How often should you replace your HVAC filter?

Why bother

A clogged filter strains the system and your bills. This is an essentials-tier job. Skip it and you're inviting the expensive kind of problem, or the dangerous kind.

Why it's a range

Pets, allergies and a cheap thin filter all shorten it; an empty house with a thick pleated one stretches it. Three months is the safe call, monthly if the dog sheds, up to six for the fat media filters. The 'change it monthly' on the box sells more filters.

What the job involves

1. Find the slot. It's either behind the return-air grille on a wall or ceiling, or in a slot on the furnace or air handler. Read the size off the cardboard edge of the old one before you bin it, so the new one actually fits.
2. Turn the system off and slide the old filter out. Check the airflow arrow stamped on the new filter's frame and point it toward the furnace or blower, the way the air travels. Backwards is a popular mistake.
3. Slide the new one in until it sits flush, close the cover, turn the system back on. That's the whole job.
4. Write the date on the filter edge or set a reminder, because in three months you will not remember doing this.

Do it yourself, or pay someone?

Yourself. A filter is $10 to $50 and the swap takes about two minutes with no tools. A pro charges around $75 to $150 in labor for the same two minutes, so only fold it into a visit if a technician's already there for a tune-up, or the slot sits somewhere you'd need a ladder and a long arm to reach.

What skipping it costs

$600 to $2,500 is the bill at the bad end. A clogged filter starves the system of air, so dirt slips past and cakes onto the cooling coil, which stops pulling heat and, on an AC, can freeze solid and drag the compressor down with it. A fouled coil runs from about $600 to replace, a burned-out compressor north of $2,000. Until then you're just paying more to run the thing: the Department of Energy puts a clean filter at 5 to 15 percent less energy than a clogged one. A $20 filter is cheap insurance against the lot.

Signs it's overdue

• Held up to a light, the filter looks gray and you can't see through the pleats, or there's a felt of dust and pet hair matted on the intake side.
• Heating or cooling bills creeping up, weak air from the vents, and rooms that won't reach temperature.
• On an AC, ice or frost forming on the coil or the refrigerant line, or the unit short-cycling and struggling to keep up in summer. That's a starved system telling you.

When to start thinking about it

A few days' warning does it. Grumpty grumbles 3 days out, then leaves you alone. Mark it done and Grumpty sets the next one, every 3 months, so you can go back to forgetting it exists.

Questions people ask