Results 1 to 12 of 12
  1. #1

    Confused with Air Filters


    I am confused about my air filter size and placement.

    The A/C unit is in the attic. The air inlet door/vent is downstairs near the thermostat. Behind the door/vent, is basically a cutout opening. It is about 20x40 in size, but looks like it was cutout with a blade. I can stick my head in there and look upward. It is basically a giant insulated chamber leading to a duct that goes into the inlet side of the A/C unit. The unit has a sticker that says the filters are located elsewhere ( or not in the unit itself) and that the correct size should be used. It doesn't say much else.

    I am assuming the filters go behind the door/vent downstairs, over the 20x40 cutout. I have been putting two 20x20x1 filters, one stacked above the other, taped together.

    What size filters should I be using, and is there possibly something missing that holds the filters?

    Thank you,


  2. #2
    Join Date
    Nov 2006
    Athens, Ohio
    Pictures would be helpful. All air must pass through the filters or they are useless.

  3. #3
    Here are the pictures in order, from the opening to the A/C unit #1-#5. #6 is with filters.

    1 - Without Filters: Name:  air_opening.jpg
Views: 221
Size:  74.3 KB
    2 - Inside of that opening, looking upward: Name:  air_intake_opening.jpg
Views: 206
Size:  14.2 KB
    3 - Attic view of duct where picture 2 goes: Name:  attic_vent_2.jpg
Views: 260
Size:  55.9 KB
    4 - Between that duct and the A/C unit: Name:  attic_vent_1.jpg
Views: 246
Size:  62.9 KB
    5 - A/C unit from there: Name:  air_unit.jpg
Views: 215
Size:  54.0 KB
    6 - The opening with filters in place: Name:  air_with_filters.jpg
Views: 209
Size:  82.2 KB

  4. #4
    Join Date
    May 2007
    DC Metro Area (MD)
    Is that the only return in your home? Can you post the diameter of the flexible duct connecting the return drop to the return plenum attached to the furnace? Hard to tell from the pictures, but looks like a 14" which may or may not be enough capacity for your system. Do you have model numbers you can post of the equipment? This will tell us sizing information.

    If you want to use one filter, you can install one at the furnace itself.

  5. #5

    Yes, that is the only return. I will measure tomorrow and get the model numbers. Where would I install the one filter?

    Thank you for the help.

  6. #6
    Join Date
    May 2007
    DC Metro Area (MD)
    Quote Originally Posted by David1000 View Post

    Yes, that is the only return. I will measure tomorrow and get the model numbers. Where would I install the one filter?

    Thank you for the help.
    There should be a 1" filter rack internal to the furnace, right between the return plenum (where the flex duct attaches) and the unit itself. You'll have to remove the access door (and peel back some foil tape) to see what size it requires. However, you will have less pressure drop using two filters as you have been doing in the past (more surface area, less velocity).

  7. #7
    The duct is 18". The label says "Filters should be installed external to the furnace casing". It is a York, but I do not see any model numbers. I am kind of expecting some filter holder or something more professional than a 40x20 cutout (and not that straight of a cutout).

  8. #8
    Join Date
    May 2007
    I don't understand. Pic 1 is a 1" FILTER GRILL. Put a filter in it! If it measures 20x40, then put a 20"x40"x1" filter in it.

  9. #9
    I cannot find 40x20 filters anywhere, including Lowes, Home Depot, Wal Mart, and local hardware stores. Do you know anywhere that sells them other special order online? Also, it doesn't seem like a filter holder, but just an opening where a holder should go. I'll do that though.

  10. #10
    Join Date
    Oct 2010
    Anderson, South Carolina, United States
    Taping two 20x20x1 filters like you have is fine as long as its secure and doesn't break in two. If you do find 20x40 filters they will likely be 4x the cost of 20x20s because they are uncommon size. The best thing you could do is have a contractor install a 5" media filter on the furnace.

  11. #11
    Join Date
    Jun 2007
    Round Rock
    It's probably a 20x36. Measure the inside opening of the part that is screwed to the wall. You'll notice a lip, the filter sits against that to keep it from being sucked in. It's a standard filter frame, which should hold a standard filter. Not sure I've seen a 20x40 before.
    I like DIY'ers. They pay better to fix.

  12. #12
    Join Date
    May 2004
    south louisiana
    it looks like a return air filter grill to me.
    in pic 1 when the return air filter grill door is opened, there is a 1" space where the filter goes.
    the lip holds it in place to keep it from falling into the return.
    2) 20x20 filters is what I'd won't find a 20x40.

    what it looks like you are doing, is putting the filter behind the lip, and in front
    of the ductboard cut out.
    oh, and you can cut the ductboard to the same size as the opening for the
    return air filter grill.

    btw, nice job locating the r/a in a corner, forming a triangular chase,
    and the ductboard sealing it air tight. also, ductboard install in attic
    looks great.
    nice supply air trunk line...from what I can see.
    nice to see duct board done well.

    if more hvac companies did this kind of work, the bad refrences to duct
    board would be much less. and using a corner for return chase...much
    easier for homeowner to give up than half a closet!

    model number is inside the access panel of the 80% gas furnace.

    best of luck
    The cure of the part should not be attempted without the cure of the whole. ~Plato

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts

Related Forums

Plumbing Talks | Contractor MagazineThe place where Electrical professionals meet.
Comfortech 365