Guides · Equipment
How often to change a furnace filter (and how to pick the right one)
The furnace filter is the cheapest part in the house that protects the most expensive one. Check it monthly and change it when it looks grey. For most houses that is every one to three months for a 1 inch filter.
How often, by filter
| Filter | Change it | Notes |
|---|---|---|
| 1 inch fiberglass or pleated | Every 1 to 3 months | Monthly with pets, smokers, allergies or a dusty renovation. |
| 2 inch pleated | Every 3 months | Check monthly in heating and cooling season. |
| 4 to 5 inch media filter | Every 6 to 12 months | Look at it every 3 months; change when grey. |
| Washable electrostatic | Wash monthly | Let it dry fully before it goes back. |
ENERGY STAR: inspect the filter every month and change it at least every 3 months. A dirty filter makes the blower work harder and can ice the coil or overheat the furnace.
What makes it need changing sooner
- Pets, especially ones that shed.
- Anyone with allergies or asthma in the house.
- A renovation, sanding, or a dusty road nearby.
- The system running most of the day, in a hot summer or a cold winter.
- A higher MERV filter, which catches more and clogs faster.
How to read the size and the rating
The size is printed on the frame: 16x25x1 means 16 inches by 25 inches by 1 inch nominal. The actual filter is about half an inch smaller each way so it slides in. Buy the nominal size printed on the old one.
MERV is how fine the filter is, from 1 to 16 for homes. MERV 8 is the usual choice: it catches dust and pollen without choking the blower. MERV 11 to 13 catches smoke and finer particles; check that your furnace is rated for it, because a dense filter on a weak blower reduces airflow.
Where the filter is
- In a slot on the return air side of the furnace or air handler, usually where the big duct meets the unit.
- Behind a return grille on a wall or ceiling, in some houses.
- The arrow on the frame points in the direction the air flows: toward the furnace.
How to never forget it
Write the size and the date on the filter frame with a marker, and buy them in packs. Then put the date somewhere that reminds you. Housemap keeps the filter size on the furnace card in your house file, logs each change and emails you when the next one is due. See the home maintenance checklist for the rest of the monthly list.
Questions
How often should I change my furnace filter?
Check it every month. Change a 1 inch filter every 1 to 3 months, a 4 to 5 inch media filter every 6 to 12 months. Change sooner with pets, allergies, dust or heavy use.
What happens if I do not change the furnace filter?
Airflow drops. The furnace runs longer and hotter, the blower motor strains, the air conditioner coil can freeze, and dust gets into the ducts. Heating and cooling bills go up.
Which way does the furnace filter go in?
The arrow on the frame points in the direction of airflow, toward the furnace or air handler and away from the return duct.
Is a higher MERV filter better?
It filters more but restricts airflow more. MERV 8 suits most homes. Use MERV 11 to 13 only if the furnace maker allows it, and change it more often.
Sources
Related guides
Get a reminder before the filter is due.
Housemap keeps the size on the furnace card, logs each change and emails you when the next one comes around. Free.