I searched for 30 minutes, and I could not find the furnace filter, is it possible to install/build a filter slot? my whole family got allergy after turning on heater this winter. Thank you!

enter image description here Lennox 80mgf4-75a https://dl.dropboxusercontent.com/u/50982191/IMG_1425.JPGenter image description here

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  • Is it mastic-ed over? Take the bottom panel off the furnace and look inside that duct. Is there a 1" channel for a filter in it? I really hope you don't find an old filter in there...
    – Mazura
    Dec 4 '14 at 4:13
  • Jimmy, thanks, I do have a filter at the return-air grill, after searching on internet and you tube, I thought I should have another one in the attic. I guess that's not true ? Dec 4 '14 at 14:10
  • Thanks Mazura, I looked, there is no moving part in that unit that looks like a filter. Dec 4 '14 at 14:14
  • Thanks Steven, I will open up that big tube and clean it myself this weekend. the house was build in 1998, air duct never got cleaned before. Dec 4 '14 at 14:15

I have a Lennox too and I hate the filter situation with a fury of 1000 suns:

enter image description here enter image description here

It's just sitting in the furnace against the return duct - previous owner left a VERY dirty one taped with aluminum tape that took me a long time to peel. I've been using masking tape. One of these days I'll find time to fabricate a filter box to sit between the return duct and the furnace.

So, I'm guessing from your pictures that you'd be in the same situation - inside the furnace, against the mastic'ed duct.

  • +1 for "fury of 1000 suns"! And for the pictures and description. Dec 6 '14 at 0:03
  • Is this the only place you have filter? for me, I don't see this filter even after I opened up the furnace, but I have a 1" filter in the return-air grill that located in the ceiling of living room. Dec 9 '14 at 20:59
  • Yes, it's the only place for me. My return is a duct that opens to a hole in the subfloor. How does the filter in the return look - very dirty? Your system is probably designed to take the filter on the return side - don't be tempted to add a second filter in the furnace, that could put strain on the blower, just like a single super-fine filter with a high MPR-value might.
    – autonomy
    Dec 10 '14 at 15:14
  • I am at work, but my return looks like this, it's in the ceiling. link Dec 12 '14 at 21:10
  • I replaced the 1" filter about 3 month ago, so it should not be too dirty. I noticed a lot of people are coughing when weather turned cold, maybe most of them got the same allergy from heater Dec 12 '14 at 21:16

Modern residential systems frequently have the filter located at the return-air grill. Look for the large grill where blower air is pulled in from the house; if the grill has small latches and hinge tabs, where it can swing open, it is probably designed to have a slim furnace filter installed behind the grill.

  • In that case you'd have to have a filter on every return, of which there could be one in every room.
    – Tester101
    Dec 4 '14 at 13:09
  • @tester101, a single return air grill and duct (with filter) can supply most residential forced-air systems. Fact. Dec 5 '14 at 7:57

Yes it would be possible to install a filter. You would need some sheet metal and know-how, or you might get lucky and find something pre-made. A qualified HVAC technician could make one easily.

You might also consider getting your ducts cleaned to help reduce the dust.

  • A duct cleaning would help in the very short term, and would be short lived. Without a filter; or regular cleaning of the furnace, the lifetime of the furnace will be reduced.
    – Tester101
    Dec 4 '14 at 13:11

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