I have a Lennox G43UF furnace and I am not sure if I need to have both a filter for a side return and bottom return. If I do (which sounds logical), where would the location of the bottom return filter be? I just bought a home and there were no filters in place.

The manual for the filter states the following: "This unit is not equipped with a filter or rack. A field−provided filter is required for the unit to operate properly. Table 1 lists recommended filter sizes. A filter must be in place whenever the unit is operating."

Table 1 per above lists the size of filter that I would need for side return (16x25) and bottom return (20x25).

I have located the side return filter which seems to be behind the tin foil in image 1 (in centre of picture) but I have no clue where the bottom return filter would go.

Any advice would be appreciated!

furnace with cover furnace without cover inside furnace panel blower motor

  • Are there filters at the other end of the air return, behind the vent covers?
    – BMitch
    Commented Oct 17, 2014 at 19:10
  • The filter should slide in the slot just where the return meets the furnace. Looks like something silver is there right now.
    – Tester101
    Commented Oct 17, 2014 at 20:21

1 Answer 1


The air filter should slide in the slot between the return air duct, and the furnace.

Air filter location

The filter should have some markings on it, to indicate which way the air should flow through it. Make sure when you install it, you install it in the proper direction.

Supporting Documentation

The installation instructions shows the location of an optional external side air filter kit, which is the slot in the photos you've provided. It also shows where, and what size the return air opening should be (14" x 23").

External air filter kit

When you slide the filter into the slot, it should completely cover the return air opening. This is why the documentation calls for a 16" x 25" filter.

Air filter in place

This model also allows for a bottom return, which you do not have. If it was installed to use the bottom return, the return ducting would attach to the bottom of the furnace (basically the furnace would sit on top of the duct). You'd then install the air filter on the bottom of the furnace.

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