What fills your air filter

Your filter only catches what flows through it. So the question is how much air moves through it each month, and what's in that air. Those two things set when it's full.

Heating and AC: how long the fan runs

Air only moves through the filter while your system's fan is running, and the fan runs mostly when you're heating or cooling. A typical 3-ton system moves about 1,200 cubic feet of air a minute. In a 2,000 square foot home, that's all of the air in the house passing through the filter four or five times an hour.

So run time is the biggest driver. A month where the system runs twice as many hours pushes about twice as much air through the filter, and catches about twice as much dust. To estimate run time for your home, we use last year's daily temperatures where you live: the further a day is from 65°F, the longer the system runs. Energy companies estimate heating and cooling use the same way. Cooling days count a little more, because most systems run the fan faster for AC than for heat.

Pollen: it comes in with you

Pollen grows outside, but it gets in through open doors and windows, on clothes, shoes, hair and pets, and through small gaps around windows and ducts. Once it's inside, it's pulled into your return vent with the rest of the air. That's why indoor pollen rises and falls with the outdoor season. In pollen months we add a bit more to what the filter collects, and more if someone at home has allergies, so the filter stays fresher when it matters most.

Wildfire smoke

Smoke particles are small enough to seep in even with the windows shut. A MERV 13 filter catches far more of them, so it also fills faster. In states with a wildfire season, we move swaps up from July to October.

Pets

Dander and hair add to what's in the air all year. One pet speeds things up by about a fifth, two or more by about a third.

Everyday dust

Skin flakes, fabric fibers, cooking and dirt tracked in from outside. Every home makes it, every month, so it's the steady base under everything else.

When it's time to swap

As a filter fills, it catches less and slows your air, which makes your system work harder. The rule of thumb is to swap every 3 months (about every 10 weeks for MERV 13), which assumes every month is average. We add up what your home actually puts into the filter, month by month, and schedule the swap when it has collected what an average filter takes in over those 3 months. That's why a busy winter or summer brings swaps closer together, and a mild spring spreads them out.