In a house now for 6 months, just learned today I should have changed my filter twice already, whoops.

I have a zoned system - two furnaces, one in the finished attic on its side, one in the basement. As far as I can tell, there is only one filter - in the attic stairway, there's a big grille on the wall, and behind that is a filter which is connected to a duct leading to the furnace. Easy. There is nothing like this at all in the basement - the ducts don't lead to any sort of grille. Is this normal?

Including a couple of photos of the furnace in the basement, from the two sides I can access. The ducts are covered in some kind of insulation, then lead out into the home.

From the front From the rear

  • 2
    Our furnace filter is between the return ducting and the furnace itself. On your 2nd pic, down there in the shadows, barely in the bottom of the picture. You might want to look there.
    – FreeMan
    Commented Mar 15, 2021 at 16:02

2 Answers 2


If the filter you know about is on a duct leading to "the furnace" in the attic, then there must be a filter associated with "the other furnace" in the basement. I've seen them in various locations on/behind/under the furnace itself, and I haven't even dealt with that many hot air furnaces (my area was boiler-centric for heating, and A/C was practically unknown.)

Figure out where the air goes towards the furnace and follow - gotta be there somewhere.

Likely a very understated edge-on view when you are looking for it.

  • Thanks, but I still can't find it when following. The ducts are covered in this insulation, I updated my original post with a couple of photos. Is it possible that the air coming into the basement furnace had been run through the attic filter/furnace?
    – bpapa
    Commented Mar 15, 2021 at 15:59
  • 1
    Wound up finding it in the bottom part after removing the cover! Glad I didn't give up.
    – bpapa
    Commented Mar 16, 2021 at 2:04

Check the basement, sometimes the return pulls air from that area also.

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