#1
Old 09-14-2013, 06:52 PM
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Furnace filter sizes

What is the deal with furnace filter sizes? My current house has three places that need filters - for downstairs, I put it inside the blower unit, and for upstairs I put it at the two intake points (because the blower is in the attic and a pain to get to). All three of those locations take different size filters, and not a single one of them matches up with ANY of the filters available at the local home improvement stores. So I end up having to buy ones that are too big and cutting them down. A couple of years ago I had an HVAC guy out to fix something, and had him replace the filters at the same time. He had to do exactly the same thing: he manually cut down too-large filters. My last house only needed one filter, but it had the same problem.

So what's the deal? Is this a conspiracy to make you pay for more filter material than you need? Is there really no standard? Or have the standard sizes changed and my houses are outdated?

Yeah, it's pretty minor in the big picture, but it's annoying and it doesn't make any sense!
#2
Old 09-14-2013, 07:21 PM
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Join Date: Jun 1999
Location: Milwaukee, WI
Posts: 26,536
Mine are standard, but you might get a reusable one. A)so you can just rinse it off and put it back in and B)some of them come in one size and you cut it down to the size you want it anyways.
I can't find a good picture, but I had one similar to this. It was fixed in one direction, and it slid in and out in the other direction. Get the right height, figure out what length you need, cut it and assemble the frame around it. Took about two minutes.

Also, you might check Amazon for your size.
#3
Old 09-14-2013, 07:37 PM
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My guess is that whoever designed or built the intake points for your furnace didn't conform to dimensions consistent with standard filter sizes. In other words, the ductwork there was probably custom and no one thought about whether it should match a standard filter size.
#4
Old 09-14-2013, 07:52 PM
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Location: Milwaukee, WI
Posts: 26,536
Are you actually mounting the filter inside the furnace next to the blower or is it outside the furnace?
If they didn't use a furnace filter holder and just left a little slot between the return duct and the furnace to slide the filter in and out of, you can use a filter that's too long instead of trying to cut one down, it's not going to hurt anything if it hangs out a little bit. (Does that make any sense?)
#5
Old 09-14-2013, 08:19 PM
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It's just a slot between the return duct and the blower. But I still have a problem, because one size (I forget the exact sizes, but I think it's 12x24) is about the right width but just too short to cover the entire opening, and the next size up (maybe 14x30?) is so long that I can't even put the cover back on the furnace unless I cut it down, and it's just barely too wide also.
#6
Old 09-14-2013, 08:43 PM
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This site sells custom sized filters. They may be able to make them to your specs.
#7
Old 09-14-2013, 08:53 PM
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Quote:
Originally Posted by FoundWaldo View Post
It's just a slot between the return duct and the blower. But I still have a problem, because one size (I forget the exact sizes, but I think it's 12x24) is about the right width but just too short to cover the entire opening, and the next size up (maybe 14x30?) is so long that I can't even put the cover back on the furnace unless I cut it down, and it's just barely too wide also.
To clarify, when I said 'slot' I meant more like this. In a case like this, if you found one that was the right height, but stuck out a few inches, it wouldn't matter. But if you have to get a door back on that's different. Without the door, you'll suck in air from around the unit instead of the rest of the house.
#8
Old 09-14-2013, 08:57 PM
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Yeah, I don't have a slot like that. I have to take the metal panel off the front of the unit, and slide a filter in on top of the air intake duct.

The idea of custom sized filters is intriguing. Thanks Chefguy!
#9
Old 09-14-2013, 09:23 PM
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Join Date: Jun 2000
Location: Dallas, TX
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Standard sized filter holders aren't that expensive. I'd imagine over the long term, it might pay to get a once-yearly whole-house filter installed (like one of these) rather than having to replace 3 non-standard filters a couple of times a year.
#10
Old 09-14-2013, 10:28 PM
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Location: Portlandia
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Quote:
Originally Posted by bump View Post
Standard sized filter holders aren't that expensive. I'd imagine over the long term, it might pay to get a once-yearly whole-house filter installed (like one of these) rather than having to replace 3 non-standard filters a couple of times a year.
Yeah, really. It's just a bit of sheet metal, and it doesn't matter whether the filter is larger than the opening, just that it's secured in place. A guy with some flashing and a pop-rivet gun could likely make short work of it.

Last edited by Chefguy; 09-14-2013 at 10:28 PM.
#11
Old 09-14-2013, 11:33 PM
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Join Date: Aug 2011
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You've just been introduced to the miracle of Intellectual Property Laws. The dimensions of a filter are intellectual property, and it is a patent violation to make a filter that is the same dimensions as somebody else's filter. So each furnace maker has to require a different size filter. It's like vacuum cleaner bags or pickle jar lids

Last edited by jtur88; 09-14-2013 at 11:34 PM.
#12
Old 09-14-2013, 11:48 PM
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Location: Milwaukee, WI
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Quote:
Originally Posted by jtur88 View Post
You've just been introduced to the miracle of Intellectual Property Laws. The dimensions of a filter are intellectual property, and it is a patent violation to make a filter that is the same dimensions as somebody else's filter. So each furnace maker has to require a different size filter. It's like vacuum cleaner bags or pickle jar lids
Kind of unhelpful since we're in GQ and that reads like a serious answer, but here's something that most people probably don't know.
You buy a furnace and a furnace filter holder separately.
The people who make the furnace having nothing to do with the filter or deciding what size will be used, that's up to the company that installed it. You decide where you want the furnace then you decide, based on where the ducting is coming in from and other things, which side the filter is going to go on and you cut out the sheet metal on that side and install the filter housing there.

There's no reason the OP can't go to an (open to the public) HVAC store and get a standard size filter holder and find sheet metal worker to install it. It's probably not worth it, but it's very doable.
#13
Old 09-15-2013, 12:01 AM
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Join Date: Oct 2008
Location: San Jose, CA
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Quote:
Originally Posted by FoundWaldo View Post
It's just a slot between the return duct and the blower. But I still have a problem, because one size (I forget the exact sizes, but I think it's 12x24) is about the right width but just too short to cover the entire opening, and the next size up (maybe 14x30?) is so long that I can't even put the cover back on the furnace unless I cut it down, and it's just barely too wide also.
If the 12X24 leaves a short gap I have an solution. Make a spacer to fill the gap. I have worked on many air handlers that the filters slid in and 1 to 2 inch space goes in after the filter to fill the gap when closing the door.
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