Help filter my furnace.

July 6, 2009 5:37 PM

Our current central air conditioner uses 20x25x2 filters, which seem to be an odd size (after searching the aisles for a bit, I asked at the local big box hardware store and the ever-so-helpful worker there implied that I was insane & there was no such thing). Anyway, I've searched online a bit & some places do indeed sell them, but I was wondering -- would it be ridiculous to just stack two 20x25x1 filters together to save myself the trouble of special ordering the correct size?
posted by zempf to home & garden (5 comments total)

The only concern I could see would be a possible flow restriction your blower isn't designed to handle. I'd make sure the first filter was a cheap Fiberglas model for its extreme flow capabilities rather than stacking a pair of super anti-allergy filters.
posted by Mitheral at 5:44 PM on July 6

No, not insane, but the 1" thick filters aren't meant to be stacked. Meaning, two of them will put up a lot more resistance to the flow of air than one 2" filter would.

So if you're going to stack two 1", make sure they're the very wispy fiberglass filters that you can see through, not the 90% capture Filtrete ones. Otherwise you may seriously impede the airflow through your AC/furnace, and that may do bad things (lead to coils freezing or to the burner cutting off due to overheat).

If I were you, rather than stacking two filters, I'd probably buy one good filter (pleated paper) and one cheap one, or use the old one you're taking out. Cut the filter material out of the old one and just keep the cardboard frame around the edge. Glue it with some Liquid Nails or similar to the frame of the new filter. I don't think it matters if you do it on the front or the back. (Maybe glue first then cut with a razor knife? Just make sure not to nick the good filter.) The result ought to be a 1" thick filter that has a 2" thick frame around the edges, so it'll fit into your 2" opening.

Only if something like that doesn't work would I consider stacking two filters. And even then I'd probably only do it until I could get a proper one, just out of an abundance of caution.
posted by Kadin2048 at 5:45 PM on July 6

I had this EXACT thought a few months ago. Same size filter and everything. So I called my HVAC guy, who urged me in the strongest terms not to do this. As I was standing in the air filter aisle at Home Despot when I asked him, I didn't grill him on his reasoning. But this guy has done commercial and residential cooling and heating for me and I've known him for almost 20 years. So I think he wasn't just trying to sell me air filters.
posted by BitterOldPunk at 5:52 PM on July 6

Don't. Just use a 1" filter and change it more often; it'll be fine.

Speaking from experience. (Multiple filters block the airflow too much and can actually be sucked into the fan as a result, causing much excitement for all involved. Not that that happened to me or anything.)

You don't really even need to pad it by that extra inch as kadin suggests; there will be enough suction to hold the 1" filter in place.
posted by ook at 6:21 PM on July 6

You can get 20x25x2 here. You can contact them for price. The 2" is a special price. I use this company. They ship one to me every 90 days without fail.

http://furnacefilterstoyourdoor.com/about_us.html
posted by JayRwv at 7:13 PM on July 6