Changing the furnace filter

A furnace filter isn't really there to clean your air. It's there to keep dust off the blower and the coil. When it clogs, the system can't pull enough air through it — the blower works harder, the air conditioning coil can ice over in summer, and the heat exchanger runs hotter than it was designed to in winter. A $12 filter protects a $6,000 appliance.

How often

A 1-inch pleated filter: every 90 days as a baseline. Monthly if you have shedding pets, allergies, construction nearby, or you're running the system hard. A thick 4- or 5-inch media filter in a cabinet: every 6 to 12 months.

Don't go by the calendar alone the first year. Pull it at 30 days and look. That tells you what your house actually does, and then you set the interval from there.

How to do it

1. 1Switch the system off at the thermostat, so the blower isn't pulling while the slot is open.
2. 2Find the filter. It sits either in a slot on the furnace itself, right where the big return duct meets the blower cabinet, or behind a return air grille on a wall or ceiling.
3. 3Before you pull it, read the size off the edge of the old filter's cardboard frame — it'll say something like 16x25x1. Write it down. That number, and not the brand, is what you buy.
4. 4Note which way the arrow on the frame points. It always points in the direction the air travels, which is toward the furnace and away from the return duct.
5. 5Slide the old one out. If it's filthy, bag it at the furnace rather than carrying it through the house.
6. 6Slide the new one in with the arrow pointing the same way the old one did.
7. 7Write the date on the edge of the frame with a marker. In three months you won't remember, and the filter will look cleaner than it is.
8. 8Close the slot cover or grille and switch the system back on.

Warning: two things that cause real damage

Warning: Never run the system with no filter in the slot, not even for a day. Everything the filter would have caught lands on the blower wheel and the coil instead, and cleaning a coil is a service call.

Warning: A very high MERV rating isn't an upgrade if your system wasn't built for it. MERV 8 to 11 suits most homes. Going higher restricts airflow, and a starved blower is the same problem as a dirty filter.

What the old filter tells you

If a 90-day filter is grey and matted at 30 days, something is feeding it — unsealed return ducts pulling from an attic or crawl space is the usual culprit. If it's filthy on one half and clean on the other, air is bypassing the frame, so the filter is the wrong size or seated badly.