how to change furnace filter (bottom return) for Lennox G43UF
The air filter should slide in the slot between the return air duct, and the furnace.

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&quot; x 23&quot;).

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&quot; x 25&quot; filter.


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.