How often should you change your AC filter?

ENERGY STAR’s advice is short: inspect, clean or change the air filters once a month in your central air conditioner, furnace and heat pump. Here is why it matters, where to find the filter, and what to look for.

Why the filter matters

The filter catches dust before it reaches the blower and the indoor coil. When it clogs, less air gets through. ENERGY STAR puts the cost plainly: a dirty filter can increase energy costs and damage your equipment, leading to early failure. It also notes that airflow problems can reduce a system’s efficiency by up to 15 percent.

A clogged filter is also one of the first things to check when an AC stops cooling well. Starved of air, a system cannot do its job, and ice can form on the indoor coil. The AC not cooling checklist puts the filter third, after the thermostat and the breaker.

Monthly means check, not always change

ENERGY STAR’s wording is inspect, clean or change. Look at the filter every month. Some months it will be fine; replace or clean it when it looks grey and dusty. In a San Diego late summer, when the AC runs long hours, it may fill faster than in a cool month. Homes near busy roads, with pets or with building work going on may see it fill faster still.

Where to find the filter

• In a return grille. A large grille on a wall or ceiling, often in a hallway, with the filter behind it.
• At the indoor unit. In a slot on the furnace or air handler, in a closet, garage or attic.
• In each ductless head. Mini split heads have their own filters behind the front panel; ask the installer to show you how to clean them.

Turn the system off at the thermostat before you take the filter out. Note the size printed on its edge and the arrow showing which way the air flows, so the new one goes in the same way.

Fitting the new filter

Buy the same size as the old one; the numbers on its edge give it. Slide it in with the airflow arrow pointing toward the indoor unit and away from the room, the way the old one sat. Make sure it sits flat, with no gap around the edge for dust to slip past. Close the grille or the access panel, then turn the system back on at the thermostat.

Keep a note

Write the date and the filter size somewhere you will see it: inside the closet door, on the return grille, or in your phone. It turns a monthly check into a habit, and it gives the contractor a quick answer at the yearly check-up.

Heating season too

The monthly advice covers the furnace and the heat pump as well as the central air conditioner. In many homes, cooling and heating share one blower and one filter, so the same check serves both. When the heat comes on for the first cool evenings, look at the filter again: it has been working all summer. A heat pump runs in both seasons, so its filter works almost all year.

What the filter does not replace

A clean filter keeps the system breathing, but it is not a tune-up. ENERGY STAR also recommends a contractor check-up each year: the cooling system in the spring and the heating system in the fall. That visit covers the parts you should not touch yourself, such as the electrical connections, the coils and the refrigerant level. Refrigerant work must be done by technicians certified under EPA Section 608. The AC maintenance guide lists everything a check-up covers.

When to call

Change the filter, wait for the system to run, and see if things improve. If the air is still warm, the system still runs without catching up, or you see ice or water at the indoor unit, it is time for a contractor.

Air Conditioning Repair San Diego is an independent referral service, not an air conditioning or HVAC company. Calls and form requests are passed to an independent local AC and HVAC contractor, who can contact you about your system; that contractor gives any quote and does any work.

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