Furnace Filter Guide: Sizes, MERV Ratings and How Often to Change
The furnace filter is the cheapest part of your HVAC system and one of the most important. We see the results of a forgotten filter on service calls in Brampton every week. Here’s everything you need to know.
Finding the right size
The size is printed on the edge of the filter you have now, something like 16x25x1. The last number is the thickness. If there’s no filter (it happens), measure the slot. A filter that’s too small lets air bypass it around the edges.
Which way does it go?
Look for the arrow on the frame. It points in the direction of airflow, toward the furnace blower.
MERV ratings in plain terms
- MERV 1–4: basic fibreglass. Protects the furnace, does little for air quality.
- MERV 8: good everyday pleated filter. Catches most household dust.
- MERV 11–13: better for allergies, pets and fine dust. In a 1-inch size, these can restrict airflow on some furnaces, so watch for longer run times or a noisy blower.
1-inch vs. 4- or 5-inch filters
Thicker media filters sit in a cabinet beside the furnace. More surface area means better filtration with less airflow resistance, and they last much longer. If you’re changing a 1-inch filter every month, a media cabinet is worth considering.
How often to change it
- 1-inch pleated: every 1 to 3 months. Monthly with pets, during renovations, or when the AC and furnace are running hard.
- 4- or 5-inch media: every 6 to 12 months, depending on the house.
- Washable filters: rinse and dry fully before reinstalling, usually monthly.
The easiest system: buy several at once and write the date on each one when you put it in.
What a clogged filter does
Overheating and high-limit trips on the furnace, frozen AC coils, higher bills, and a blower motor that wears out early. Our post on why ACs freeze up explains the AC side.
Want a media filter cabinet or better filtration? See our indoor air quality services or contact us at 647-657-2977. We serve Brampton and the GTA.
