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Furnace filter week: what the old one tells you about your air
Change the furnace filter about six weeks after the heat comes on, which in Northwest Indiana is the middle of November, and look at the old one: whatever is on it was in the air. The EPA's guide to air cleaners in the home says manufacturers typically recommend replacing a filter every sixty to ninety days, sooner if it comes out heavily soiled, and that a filter rated at least MERV 13, or as high as the system allows, catches the small particles a basic mesh filter misses. Then take the return vent cover off and wipe the inside of the grille, where a felt of dust builds that a cloth on the outside never reaches.
By the middle of November the furnace in a Valparaiso home has been running for about six weeks, pulling every room's air through one filter several times a day. The old filter is the most honest report on your indoor air you will get all year, and it goes in the bin without anyone reading it.
What the filter is telling you
Pet hair means the vacuuming is not keeping up with the dog. A thick grey felt means the home is dustier than it looks, usually from a return vent that has not been cleaned. A dark edge means candles or the fireplace. Fine white dust means somebody has been sanding drywall. All of it was in the air you breathe before the filter caught it, and the filter only catches a share.
How often, and which one
The EPA's Guide to Air Cleaners in the Home gives two pieces of advice that are worth repeating every November:
- Replace filters on the manufacturer's schedule, which is typically every sixty to ninety days, and more often if they come out heavily soiled
- When choosing one, pick a filter rated at least MERV 13, or as high a rating as the system's fan and filter slot can take; most furnaces can handle MERV 13 as long as the filter is changed frequently
The rating matters because the cheap blue mesh filter protects the furnace and not much else. Filters at MERV 13 and above have to show they remove at least half of the smallest particles tested, which is the dust that stays in the air and ends up on the furniture.
The return vent is the other half
The filter sits behind the return, and the return grille itself collects a felt of dust on the inside that a cloth on the outside never touches. Take the cover off, wash it in the sink or wipe both sides, vacuum the opening, and put it back. Then the floor vents, with the crevice tool. It is ten minutes and it does more for the dust on the furniture than another pass with the duster.
What I do on a November visit
I note the filter's date on the visit note, wipe the return cover inside and out, and vacuum the floor vents. Changing the filter itself is the owner's job unless they have asked me to keep a spare, and I will say when I see the old one has gone grey. House cleaning in winter here is partly air cleaning. Most of the dust that lands on the shelf came out of a vent.
Sources
- Guide to Air Cleaners in the Home, US Environmental Protection Agency
