My wife and I just purchased / moved into a new house and I was going through each of the systems and found the HVAC filter, it was literally destroyed (it had filled with crap and crushed itself in half. So essentially we had no filter.).

So I want to replace obviously, and reading up on filters it seems a MERV 11 or 13 would be perfect, I was also reading that a thicker filter can work better while not impacting air flow as much, so I was hoping to go with something in the 4"-5" range like this.

Here is where I am confused, the ductwork must be custom, there is a sliding door made of just bent tin. HVAC Overall System

So I open that up, and find the broken down filter, which is marked as 20x20x1. However I have a feeling that either one or both of these are true: 1. They used the wrong type of air filter 2. I can use a thicker air filter

Reasons being: - the actual hole going into the furnace is 10" H x ~19.5" W - There is a (what I assume is retention tab) tab about 5.25" off the hole wall, making me think that I can use a filter of ~5" depth.

Here are all the measurements Dimensions of filter cavity

And a picture of the filter cavity interior filter cavity interior

Finally, while I don't think it matters, this is the furnace model: Furnace Model

( It's carrier 58CVA135---16122 )

My questions then are: 1. What size filter should I be using? 2. And is a thicker filter best for this setup?

I really appreciate any help! Scratching my head on this one.

  • The huge size of that cover leaves me wondering if there was a electrostatic filter in that space, sure higher merv #s filter better but the correct filter for your unit should be written or stamped close to the filter location, the wrong filter can cause many problems including a shorter service life for the furnace. The furnace has a recommended size but your sheet metal installer is the one that specs the actual size.
    – Ed Beal
    Jun 7, 2019 at 20:39

1 Answer 1


I do not believe that that large hole shown in the next to the bottom picture is for a filter. If you look to the left side of the picture that grey wavy item is probably the filter. To access the filter you will probably have to remove the bottom furnace door. You could also have a filter mounted in the bottom of the furnace. I have no idea what that access hole is for but it probably is not for a filter or electronic air cleaner. I am sure glad the person that installed that return drop duct never worked for me. It may be a good idea to have a reputable HVAC company inspect the whole furnace and duct installation and correct any deficiencies. my 2 cents

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