Furnaces filters can be hard to find sometimes. Here are some places to check.
- Behind return air grills.
- In the return duct, it could be almost anywhere between the return air grill and the furnace. Check for openings in the duct or pieces of sheet metal that look like they are removable. These are sometimes hard to find.
- Unfortunately people sometimes think they don't need a furnace filter. If you still can't find yours maybe there isn't one. Most furnaces and air conditioners have an opening where a filter can be installed.