Furnace filter size by brand
Lennox furnace filter size
Most furnaces do not print the filter size on the furnace label. The filter is sized to the opening the installer built, so the reliable answer is the edge of the filter you have now.
Where to look
- A slot on the side or bottom of the furnace, behind a small cover.
- A rack on the return duct right next to the furnace.
- Behind a return grille in a wall, ceiling or floor. Some homes have more than one.
- A deep media cabinet a few inches thick between the duct and the furnace.
Lennox sells 5-inch media filters, including 16x25x5 and 20x25x5, for its media cabinets. If your system has a cabinet a few inches deep between the return duct and the furnace, match the size on its door or on the old filter.
The model number tells an installer the cabinet width, but the filter follows the return duct, so it does not give you a filter size on its own.
The rating plate
The furnace model and serial number are on a rating plate, usually inside the blower door. Your HVAC company can use them to look up the cabinet, but the printed edge of your current filter is still the quickest answer.
Not sure? Check the printed edge of the current filter, or measure the slot and use the size finder.
Lennox filter questions
What size filter does a Lennox furnace take?
It depends on how the return duct was built, not on the Lennox badge. Read the size printed on the edge of the filter you have now, or measure the slot and round each side up to the next whole inch.
Where is the filter on a Lennox furnace?
Check a slot on the side or bottom of the furnace, a rack on the return duct right next to it, any return grilles in the walls or ceiling, and a deep media cabinet if there is one.
Can I find the filter size from the Lennox model number?
Not reliably. The model number describes the furnace cabinet, while the filter is sized to the opening the installer built. Your HVAC company can look it up from the model and serial number on the rating plate.