Furnace air filter — check it sooner if the fan stays on

The months printed on a furnace air filter assume the fan runs only when heating or cooling. If your thermostat fan is set to On, the filter works all day and loads up faster.

Match furnace air filter checks to your fan setting

1. Look at the thermostat fan setting: Auto runs with heat or cool, On runs constantly.
2. If it is set to On, pull and look at the filter about halfway through its rated life.
3. Hold it up to a light. If light barely passes through, swap it now and note the date.

That is the beat. The fan setting changes the schedule, not the filter type.

More help

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