4-inch media filters
A 4-inch media filter is a thick filter, about 4 inches deep, that sits in a cabinet beside the furnace. Change it every 3 months.
Find the right fit
- Model number. The furnace model label or the filter cabinet label lists the size the cabinet takes.
- Size or part number. The size printed on the old frame, like 20x25x4, is what you match. The last number is how thick it is. Some cabinets take a filter closer to 5 inches thick: if the old one says 5, get a 5.
- Two or more retailers. Look the model or part number up at two or more retailers. Buy the one listed as a fit for your model, and check the printed size or part number on the box matches.
What to look for
- Thick filters carry the same MERV rating, the dust-catching score. Match what came out unless your furnace manual says otherwise.
- A frame that fits snugly, with no gaps around the edges.
- The airflow arrow points toward the furnace.
- Keep one spare on the shelf.
When to swap
Every 3 months, or as often as the label on the filter says.
Where to buy
Set a reminder for the next one in the Reminder Tool or the home hunt.