Every 1 to 3 months
Check the filter monthly and replace it when it looks gray and clogged, at a minimum every three months. Go monthly if you have pets, allergies, or a thin one-inch filter; thicker pleated filters in a low-use home can stretch closer to three.
A furnace filter is the cheapest, easiest piece of home maintenance there is, and it is also the one people forget until the system is straining. The filter's whole job is to keep dust out of the air and out of the equipment. When it clogs, air cannot move through it, so your furnace or AC works harder for the same result, your bill creeps up, and dust starts collecting on the parts that are expensive to replace.
None of that announces itself. There is no light, no chirp, no leak. That is what makes it a perfect example of the kind of quiet, recurring upkeep that is easy to know about and easy to let slide.
How to do it
- Turn the system off at the thermostat before you start.
- Find the filter slot, usually in the return air duct or the furnace cabinet.
- Read the size printed on the old filter's cardboard frame, such as 16x25x1, so you buy the right replacement.
- Slide the old filter out gently so you do not scatter the dust, and put it straight into a bag.
- Wipe any loose dust from the slot and the area around it.
- Slide the new filter in with the airflow arrow on the frame pointing toward the furnace and away from the return duct.
- Check that it sits flush with no gaps at the edges, since air that slips around the filter is not being filtered.
- Turn the system back on and note the date on the filter frame or your calendar.
Common mistakes
- Installing the filter backward. The airflow arrow has to point toward the blower, or you strain the system.
- Buying the highest MERV rating you can find. Confirm your system can handle it; a too-dense filter restricts airflow. Ask an HVAC tech if unsure.
- Waiting the full three months out of habit. With pets or heavy use a filter can clog in under a month, so check monthly.
This is exactly the kind of thing that is easy to know and easy to forget. Adulting can put it on a cadence so it surfaces when it is next due, instead of when something goes wrong. Free to start.
Sources
Questions people ask
- What happens if I never change it?
- A clogged filter chokes airflow, so the system runs longer and costs more to operate, and dust builds up inside the equipment and shortens its life.
- Do more expensive filters last longer?
- Not necessarily. Thicker pleated filters can go longer than thin flat ones, but a high MERV rating your system cannot handle restricts airflow. Match the filter to your equipment and check it monthly regardless.