Can't find the right size furnace filter. What now?

September 29, 2020 10:55 AM

I need help determining the least-bad option for dealing with the furnace filter in my home.

We live in the Pacific Northwest, and are fortunate to have A/C, which we ran pretty much non-stop when the air was bad due to the recent fires. We had one spare air filter, which we swapped in a few days in. It desperately needs to be replaced now. There are absolutely no filters available to buy anywhere in the area, so I ordered some online. Even those were scarce, and I had to pick a brand that's not the one we usually get. They've finally arrived, but... they're too small. There's about a half inch gap on all sides. Even though the listed dimensions match, I now know that Honeywell filters are physically larger than advertised. Great. I know that using the wrong sized filter is bad for the furnace, but is it worse than continuing to use the old smoke-clogged one? Is there something else I should do? I will replace it with a correctly-sized one ASAP, but that could be another 1-2 weeks.

posted by Sibrax to Home & Garden (13 answers total)

I’d stick weatherstripping on the sides.
posted by sageleaf at 11:02 AM on September 29, 2020 [4 favorites]

I'd cut a piece of plywood or something the size of the filter opening, then cut out the center to an appropriate size for the filter you have. Probably can't get the cover of the air return to close but at least the filter is filtering.

Good Luck with it.
posted by sandpine at 11:06 AM on September 29, 2020

I personally would be more concerned about a heavily clogged filter than a slightly smaller but clean filter. If you get creative with duct tape and cardboard, you can probably make a frame large enough to keep it in place until the correct one arrives. Tape around the edges to get a good seal and you should be fine - just make sure it is secure so it doesn't get sucked into the furnace itself.
posted by _DB_ at 11:10 AM on September 29, 2020 [3 favorites]

In a pinch you can try vaccuming out the used filter so that you can continue to use it long enough to get a replacement.
posted by mezzanayne at 11:14 AM on September 29, 2020 [1 favorite]

Was your older filter jammed in there? Mine rests on little metal pieces but I didn't think it was supposed to be a super tight fit.

I'd use the new filter and either put some cardboard or weatherstripping around it (my guess is that's available locally) or just not worry about the gaps. I have some standard weatherstripping in my house and I just measured it -- it's about 1/4" thick. Or cut some coardboard pieces off of a box and use some duct tape to add a few layers.

Seems like having a clean filter with gaps is better than sending all the air in your house back through all those particles. I am not a scientist, just a homeowner in the PNW who has been dealing with the same thing. (I got my new filters a few days before the smoke cleared, and I did swap the dirty one out again, even though it was just a few days old; it was very dirty.)
posted by bluedaisy at 11:48 AM on September 29, 2020

As long as it fits securely (isn't likely to be sucked through), I wouldn't worry about it half as much as I'd worry about anything I put in to make it more secure might get sucked through. A lot of cheap furnace filters don't filter much beside chunks, so a bit of leakage probably isn't the end of the world.
posted by wotsac at 11:53 AM on September 29, 2020

Best answer: I had great luck contacting HVAC contractors during the smoke-pocalypee. Box stores sold out fast, but individual contractors still had plenty in stock (even towards the end of the smoke).
posted by furnace.heart at 12:11 PM on September 29, 2020 [2 favorites]

Similar to all these ideas: weatherstripping, and wrap it entirely in duct tape to make sure it doesn't go into the system. That should still allow for a bit of a compression fit as long as you start with relatively low density foam.
posted by MillMan at 12:22 PM on September 29, 2020

Best answer: The Honeywell brand has broad distribution through HVAC channels, less so they retail channels, so I would explore local HVAC companies.

Longer term, you could look into having an HVAC company make or augment the channel where the filter goes, to accept a more easily available filter size (note: this is not really recommended by the manufactures, but some people do this to avoid the cash grab that is obscure filter sizes).

You have probably looked, but Home Depot has a good online catalogue of sizes and may be able to ship quickly to a local store for you.
posted by walkinginsunshine at 12:47 PM on September 29, 2020

Response by poster: Yeah, I have open pickup orders with Home Depot and Lowes that aren't expected to arrive for another week.

I hadn't thought of checking with local HVAC companies. We just called the place that installed our A/C and they have filters! That's amazing! I appreciate the DIY solutions as well, but feel much better about having the real deal. Thanks for the assistance, everyone!
posted by Sibrax at 1:13 PM on September 29, 2020 [7 favorites]

For another DIY, you might be able to remove the innards of one of your old filters by removing the outer mesh on one side, for example, and placing your entire new smaller filter inside the frame of the old one and tape the frame of the new to the old frame along the edges, being careful to seal any gaps.
posted by jamjam at 12:31 AM on September 30, 2020

FWIW, filters do not fit tightly into the slot. There needs to be a bit of freeplay in order to remove them easily.
posted by Thorzdad at 5:15 AM on September 30, 2020 [1 favorite]

You've got this under control Sibrax but for anyone who might be coming to this later: Never run a furnace with A/C with a filter that doesn't completely cover the hole. It's fine for it to be loose in the channel but it must cover the entirety of the opening. A dirty filter is much better than a gap because the fins on an A/C evaporator coil are only 1-2mm apart. Hair, lint, dandruff, etc will gladly take the route thru any gaps and it takes very little to plug the coil up and then it is usually a major undertaking to clean it. A dirty filter might cause freeze ups but it that can be fixed easily by changing the filter.
posted by Mitheral at 1:55 AM on October 2, 2020