Heating & cooling

Change your HVAC air filter

The air filter is one of the easiest parts of your heating and cooling system to look after, and one of the easiest to forget. When it clogs, the system has to work harder, which can mean more dust, higher bills and extra wear. Once you know where it is and which way it faces, changing it takes just a few minutes. It's also the first thing an HVAC technician checks on a lot of service calls, so it's worth ruling out yourself.

Hands sliding a clean pleated air filter into the filter slot of a furnace, with the airflow arrow pointing toward the furnace and an old dirty filter behind
The arrow on the filter frame points toward the furnace.
Quick answerFind the slot by the furnace or the big return grille, match the size printed on the frame, and put the new one in with the arrow pointing toward the furnace.
Worth checking firstPull the old filter out and read the size printed on the frame before you buy, not the size you think it is. A 16x25x1 and a 16x25x4 look similar on a shelf and won't swap. Note the arrow direction while it's out, too.

Good to knowFilter scheduleQuick fixRenter-friendlyGood first repairNo toolbox neededSummer checklist

Mike TurnerWritten and reviewed byMike TurnerUpdated Sources and safety notes
Easy5 min$8–30
Diagram of which way an HVAC air filter goes, with the frame arrow pointing toward the furnace
Which way does an air filter go? The arrow on the frame (2) always points toward the furnace or air handler (3), in the direction air flows from the return grille (1).

Why a dirty filter matters

Every bit of air your furnace or AC moves goes through that filter first. As it clogs, the blower has to pull harder to get the same air through, which means longer run times and more wear. A badly clogged filter can even make an AC coil freeze or a furnace overheat and shut itself off.

Filter mistakes to avoid

The filter is clogged

Dust, pet hair and pollen build up until the blower is straining to pull air through it.

It's the wrong size

A filter that's even half an inch small lets air sneak around the edges, dust and all.

The MERV is too high for your system

Very fine filters catch more, but some older systems can't pull enough air through them.

It's been installed backwards

Filters are built to catch dust from one side. The arrow on the frame points toward the furnace.

How often should you change it?
Your home1-inch filter4-inch filter
One or two people, no petsEvery 2 to 3 monthsEvery 6 to 12 months
Pets or allergiesEvery month or twoEvery 3 to 6 months
Construction or a very dusty seasonCheck monthlyCheck every 2 months
Vacation home, lightly usedEvery 6 monthsOnce a year
Turn the system off firstTurn the system off at the thermostat before pulling the filter, so the fan doesn't suck loose dust straight in.

Swap the filter

0 of 5 done
  1. Find the filterIt's usually in a slot beside the furnace, or behind a big return-air grille in a wall, ceiling or hallway.
    Furnace with a filter slot on its side and a ceiling return grille, the two usual filter spots
  2. Read the sizeIt's printed on the frame, something like 16x25x1. Snap a photo so you never have to pull it out at the store again.
    Close-up of an air filter frame showing its printed size 16x25x1
  3. Pick a ratingMERV 8 is right for most homes. MERV 11 to 13 catches more pollen and pet dander, but on older systems these denser filters can restrict airflow, so check your manual.
    MERV rating scale from 1 to 16 with MERV 8 highlighted as right for most homes
  4. Swap it, arrow toward the unitSlide the old one out, slide the new one in with the arrow pointing toward the furnace, and close the cover.
    Something went wrong?
    • The filter doesn't fit Double-check the size on the old frame and the slot. The printed size is rounded; the actual filter is often a little smaller.
    New air filter sliding into its slot with the frame arrow pointing toward the furnace
  5. Write the date on the frameChange 1-inch filters every 1 to 3 months, or as the filter's packaging and your system's manual suggest. Check monthly if you have pets, allergies, or dusty construction nearby.
    Marker writing today's date on the edge of a new air filter frame

Watch it done

How to Change a Furnace Filter the RIGHT WayBy Silver Cymbal on YouTube. Their method may vary a little from ours.
Watch on YouTube
How you'll know it's fixed
  • The new filter fits the slot snugly with the arrow pointing toward the furnace
  • The cover is closed
  • The date is written on the frame
  • Air flows steadily from the vents

Airflow still weak? Check for blocked vents and grilles.

If airflow is still weak

If airflow is still weak with a new filter, check that supply vents and return grilles aren't blocked by furniture or rugs. If the furnace whistles, rattles or squeals even with a fresh filter, see what furnace noises mean.

Ice on the refrigerant lines or indoor coil, even after a filter change, means it's time to call a technician.

Make it a habitBuy filters in a multi-pack and set a phone reminder. The pack price is usually far lower, and the reminder is what actually makes it happen.
Renting?If you rent, check your lease. Some landlords change filters themselves; others expect tenants to.
Call an HVAC tech ifA burning smell, ice on the indoor coil or refrigerant lines, or weak airflow that doesn't improve with a fresh filter.Fix it yourself or call? Compare the cost →

What a pro typically charges (a rough estimate; prices vary by area): Filters cost $8–$30. An HVAC tune-up, which includes a filter check, typically runs $90–$200.

Find an HVAC technician near you

Other heating and cooling problems

Air filter questions

What happens if I never change my air filter?

Your system works harder, energy bills rise, dust increases, and a badly clogged filter can cause the coil to freeze or the furnace to overheat.

Does a higher MERV rating always mean better?

Not always. Higher MERV filters catch more, but they restrict airflow. Many older systems work best with MERV 8.

Where is my HVAC filter?

Check beside the furnace or air handler, in a slot on the return duct, or behind a large return-air grille in a hallway, wall or ceiling.

Can I wash and reuse my HVAC filter?

Only if it's sold as a washable filter. Standard pleated and fiberglass filters are disposable; washing them ruins them.

Which way does the air filter go?

Put it in with the arrow on the frame pointing toward the furnace.

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About the authors

GrabTheHammer Team

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  • Safety first
  • Tells you when to call a pro

We're a small team of writers and editors who check every guide against manufacturer instructions and common code requirements. Our goal is to help you understand the problem, tackle what you safely can and know when to call a pro.

Checked against: Manufacturer instructions for the parts and products involved, ENERGY STAR.

How we write and review →
A helpful next stepFurnace turns on and off too oftenChange the filter, open the vents, check the thermostat, and clean the flame sensor on a gas furnace.