How often should I change my furnace filter? 1-inch vs 4-inch

There is no single number because the filter is catching whatever is in your house air. A two-person condo with the furnace fan on AUTO loads a filter slowly. A Newmarket house with a dog, three kids and a basement being drywalled can fill a 1-inch filter in three weeks. The calendar is a reminder to look; the filter itself tells you when to change it. Natural Resources Canada puts it the same way: change or clean filters about every three months, or as they become loaded.

How often should I change a 1-inch furnace filter?

Every 1 to 3 months, checked monthly from October to April. The thin 1-inch pleated filters most GTA homes have sit in a slot beside the furnace, usually between the return duct and the blower. They have little surface area, so they load up quickly and start to restrict airflow long before they look completely black.

• Every month: homes with pets that shed, smokers, an active renovation, or the fan set to ON all the time.
• Every 2 months: a typical family home through the heating season.
• Every 3 months: a small, low-dust home with the fan on AUTO.
• Right after: drywall sanding, flooring removal or any dusty job, even if the filter is new. Fine dust loads a filter in days.

How often should I change a 4 or 5-inch media filter?

Every 6 to 12 months for most homes. A thick media filter sits in a metal cabinet between the return duct and the furnace, with a door you slide it out of. The deep pleats give it many times the surface area of a 1-inch filter, so it lasts longer at the same airflow. Aprilaire, a common brand in Ontario homes, rates its MERV 10, 11 and 13 media filters for about a year and suggests checking at six months if the fan runs constantly. In a home with pets or construction dust, plan on six months, not twelve.

How do I tell if my furnace filter is dirty?

Pull it out and hold it up to a light. If you cannot see light through the media, or the intake side is grey with a layer of dust in the pleats, change it. Other signs the filter is too loaded:

• Weaker airflow at the supply registers, especially in rooms far from the furnace.
• More dust settling on furniture than usual.
• The filter is bowed or sucked inward toward the blower.
• A whistle at the filter slot when the fan runs.
• The furnace heats for a few minutes, then blows cool air or shuts off. A clogged filter can make the heat exchanger overheat, and the high-limit switch shuts the burners off to protect it.

Which way does the furnace filter go?

The arrow on the frame points toward the furnace, in the direction the air flows. Air comes from the return duct, through the filter, and into the blower. In a standard upflow furnace in a basement, that usually means the arrow points up or toward the furnace cabinet. The size, such as 16x25x1, is printed on the frame of the old filter; buy the same size. A filter that is too small lets dirty air bypass it and coat the blower and the air conditioning coil.

Is a MERV 13 filter bad for my furnace?

Not by itself, but a 1-inch MERV 13 filter can restrict airflow more than some furnaces are designed for. MERV is the rating from the ASHRAE 52.2 test standard: the higher the number, the smaller the particles it catches, and the more resistance it can add to the airflow. In a 4 or 5-inch cabinet, MERV 13 is usually fine because the deep pleats spread the air over a much larger area. In a 1-inch slot, MERV 8 to 11 is the safer choice for most older furnaces.

If you want MERV 13 for allergies or wildfire smoke and your furnace has only a 1-inch slot, watch for the warning signs above, change the filter more often, or have a technician measure the static pressure across the system. The furnace's rating plate and installation manual list the maximum external static pressure it is built for, and the technician can tell you whether a higher MERV filter, or a media cabinet, fits within it.

What happens if I never change the furnace filter?

Less air moves through the system, so the furnace runs longer and works harder to heat the house. The heat exchanger runs hotter, the high-limit switch can start shutting the burners off, and the blower motor strains. Dust that gets past a collapsed or missing filter coats the blower wheel and the air conditioning coil above the furnace, which then cools poorly next summer. A clogged filter is one of the first things a technician checks when a furnace short-cycles or blows cold air.

Should the furnace filter be changed when the furnace is serviced?

Ask about it when you book. A yearly maintenance visit in early fall is the right time to start the season with a clean filter, and to have the technician check that the filter you use suits the furnace. On Doyo, Furnace Maintenance is $206.50 all-in, done by a technician whose gas certificate Doyo checks before they are offered heating work. Between visits, the filter is yours to change.