I just bought a house that has a two-year old Carrier central air/ furnace unit. The filter holder slot has a convex bend on the side closer to the furnace which makes little gaps between the filter's cardboard frame and the metal filter holder. When the fan is running I can feel a little air flowing into these gaps rather than through filter.

Here is a picture:

enter image description here

I'd estimate the top gap is less than 1/8" and lower is maybe 1/4". This does not change when the fan kicks in, i.e. neither the filter nor the metal frame flexes to close the gap. When the fan is running I can feel a small amount of air flowing into the gaps, which would obviously not be getting filtered.

Is this a big deal? I assume like 95%+ of the air coming in the main return duct is flowing through the filter as intended, not flowing out around it and into those gaps.

Is it worth trying to bend the right side of the frame to be more flat, or having a service tech in to straighten it?

Note that there is no door that closes to cover the filter slot. It's permanently open by design AFAIK.


More Pictures

Filter insertion area, filter removed:

enter image description here

Filter slot viewed from inside:

enter image description here

So the filter's cardboard frame roughly sits where these red lines are:

enter image description here

4 Answers 4


There should be an air filter holding frame there. It looks like someone tried to make one or they literally got the cheapest filter frame I have ever seen. These things should be a much thicker gauge and should firmly house the filter.

The other issue is that this should be fully enclosed with a solid cover. There are a multitude of problems here, easily resolvable. You need a filter holder (pan and a door). Any AC company can install this in an hour and probably have parts on hand.

You can install this yourself if you don't mind shopping around and finding the right sizes and doing so minor duct cutting and sealing.

enter image description here

So based on your picture the issue isn't the effectiveness of the filter - yea that's a very very minor issue - the main issue is the inefficiency of your unit based on lost flow and disrupted flow caused by all of the "cracks". All of the things in yellow should be caulked/sealed/taped and the filter holder should be rigid so these things can be done right.

Addendum based on comments: An example of a filter holding frame is here. I don't know this brand, just did a search. Should be very rigid, should have a "door", should have a section that the filter slides into. This is not rocket science and there are many different brands/types available.

When doing this you have two options:

  1. You get a good, rigid, thick gauge holder.
  2. Custom holder is securely fastened on both sides and sealed. (This is usually thinner gauge)

You have neither. Not really a huge deal but needs to be fixed.

  • I've added some more pictures to my post. There is what I would call a filter frame/holder composed of sheet metal, which you can see clearly in the last two pictures. Are you saying there's an additional item that should sit inside that, in which the filter itself should sit? If so, can you provide a link to or picture of an example product? Thanks.
    – SSilk
    Commented Oct 1, 2020 at 21:20
  • And yes, it appears there should be a cover. I see two screw holes for it, but no cover in sight. I'll follow up with the furnace installation company and see if they can provide one. The unit's only a couple years old.
    – SSilk
    Commented Oct 1, 2020 at 21:21
  • I will add more info based on your comments.
    – DMoore
    Commented Oct 2, 2020 at 6:50

That is going to cause a bit of leakage depending on how tightly the filter gets sucked against the frame when the air handler is running.

Usually there is a foam gasket there to help with sealing. But it's possible yours didn't have one or it was left off at installation.

You may want to adjust this to help force all the air to go through the filter. As it is most will but the larger the gaps the less effective the filter will be.

I've NEVER seem one of these that has no door, however. That reduces the effectiveness of your air return duct.


When the furnace fan starts drawing air through the filter it will conform to the housing, the bend may flatten out under pressure also but normal paper filters do flex and seal.

The end should be sealed with a “flap” of metal that snaps in or is screwed into place and that will help Chanel the air flow and pull the duct flat.

If this is in a garage the open intake would be a code violation. In a utility closet then it would depend if it is a sealed combustion chamber or not.

  • Regarding the filter bending to conform: it does not flex visibly to close those gaps when the fan starts. I started the A/C to get the fan going to see if flex and it did not. Or are you suggesting the filter will slowly bend over time to conform to the curve in the metal?
    – SSilk
    Commented Oct 1, 2020 at 20:37
  • Regarding possible code violation: this is in an unfinished basement. Concrete walls and floor, wood ceiling, door at top of stairs so basement is closed off from rest of house. Also I'm in Ontario Canada if that affects the code.
    – SSilk
    Commented Oct 1, 2020 at 20:39
  • In a basement it would not have to be blocked off but a garage normally it would. If the opening is sealed it forced all the air through the filter it can’t come in from the side and normally pulls the filter in. The problem I see is the ducting is hard mounted to the furnace. I was taught to add a decoupling / flex point that removes the stress from the duct work to furnace connection it also makes things less noisy as the fans are no longer hard coupled to the large sheets of metal that vibrate. The holders are usually folded on site so they vary in quality even from the same guy.
    – Ed Beal
    Commented Oct 1, 2020 at 22:59

You'll find out that these AC filter are not built to exact measurements. I've seen the thickness vary by as much as 1/4". If this bothers you, get some weather stripping from your home store and shove it in between the filter and the metal holder. It will bend the filter enough to close that gap. Any stripping would need to be rated for air handlers.

  • I would not put something behind unless it was done on all 4 sides. Also the weather stripping needs to be plenum rated some guys skimp on this but know many years there was a large lawsuit because “automotive” weatherstripping was used but that was in a commercial building. You don’t want anything that could generate excess or toxic smoke.
    – Ed Beal
    Commented Oct 1, 2020 at 23:10

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