Furnace Filters Explained
The filter is the cheapest part of a heating system and the one most often neglected. In a long heating season it matters more than most people think.
Request an HVAC technicianFind the filter
Most filters slide into a slot beside or below the blower. Note the size printed on the frame before buying a new one.
Which way it goes
The arrow on the frame points in the direction of airflow, toward the furnace.
How often
Check monthly in winter. Basic filters often need changing every month or two; thicker media filters last longer.
Denser isn't always better
High-rated filters trap fine dust, but some furnaces can't push enough air through them. Ask a technician what your blower can handle.
Why it matters
A clogged filter makes the furnace overheat and shut off, freezes AC coils in summer and lets dust coat the blower.
Renovation dust
Drywall dust clogs filters fast. Change the filter often during work, or cover returns.
More help
See maintenance and tune-ups for a full check.
Write the date on it
Write the install date on the filter frame with a marker. It's an easy way to remember when it went in, especially in a long heating season.
Pets and dust
Homes with pets, recent renovations or gravel roads nearby load filters faster. Check them more often and keep spares on hand through the winter.
Summer counts too
If you have central air or a heat pump, the filter keeps working all summer. Don't forget it once the heating season ends.
Washable filters
Some homes use washable filters. They need a thorough rinse and must dry completely before going back in, or they can grow mould and restrict airflow.
More heating and cooling guides
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