Furnace Filters 101: Size, Timing and Which Way It Goes

Find the right size

You don't need to measure anything. The size is printed right on the edge of the filter that's in there now, something like 16x25x1 or 20x20x1. The first two numbers are the width and height, and the last one is the thickness in inches. Some homes have more than one filter, like one at the furnace and another behind a return grille in a hallway or ceiling, so check both places.

How often to change it

A standard 1-inch filter usually needs changing every one to three months. Change it closer to every month if you have pets, smokers, a dusty house or someone with allergies, and during the stretches when the heat runs a lot. Thicker media filters, like 4-inch or 5-inch ones, last longer, often several months. When in doubt, pull it out and hold it up to a light. If you can't see light through it, it's time.

• Pets, dust or allergies: check monthly
• Normal household: every two to three months
• Thick media filters: follow the label, and still check it every season

Which filter to buy, and which way it goes

It's tempting to buy the thickest, highest-rated filter on the shelf, but a 1-inch filter with a very high rating can choke the airflow on many home systems. That makes the furnace run hot and can trip its safety switches, which is the same short-cycling problem I wrote about this week. For most homes, a mid-range pleated filter is the sweet spot. Then look for the arrow printed on the edge: it shows which way the air flows, and it should point toward the furnace.