My friend has just moved into a house that was previously abandoned. His furnace is a Rheem Criterion 2 brand/model. We can't tell where the furnace filter should go or what size it is. Can anyone help me out?

Measure the width of the furnace. If it is 21" wide, it takes two 12x16 filters. If it is 24 or 24 1/2" wide, it takes two 16x20 filters. Assuming it is a downflow model with the two filters in a V pattern at the top of the top compartment.

Measure the width of the furnace. If it is 21" wide, it takes two 12x16 filters. If it is 24 or 24 1/2" wide, it takes two 16x20 filters. Assuming it is a downflow model with the two filters in a V pattern at the top of the top compartment.

I'm sorry. I did not proof my answer. If it is 21" wide, it takes two 12x20 filters... not 12x16.