How often should I change my HVAC filter?

Most homes need a new one-inch HVAC filter every 90 days. Pets, allergies, or year-round system use shorten that to 30 to 60 days. Thicker 4-inch or 5-inch media filters last 6 to 12 months. The simplest check: pull the filter, hold it up to a light, and look through it.

How do I know when my HVAC filter needs changing?

The 90-day mark is the calendar answer. The eyeball answer is simpler. Pull the filter, hold it up to a light, and look through it. A clean filter shows light clearly. A spent one looks like a grey felt sheet.

Past the look, the house tells you. The system runs longer to hit the same temperature. Air comes out of the vents weaker than usual. Dust builds up on returns and registers faster than it used to.

Pets and allergies push the schedule. A house with two dogs and a cat will load up a one-inch filter in about a month. Same goes for any house running the HVAC most of the year (most of the South, summer in the Midwest, year-round in Phoenix).

If the old filter has a date written on it in pencil, that’s the previous owner or the last contractor leaving you a note. Trust it as a starting point and check yourself in a month.

What size HVAC filter do I need?

The size is printed on the side of the existing filter. It looks like 16×25×1 or 20×20×1. That’s width by length by thickness, in inches. Write it down before you go to the hardware store.

If the filter is missing, the slot itself has the dimensions stamped or stickered inside. Check the access door at the air handler, or the grille on a return-air vent.

Standard one-inch filters fit most homes built since the 1990s. Older homes, or homes with a media-filter cabinet (a wider slot at the air handler), take 4-inch or 5-inch filters in sizes like 16×25×4 or 20×25×5. These are not interchangeable. A one-inch filter in a 4-inch slot will flap, bypass air, and let dust skip the filter entirely.

Buy two while you’re there. The two-and-a-spare rule: keep one in reserve so you never miss a change because you forgot to order.

What MERV rating should I use?

MERV is a number from 1 to 20 that measures how fine the filter is. Higher number, smaller particles caught. Higher number also means more resistance, which can stress a residential blower if the system wasn’t designed for it.

For most homes, MERV 8 to 11 is the right range. It catches dust, pollen, and pet dander without choking the system. MERV 13 is the upper end of what most residential units handle comfortably and is worth the step-up if anyone in the house has allergies or asthma. The EPA’s guidance on home air filters is the cleanest reference if you want to read more.

Past MERV 13, you’re into commercial territory. Don’t put a MERV 16 filter in a residential unit unless your HVAC was specifically spec’d for it. The blower will work harder, run hotter, and the filter itself will load up faster than the rating suggests.

If the existing filter doesn’t say MERV, check the box for the equivalent. Home Depot uses FPR. 3M Filtrete uses MPR. They’re rough analogs, not exact translations, but a FPR 7 or MPR 1000 lands you in the MERV 11 neighborhood.

How do I actually change an HVAC filter?

Three steps. The whole job runs under five minutes once you’ve done it once.

First, turn the system off at the thermostat. Not strictly required for a quick swap, but it keeps loose dust from getting pulled into the blower while the slot is open.

Second, find the filter. Two common locations: a return-air grille on a wall or ceiling (held in place by clips or hinges), or a slot at the air handler itself, in the basement, attic, or utility closet. Pull the old filter straight out.

Third, put the new one in with the arrow pointing toward the blower. Every filter has an airflow arrow printed on the cardboard frame. The arrow follows the air’s path: from the room, into the filter, into the system. At a return-air grille, the arrow points into the wall. At the air handler, the arrow points toward the unit.

Then turn the system back on. Done.

What happens if I skip a filter change?

Three things happen, in order.

Air quality drops first. The same air keeps recirculating through a saturated filter, and the dust load in the house climbs. Allergy sufferers feel it within a week or two.

Then the system works harder. Restricted airflow means the blower runs longer to move the same volume of air. ENERGY STAR calls regular filter changes one of the cheapest ways to keep an HVAC system running efficiently, and a clogged filter can drive up energy use 5 to 15 percent while the house takes longer to hit the set temperature.

Then components fail. A year of restricted airflow can crack the heat exchanger, ice the evaporator coil, or burn out the blower motor. A coil replacement runs $1,500 to $3,000. A blower motor runs $400 to $900. A new air handler is $2,500 and up. The filter that would have prevented all of that costs $20.

A neglected filter is the cheapest expensive mistake in homeownership.

A note from Kempt

Filters are exactly the kind of small, dated, easy-to-forget thing Kempt is built for. Kempt remembers the size that fits the slot, marks the next change on the calendar, and sends a plain reminder when the 90-day window is up, with the size printed in the message so you can buy the right one without hunting for it.