The furnace filter is the cheapest part of a heating and cooling system and the one homeowners can look after themselves. It protects the furnace blower and the air conditioner coil from dust, and it cleans the air the house breathes. A clogged one makes the furnace work harder and run hotter; the wrong one can choke airflow. Here is how often to change it, and which kind to buy.
The short answer
- 1-inch pleated filter: every 1 to 3 months.
- 1-inch fibreglass filter: every month. These protect the furnace but catch little dust.
- 4- or 5-inch media filter: every 6 to 12 months.
- Washable or electrostatic filter: wash monthly, and replace when it no longer comes clean.
- HEPA bypass unit: follow the manufacturer, usually yearly for the main filter.
Change it more often if you have pets, if anyone smokes indoors, during and after renovations, in a new house with construction dust, or if the furnace fan runs all the time. Check it monthly until you know how fast it loads up in your house.
How to tell when it needs changing
Pull it out and hold it up to a light. If light barely passes through, or the pleats are grey and caked, change it. Do not go by colour alone: a filter can look only lightly grey and still be restricting airflow if it has trapped fine dust deep in the media. Marking the date on the frame with a pen saves guesswork.
Where the filter is
Most filters slide into a slot between the return duct and the furnace, often behind a small door or a removable panel near the bottom of the furnace. Some furnaces have the filter inside the blower compartment. Media filters live in their own cabinet attached to the side or bottom of the furnace. The airflow arrow on the filter frame should point toward the furnace, in the direction the air flows.
Sizes
The size is printed on the frame of the current filter, such as 16 x 25 x 1. The first two numbers are the face size in inches and the third is the depth. The real size is often a little smaller than the nominal one, which is normal. Buy the nominal size printed on the old filter.
MERV ratings explained
MERV, the Minimum Efficiency Reporting Value, rates how well a filter catches particles, on a scale from 1 to 16 for residential filters.
- MERV 1–4: basic fibreglass. Protects the equipment, does little for air quality.
- MERV 6–8: standard pleated filters. A good default for most homes.
- MERV 11–13: catches finer particles, including much of the pollen, pet dander and smoke. Good for allergies, as long as the system can handle it.
- MERV 14 and up: close to hospital grade. Usually too restrictive for a 1-inch slot in a home furnace.
Some retail brands use their own scales instead of MERV. Look for the MERV equivalent on the package.
Why a high-MERV 1-inch filter can be a problem
A thin filter with very fine media has a small surface area and resists airflow, especially as it loads up. Many furnaces, particularly older ones with undersized return ducts, struggle to pull enough air through it. Low airflow makes the heat exchanger run hotter, can trip the high-limit switch and shut the furnace off, and in summer can freeze the air conditioner coil. If you want better filtration, a 4- or 5-inch media filter cabinet gives you MERV 11 to 13 filtration with much less restriction, and lasts far longer.
Common mistakes
- Installing the filter backwards, against the airflow arrow.
- Leaving gaps around the filter, so air bypasses it entirely.
- Running the furnace without a filter "for a day". Dust goes straight onto the blower and coil.
- Buying the highest MERV available for a 1-inch slot on an older furnace.
- Forgetting the filter during renovations, when it loads up in days.
New houses and renovations
A newly built home, common in Binbrook, Waterdown, Caledonia and Milton, often has drywall dust, sawdust and debris in the ducts for months after move-in. Change the filter every few weeks for the first season. The same goes during any renovation: seal off return grilles in the work area, run the fan as little as possible, and replace the filter as soon as the dusty work is done. A filter that has loaded up with fine drywall dust can restrict airflow badly while still looking only lightly grey.
Buying and storing filters
Buying a year’s worth of filters at once saves trips and makes it easier to change on time. Store them flat and dry so the frames do not warp. Reusable washable filters save money over time but need to be washed and dried completely before going back in, and they rarely filter as well as a good pleated filter.
Filters and heat pumps or AC
Air conditioners and heat pumps depend on airflow across the indoor coil even more than a furnace does. A clogged filter reduces cooling, can ice up the coil and makes the compressor work harder. If you have a heat pump, the filter is working twelve months a year, so check it more often.
Beyond the filter
If the house is still dusty with a good filter changed on time, the cause may be leaky return ducts drawing in basement air, or debris already in the ducts after a renovation. Duct cleaning helps in that case, and air quality upgrades such as a media filter cabinet or an HRV can make a lasting difference.
Want it checked?
A yearly furnace tune-up includes checking the filter, the blower and the airflow through the system. Send one request and a licensed contractor in your area will book a visit.
Questions people ask
How often should I change a 1-inch furnace filter?
Every one to three months for a pleated filter, monthly for fibreglass. Check it monthly at first to learn how quickly it loads up in your home.
What MERV rating should I use?
MERV 6 to 8 suits most furnaces with a 1-inch slot. For better filtration, MERV 11 to 13 in a 4- or 5-inch media cabinet avoids restricting airflow.
Can a dirty filter stop my furnace?
Yes. Restricted airflow makes the furnace overheat, and a safety switch shuts the burner off. Changing the filter often brings it back.
Which way does a furnace filter go?
The arrow on the frame points toward the furnace, in the direction the air flows through it.