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This is the first time I am replacing the filter for my furnace. The old filter is 20"x20"x1" and I got a new one that's 20"x20"x4" since there's a 4" slot. However, there's a little plastic thing blocking the 4" filter from going in. I can probably unscrew and remove it but there's probably a good reason it's there. Photo of filter slot with plastic protrusion on sidewall circled in red

Closeup of white plastic protrusion on side wall of slot

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    Apparently the slot isn't a true 4", which is probably part of why a 1" filter was installed. Try 2"?
    – keshlam
    Commented Oct 24, 2023 at 0:43
  • The thinner/cheaper a filter you use, the cheaper it is to cool and warm your house. If you have air quality issues, it's a lot better to buy a stand-alone filter than to lower the efficacy of your HVAC. Since it consumes a lot of power, even a small difference can be reflected in your bill.
    – dandavis
    Commented Oct 24, 2023 at 4:07
  • @dandavis Do you happen to have a reference site that goes into the details of what you are saying? It would be interesting to learn the savings potential versus the cost of buying, running, and maintaining separate filtering units. An HVAC system designed properly for an entire structure will have the economy of scale and possibly do a much better job than a couple stand-alone filters sitting in corners. It will be interesting to read if someone or some company has performed a quality analysis of the issues involved.
    – End Antisemitic Hate
    Commented Oct 24, 2023 at 10:11
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    @dandavis That's just not true. Some wide filters are designed for the same filtration as their narrower brethren, but with easier airflow and longer filter life, especially as it takes them longer to get filled with filtered debris that restricts flow.
    – Joel Coehoorn
    Commented Oct 24, 2023 at 15:36
  • @EndAnti-SemiticHate No, just my own experience. In the cold snap a few years ago our furnace struggled to keep up, got down to 54 inside. We replaced our 5" filter (with another 5") and it started slowly crawling back up, but still under 60 after 4 hours. On a whim, I replaced the 5" filter with a cheap 1" I had around, and it reached 68 in about an hour. Ergo, my furnace can put far more heat into the house with a "barely legal" filter than a "good" one. There's some caveats, ideally fan speed might need to be lowered or hysteresis increased to stop short-cycling, but my results speak aloud.
    – dandavis
    Commented Oct 24, 2023 at 21:32

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The thin (1") slot is for a standard passive filter.

The larger slot is for an electronic air cleaner. A much more complex and expensive beast. I have them in my furnace. They haven't really worked in many years, but they are still there, and I clean them (since they do catch some of the dust) when I clean the other filters. I have metal mesh filters rather than the disposable filters, so they get a good cleaning instead of replacing.

My hunch is you had one and it stopped working and someone (e.g., previous owner) removed it. Something like this Honeywell:

Honewell air cleaner

When the main section that is in the air path is removed, it looks like the picture in the question.

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  • You are right, so I got an air cleaner that takes a renewable filter. But if I remove the plastic, I can fit the 4" single use filter perfectly, is that what you did or you installed a filter housing in a different section of the duct?
    – Ray Cheng
    Commented Oct 24, 2023 at 1:27
  • Mine has the BIG middle thing, and it has a washable thin metal mesh filter next to it, similar to the size of a typical disposable filter (actually maybe a little thinner than the disposable). It goes in a slot right next to the BIG middle thing - your picture shows a similar slot, actually slots on both sides of the BIG middle thing.
    – manassehkatz-Moving 2 Codidact
    Commented Oct 24, 2023 at 1:37
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    I use nice large disposable filters that slide into the slot where the electrostatic precipitators went. I think the question is more how to spec the filters they buy to fit the slot properly, which can take some trial and error.
    – Jon Custer
    Commented Oct 24, 2023 at 14:55

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