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Our Carrier Infinity gas furnace (model 59MN7A080V) was installed more than 10 years ago. I have only recently taken over as "manager" of the furnace and have no clue how to properly install the air filter (we have both 1" and 3.5" thick ones). The original installer did a poor job so I am unwilling to ask them, but don't want to find someone else for a service call just yet.

My issue: I think our current air filter is not properly installed. As shown in the photos, the air filter box is below with air traveling up into the blower. The 1" thick filters used until now have just been placed loosely in the bottom of the air box, under the J bracket visible in the middle (it is 3.5" high). There are also one or two small round holes at the very back end of the air box, one probably lining up with the bottom of the J bracket. There is a long metal rod with a square hook at the end, currently unused (see separate photo).

Air Filter Box Area Open Filter Box showing filter sucked up at back by blower Mystery Rod

When the blower is on, as in the photos, the back end of the filter gets sucked up toward the blower, but the front is held down by the J bracket. The filter does not seem sized to fully fill the opening and seal against the box walls. Do I need more/different brackets or am I just installing the filter improperly. Would a 3.5" thick filter work better, maybe fitting in the J bracket?

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  • How have filters been replaced over the last 10 years? Where does that bar in the last picture normally go in the filter compartment (i.e. before it was leaning against the door for its beauty shot)? I'd say that big metal tab in the middle of the "filter installed" pic is going to preclude putting in a thicker filter.
    – FreeMan
    Feb 23, 2022 at 13:19

2 Answers 2

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The 1" thick filters used until now have just been placed loosely in the bottom of the air box

Right, have you seen the price of a 3.5" filter? Something is better than nothing for sure. FYI, all filters are a nominal size and are unlikely to be a perfect fit; "nominal" differs between manufacturer as well so good freakin' luck trying to find a perfect filter.

FWIW, that's an FPR 9 (MERV 11-ish) filter at 1 inch which should probably be replaced monthly or else the blower motor will be working extra hard and you will send the blower to an early grave.

Anyways, it sounds like you want the easiest solution possible so:

  1. Clean the dust from the sides of the chamber
  2. Cut 4 pieces of foam weatherstrip or more if you'd like

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  1. Adhere the weatherstrip to the front and back of the box. Warning: dimensions and angles not drawn to scale.

enter image description here

  1. Insert your filter and the foam should keep it from going diagonal

After further consideration you should probably place the foam pads higher and place the filter on top of the pads. Obviously, you'll have to remove that J-bracket.

This way you don't run the risk of having the foam adhesive come loose and make its way into the blower motor.

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  • Great idea on the weatherstripping to support a filter! Since I went out and got a Filtrete 3.5" thick filter, I think I will try setting the supports for that thickness. Hopefully with the thicker filter, replacing filter twice a heating season (winter in central California) will be OK for the blower motor.
    – Armand
    Feb 23, 2022 at 15:58
  • @Armand Sounds good, up to you. The only true way to test if a filter needs to be replaced is to take static pressure measurements.
    – MonkeyZeus
    Feb 23, 2022 at 16:02
  • Hah! I'm already thinking about what cool tool/device purchase would be required, but I've got to hold myself in check until after I finish dealing with the house's 1968 split-bus electrical service panel :)
    – Armand
    Feb 23, 2022 at 16:10
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According to the owner's manual for that model (pg. 7-8), you can use either the 1" (16 x 25 x 3/4") or what might be sold as a 4 or 5" (16 x 25 x 4 5/16") filter. Internet search seems to indicate that the 5" filter is the "preferred" one. The manufacturer recommends a "washable" filter which might prove to be cost-effective over time.

I guess that when using the 1" filter, the "hook bar" (from the last photo) is inserted into one of the holes at the rear of the compartment, and the other end clips into the "J-hook" at the front to hold the filter down. When using the 5" filter would require removing or "modifying" the bracket at the front of the filter compartment.

You might have better results (both in performance and fitment) if you use a little bit higher quality filters. The HD house-branded filters, in my experience, are pretty flimsy and struggle to hold their shape as the air flows through them.

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  • Ahh, makes sense about the hook bar's use. Thanks for the general owner's manual link as well - the lists are helpful! PS - apparently a replacement hook bar now sells for $100 apiece online!
    – Armand
    Feb 23, 2022 at 16:08
  • Selling the hook bar, @Armand, could well pay for a long-life, reusable, washable filter!
    – FreeMan
    Feb 23, 2022 at 17:40

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