How often to change your furnace filter

How often should you change a furnace filter?

There is no single schedule, because dust loads differ. A thin disposable filter in a small household may last two or three months. In a house with pets, smokers, renovation dust or a lot of carpet, it may clog within a month. Thick pleated or media filters hold far more dirt and often last half a year or longer, if the manufacturer says so.

The most reliable method is to look. Pull the filter out and hold it against a light. If you cannot see light through it, or it is visibly grey and furry, replace it. Many people check on the first of each month through the heating season and write the date on the frame of the new one. During long cold spells, when the furnace runs for much of the day, filters fill faster. If you run the furnace fan all year or use central air in summer, the filter works then too, so it may need changing in the cooling season.

What size filter does your furnace need?

Look at the old filter first. The size is printed on the cardboard frame as length by width by thickness, in inches, for example 16 x 25 x 1. Those are nominal sizes, so the real filter is usually slightly smaller than the label. Buy the same label, not a tape-measure size.

If the filter has been thrown away, check the slot in the furnace or return duct and the furnace manual. The filter may sit in a slot beside the furnace, in a grille in the wall or ceiling, or in a cabinet at the base of the furnace. Make sure the new filter fits snugly, with no gaps for air to slip around it. If the size is unusual, filter suppliers and HVAC contractors sell custom sizes.

How do you change the filter step by step?

1. Turn the thermostat to off, or switch off the furnace at its switch, so the blower does not run while the filter is out.
2. Open the filter slot or grille and slide out the old filter. Note the arrow on its frame.
3. Wipe away loose dust around the slot with a dry cloth or vacuum.
4. Slide in the new filter with the arrow pointing in the direction of airflow, toward the furnace and the blower.
5. Close the cover firmly, and restore power and the thermostat setting.
6. Write the date on the new filter frame or set a reminder for the next check.

Never run the furnace without a filter. Dust will coat the blower and the heat exchanger, and it shortens the life of both. Reusable washable filters exist, and they must be fully dry before they go back in.

Why does a dirty filter matter?

A clogged filter restricts airflow. The blower works harder, the furnace runs longer to heat the same rooms, and the heat exchanger can overheat and trip its safety limit, which shuts the burner off. Rooms heat unevenly, energy use can rise, and parts wear out sooner. Dirty filters are among the most common reasons for a furnace that is not heating, as covered in the furnace troubleshooting guide.

Filters also affect the air you breathe, since they catch dust, pollen and pet hair, though a furnace filter is not an air purifier. Denser filters catch smaller particles but resist airflow more, which is why the rating on the box matters: see the guide to MERV ratings before upgrading. A furnace that is not designed for a dense filter can suffer.

Where does the filter fit in yearly maintenance?

It is the simplest job on the list and the one most often forgotten. Put it in the same routine as the smoke alarm check and the autumn tune-up. The heating maintenance checklist shows how the filter fits in with other tasks, and the annual tune-up covers what a technician does beyond it. If you want to lower winter bills as well, the guide to lowering heating bills has other low-cost steps.

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