How to Change Your HVAC or Furnace Filter
Updated September 18, 2026
Can I do this myself?
Safe to DIY
Yes, changing an HVAC filter is one of the safest and most useful home maintenance jobs. Just turn the system off first and make sure any access panel goes back on properly.
The air filter in a furnace, air handler or central AC system traps dust, lint, pet hair and other particles before they reach the blower and coils. A clogged filter chokes airflow, which makes rooms heat or cool unevenly, raises energy use, and can cause a furnace to overheat or an AC coil to freeze.
Changing it takes a few minutes and no special tools. The keys are finding where the filter lives, buying the right size, putting it in facing the right way, and choosing a filter that suits your system. Many people assume the highest-rated filter is best, but a very dense filter can restrict airflow on systems not designed for it.
This guide is for ducted systems with a disposable or washable panel filter. If you have a ductless split AC, its filters are the washable mesh screens behind the indoor unit’s front panel, covered in our AC guide.
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Signs of the problem
- The filter looks grey, matted or covered in dust
- Weak airflow from the vents
- Rooms heat or cool unevenly, or the system runs longer than usual
- More dust on furniture than normal
- The furnace shuts off early or the AC coil freezes
Common causes
Filter past its change interval
Most commonDisposable filters fill up over time. Common intervals range from about one month for thin filters to several months for thicker media filters, but the right schedule depends on your home and the filter’s instructions.
Heavy dust load
Most commonPets, renovation work, smoking, dusty climates and running the system constantly in extreme weather all fill filters faster than the label suggests.
Wrong filter size or type
SometimesA filter that is too small lets air bypass it around the edges, carrying dust into the coil. A filter that is too restrictive for the system reduces airflow even when it’s new.
Safety first: before you start
- Critical:Turn the system off at the thermostat, and switch off the furnace or air handler service switch, before removing the filter.
- Wear a dust mask if you have allergies. Old filters release dust when handled.
- Use a stable step ladder for ceiling-mounted return grilles, and don’t overreach.
- Never run the system without a filter. Dust will coat the coil and blower quickly.
- Make sure any blower or filter access door is fully closed afterwards, or the furnace safety switch may keep the system off.
Tools & materials
Tools
- Flashlight
- Screwdriver (for some grilles)
- Step ladder (for ceiling returns)
- Marker
Materials
- Replacement filter of the exact size
- Rubbish bag for the old filter
- Dust mask (optional)
Step-by-step fix
- 1
Find the filter
Common locations are a slot between the return duct and the furnace or air handler, a door on the front of the unit, or behind a large return-air grille on a wall or ceiling. Some homes have several return grilles, each with its own filter. The owner’s manual will show where yours goes.
Tip: Follow the largest duct into the unit. The filter is usually on the return side, where air enters before the blower.
- 2
Turn the system off
Set the thermostat to Off, and flip the service switch on or near the furnace or air handler to off. This stops the blower pulling dust in while the filter is out and keeps fingers away from moving parts.
- 3
Remove the old filter and read the size
Slide the filter out carefully, keeping the dirty side facing up so dust doesn’t fall. The size is printed on the frame, such as 16 x 25 x 1 in inches or its metric equivalent. Note the nominal size (what you buy) and the thickness. Buy the same size.
- 4
Choose the right rating
Filters are rated for how fine a particle they catch, commonly with MERV numbers in North America or ISO 16890 or EN 779 classes elsewhere. Mid-range ratings suit most homes. Higher-rated, denser filters catch more but can restrict airflow on some systems, so check your system’s manual or ask a technician before going high.
Warning: A filter that is too restrictive can cause the furnace to overheat or the AC coil to freeze, the very problems filters are meant to prevent.
- 5
Check the airflow arrow
Every filter has an arrow on the frame. It must point in the direction the air flows: toward the furnace or air handler and away from the return duct. For a wall or ceiling return grille, the arrow points into the wall or ceiling, toward the duct.
- 6
Insert the new filter
Slide it in fully so it sits square with no gaps around the edges. Replace the slot cover or close the grille, and refit any screws. Write the date on the frame edge with a marker so you know when it was changed.
- 7
Turn the system back on
Close any access doors firmly, turn the service switch back on, and set the thermostat. Check that air flows normally from the vents after a few minutes.
- 8
Set a reminder
Put a reminder in your phone or calendar based on the filter’s recommended interval. Check it monthly at first, especially in peak season or with pets, and adjust your schedule to how dirty it gets.
Stuck on a step?
Point your camera at it and a Fixwise expert voice will guide you through it, safely.
How to check it's fixed
- The filter sits snugly with the arrow pointing toward the unit
- Air flows strongly from the vents
- The system starts and runs normally, with all access doors closed
When to stop and call a professional
- You can’t find the filter, or the unit doesn’t appear to have one
- Dust is caked on the blower or coil, suggesting it ran without a filter
- Airflow is still weak with a new filter (possible duct, blower or coil problem)
- The furnace or AC won’t restart after the filter change even with the doors closed
How to prevent it
- Check the filter monthly and change it when it looks grey or on the label schedule.
- Keep spare filters of the right size at home.
- Change filters more often during renovation work, with pets, or in dusty seasons.
- Keep return grilles clear of furniture and curtains.
Frequently asked questions
Which way does the furnace filter arrow go?
The arrow shows airflow direction, so it points toward the furnace or air handler and away from the return duct. On a wall or ceiling return grille, the arrow points into the grille, toward the duct behind it.
How often should I change my HVAC filter?
Follow the filter’s label, often every 1 to 3 months for common filters. Check monthly and change it sooner if it looks grey, if you have pets, or if the system runs a lot in very hot or cold weather.
Is a higher MERV filter always better?
Not always. Higher-rated filters capture finer particles but can reduce airflow on systems not designed for them. Check your system’s manual or ask a technician for the highest rating it can handle comfortably.
Can I run my furnace or AC without a filter?
Only for a very short time in an emergency. Without a filter, dust coats the blower and coil, reducing efficiency and potentially causing costly damage. Replace it as soon as possible.
Can I wash and reuse my filter?
Only if it’s sold as a washable or permanent filter. Disposable pleated or fibreglass filters must be replaced. Washable filters must dry completely before going back in.
This guide is general information, not professional advice. Homes differ, so follow your product manuals and local codes. If anything looks different from what's described, or you feel unsure, stop and call a qualified professional. In an emergency, leave the area and call your local emergency number.
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