this is located. – FreeMan Oct 1 '20 at 15:56

Those gaps are normal and don't usually cause a problem. When the furnace is running the air pressure forces the filter against the downstream face of the slot, effectively sealing the opening.

Also, it's not critical that you have a complete seal. The air in your home isn't (or shouldn't be) particularly dusty. It's important that most of the dust gets caught before it passes through the fan and heat exchanger(s), but it's not required to have clinically clean air.

They're pretty much all this way. If the filter was to fit tightly enough to create a seal against both faces it would be extremely difficult to install them. The alternative would be some sort of clamping mechanism like you'd see on a car engine, but it's just not that critical.

  • Pic #3 was taken with the furnace on, and it doesn’t seem to be making a difference. That said, my filters do seem dirty when I replace them, so I guess they must be doing some good. Thanks! – Jack Sep 30 '20 at 23:44
  • That's because you had the panel open. With it closed there's more pressure against the filter. – isherwood Oct 1 '20 at 0:16
  • Ah, that sounds like the real problem. There's no "panel" -- this is outside the furnace enclosure, and the gap for the filter doesn't have any cover to shut. I've added a picture to demonstrate. There's even a matching 1"+ gap on the rear side of the duct, as seen in picture #2. – Jack Oct 1 '20 at 13:43
  • Ah. Yes, that slot is usually at the top of the duct and there's a panel that lays in place over it. If you don't need that air evacuation you might look at how you can close it. I'm not sure whether that's appropriate for your overall HVAC system plan. – isherwood Oct 1 '20 at 13:53
  • Thanks! I just ordered this product to help seal up the gaps. The product images look just like my setup -- filter slides in from the side, with no cover. amazon.com/AllergyZone-AZFL-100-Filter-Lock-Cover/dp/B00PKS6MT0 – Jack Oct 1 '20 at 17:50

That slot is designed for a cut-to-fit filter. You can have your HVAC people cut a new slot that will fit a standard (better) filter.

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    There is custom fit furnace filter material available? That's very unexpected. Could you point to where one might find this available to purchase? I've not heard of that in the US. – FreeMan Mar 12 at 18:43
  • I have seen some cut-to-fit filter media. – Jim Stewart Mar 12 at 21:01

The opening around this filter would allow air to enter from the space the duct is in. This air will be from the non-conditioned space and should not enter the air handler even if it passes through the filter. It will cause the filter to get clogged sooner.

I would seal it with wide tape whenever a new filter is put in. Tape could stop the filter moving toward the air handler (by adhering to the edge of the filter) so cover inside of the tape that could stick to the cardboard edge of the filter. You could cut a piece of sheet metal to cover the opening and seal that with tape.

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