How often should you change your HVAC filter?

Short answer

Change the HVAC filter every 1 to 3 months. A thin filter in a home with pets or allergies needs the monthly end of that range. A thick media filter, the 4 or 5 inch kind that sits in its own cabinet, lasts every 6 to 12 months. It's a 5 to 10 minute job, and the filter costs $5 to $50 depending on its size and thickness.

Why it matters

The filter is the only thing standing between the dust in your house and the inside of your furnace or air handler. As it fills, air has a harder time getting through. The blower runs longer to move the same air, your energy bill creeps up, and the rooms farthest from the unit stop getting enough heat or cooling.

A badly clogged filter does worse than that. In cooling season, too little air across the indoor coil lets it get so cold that it freezes solid, and the system stops cooling until the ice melts. In heating season, a furnace starved of air overheats and shuts itself off on its safety switch. Either way you're calling for service on a problem a fresh filter would have prevented. Over years, the strain shortens the life of a system that costs $5,000 to $12,000 to replace.

When to do it more often

Pets or allergies in the home: go to every month. Fur and dander load a filter fast, and an allergy sufferer gets the most out of a clean one.
A 4 or 5 inch media filter: the other direction. Its pleats hold far more, so every 6 to 12 months is plenty.
A vacation home, or a system that rarely runs: every 6 months. A filter only loads when air moves through it.
Construction dust, a smoky summer, or a new puppy: check it monthly until things settle down.

What you need

A replacement filter in the size printed on the edge of the old one, including the thickness

Steps at a glance

Turn the system off at the thermostat so the blower doesn't start mid-change.
Find the filter slot: in the return grille on a wall or ceiling, or in the cabinet beside the furnace or air handler.
Slide the old filter out and note which way the airflow arrow points.
Slide the new filter in with its arrow pointing the same way, toward the unit.
Close the cover and turn the system back on.
Write the date on the edge of the new filter.

You're done when the arrow points into the unit and the cover sits flush with no gaps around the filter.

Signs it's overdue

The filter looks gray or clogged: hold it up to a light. If you can't see light through it, it's done.
Weak airflow from the vents, especially in the rooms farthest from the unit.
Dust builds up faster than usual on furniture, because dirty air is slipping around a loaded filter.
Higher energy bills with no change in the weather or the thermostat setting.

DIY or pro

This is the easiest job on the whole maintenance list. There's nothing to unscrew and nothing to adjust, and the hardest part is remembering to do it. Most people never pay anyone for it.

A technician will change the filter during a tune-up, and a service call just to change a filter runs $75 to $150, which buys a lot of filters. Call someone only if you can't find the filter slot at all, which happens in some older homes where the return is in an odd place, or if the slot is in a crawl space or attic you'd rather not enter.

FAQ

Is it better to change the filter every month?

For a thin filter in a busy home, yes. For an empty house or a thick media filter, monthly changes throw away filters with plenty of life left. Check it monthly and change it when it looks gray.

What happens if you don't change the HVAC filter?

Airflow drops, bills rise, and in the worst case the indoor coil freezes or the furnace overheats and shuts down. Over time the extra strain wears out the blower.

Can I wash and reuse a disposable filter?

No. Water ruins the pleated material, and a damp filter grows mold. Only filters sold as washable are made for it.

Which way does the arrow go?

Toward the furnace or air handler, in the direction the air travels. On a return grille, that means pointing into the wall or ceiling.

Does a thicker filter really last longer?

Yes. A 4 or 5 inch media filter has many times the surface area of a thin one, which is why it can go every 6 to 12 months while a thin filter needs changing every 1 to 3 months.