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.
Best Answer
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.