Furnace filters: which one, how often, and why it matters

It's the cheapest part of your heating system and the one most often forgotten. A plain guide to filter sizes, MERV ratings, and how often to change yours.

A clogged filter makes your furnace work harder, heat less evenly and wear out sooner. It's also one of the few heating jobs nearly anyone can do in five minutes, without tools.

Find your size

The size is printed on the edge of the filter you already have, as width × height × depth, for example 16 × 25 × 1. Filters are labelled with a rounded "nominal" size, so buy by the label rather than a tape measure. The filter usually sits in a slot where the return duct meets the furnace, or behind a grille on a wall or ceiling.

Choosing a MERV rating

MERV (minimum efficiency reporting value) describes how much fine dust a filter catches. Higher catches more, but also restricts airflow more.

• MERV 8: a solid default for most homes.
• MERV 11 to 13: better for allergies, pets or wildfire smoke, if your system is designed for it.
• Above 13: usually meant for commercial systems; ask an HVAC technician first.

If you're not sure what your system can take, check the furnace manual or ask at your next tune-up.

How often to change it

For standard 1-inch filters, every one to three months is the usual guidance. Change it more often with pets, allergies, renovation dust or a busy household. Thicker 4- to 5-inch media filters can last six months or more. The filter maker's guidance on the package comes first.

How to swap it

1. Turn the system off at the thermostat.
2. Slide out the old filter and note the arrow showing the airflow direction.
3. Put the new one in with the arrow pointing toward the furnace, away from the return duct.
4. Close the panel, turn the system back on, and write the date on the filter's edge.

Signs it's overdue

A filter that's grey and caked, rooms that heat unevenly, more dust than usual, or a system that runs longer than it used to. If a fresh filter doesn't help, book a technician. Send Handyman a photo of the filter's edge and it remembers the size and reminds you when the next one is due. Try it free.