How to replace your furnace filter (and how often, in Canada)

The single most important furnace maintenance task — costs $10 and takes 5 minutes.

Last reviewed May 31, 2026 by the EveryDIY.ca editorial team

5 min $10–$30 CAD CAD pricing

Why it matters

A clogged filter chokes airflow, drops efficiency 5–15%, and causes the furnace to short-cycle. It's the leading cause of "furnace won't turn on" service calls.

How often

1" filters: every 1–3 months
4" or 5" media filters: every 6–12 months
Homes with pets or renovations: more often

Set a reminder: first of every month, glance at the filter. If you can't see light through it, replace.

How to replace

Power off the furnace at the wall switch.
Open the filter compartment — it's a slot on the return-air side, usually with a removable cover.
Note the airflow arrow on the old filter. The new one points the same way (toward the furnace).
Slide old out, slide new in.
Close the cover, flip the power back on.

Which filter to buy

| Use | MERV | What it catches | |---|---|---| | Basic | 8 | Dust, pollen | | Allergies | 11 | Pet dander, mold spores | | Wildfire smoke season | 13 | Fine particles |

Don't go above MERV 13 on a residential furnace — too restrictive, can damage the blower.

Common Canadian sizes: 16x25x1, 20x25x1, 16x20x1. Check the old filter — size printed on the side.

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Editorial note. Wear appropriate PPE. When in doubt — especially with electrical, gas, or structural work — hire a licensed Canadian tradesperson. See our safety policy.