# Furnace Filter-Right or Wrong? **URL:** \ **Category:** Factual Questions\ **Created:** [December 23, 2016, 8:59pm UTC](https://boards.straightdope.com/t/furnace-filter-right-or-wrong/775477 "2016-12-23T20:59:18Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![crocket104](https://avatars.discourse-cdn.com/v4/letter/c/9de0a6/32.png) [@crocket104](https://boards.straightdope.com/u/crocket104)\ **Post date:** [December 23, 2016, 8:59pm UTC](https://boards.straightdope.com/t/furnace-filter-right-or-wrong/775477/1 "2016-12-23T20:59:18Z")
I was told by the installer of my 67000 BTUH, Revolv EB20D, Electrical furnace that I could only use a flat filter. Could anybody tell me is this is correct? ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [December 23, 2016, 9:06pm UTC](https://boards.straightdope.com/t/furnace-filter-right-or-wrong/775477/2 "2016-12-23T21:06:49Z")
Looks that way … here’s the Owner’s Manual {PDF} … it says you might have two filters … I suggest finding them, writing down their numbers … and buying some … clean them often … That’s a nice unit … smart move on your part … ---
**Author:** ![crocket104](https://avatars.discourse-cdn.com/v4/letter/c/9de0a6/32.png) [@crocket104](https://boards.straightdope.com/u/crocket104)\ **Post date:** [December 23, 2016, 9:23pm UTC](https://boards.straightdope.com/t/furnace-filter-right-or-wrong/775477/3 "2016-12-23T21:23:43Z")
Thanks for the quick response!! Merry Christmas!! ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [December 24, 2016, 12:15am UTC](https://boards.straightdope.com/t/furnace-filter-right-or-wrong/775477/4 "2016-12-24T00:15:07Z")
“A sorry excuse to pick a man’s pocket every twenty-fifth of December” – Dickens ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [December 24, 2016, 4:38am UTC](https://boards.straightdope.com/t/furnace-filter-right-or-wrong/775477/5 "2016-12-24T04:38:31Z")
Gosh, that’s kinda Bah Humbug of you there **wolf** old buddy. Where’s Tom Lehrer’s Christmas Carol when you need it? Right here of course: [Tom Lehrer - A Christmas Carol - YouTube](https://www.youtube.com/watch?v=DtZR3lJobjw) . Listen and behold the proper spirit of the season. ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [December 24, 2016, 4:55am UTC](https://boards.straightdope.com/t/furnace-filter-right-or-wrong/775477/6 "2016-12-24T04:55:27Z")
Sir Isaac Newton was born on Christmas Day … I guess there’s that … and the OP is going to be nice and warm … ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [December 24, 2016, 5:23am UTC](https://boards.straightdope.com/t/furnace-filter-right-or-wrong/775477/7 "2016-12-24T05:23:28Z")
> [@watchwolf49](#): > > Looks that way … here’s the Owner’s Manual {PDF} … it says you might have two filters … I suggest finding them, writing down their numbers … and buying some … clean them often … > > That’s a nice unit … smart move on your part … Semi-nitpick. It can be operated in upflow or downflow mode. In upflow mode (which is how most furnaces run) it uses a 20x20x1 filter, located on the side. In downflow mode, it uses a 16x20x1 filter, located near the top behind the access panel. One filter, just two locations depending on how it’s set up. Anyways, if by ‘flat’, you mean 1" as opposed to a big 4" wide filter, then yes\* that would appear to be the case. \*If you _really_ wanted to use a 4" filter, you can have someone install the housing for one to accept that, or even just have them make one out of duct. Depending on the layout of your existing ductwork, it may or may not be trivial to get it between the return and the furnace. They look like [this](http://media2.airfiltersdelivered.com/media/catalog/product/cache/2/image/512x512/9a69726f1e7e95793254439d445924bf/t/t/tt-mac-1722_2.jpg), at an HVAC supply house, they’re purchased separately from the furnace and mounted to the side of it.