How to change your AC filter (and how often in Arizona)

In most Arizona homes you should check your AC filter every month and replace a standard 1-inch filter every 1 to 3 months. Dust, pets and a system that runs nearly nonstop from May through September clog filters faster than in cooler states. The swap takes about five minutes and needs no tools.

Time: about 5 minutes. Skill: easy.

What do you need?

• A new filter in the right size
• A marker to write the date on the frame
• A flashlight if the filter is in a dark closet

The size is printed on the cardboard edge of the old filter, for example 16x25x1. Those numbers are length, width and thickness in inches. Write the size on a note in your phone so you can buy the right one every time. If you cannot find a printed size, measure the slot and match it, and ask a pro if the numbers do not look standard.

How do you change an AC filter?

1. Turn the thermostat to Off so the system is not running while you work.
2. Find the filter. In many Valley homes it sits behind a large grille in a hallway ceiling or wall. In others it is inside the air handler in a closet or garage. If yours is in the attic, see the heat warning below.
3. Open the grille. Most swing down or slide out. Support the grille so it does not drop on you.
4. Slide the old filter out. Note which way the arrow on the frame points before you pull it.
5. Look at the old filter. A thick gray blanket of dust tells you that you are on the right schedule or overdue.
6. Slide the new filter in with the arrow pointing the same direction as the old one. The arrow points toward the blower and away from the room the air is pulled from.
7. Close the grille, write the date on the frame edge or on a calendar, and turn the thermostat back to Cool.

Heat warning: a summer attic can be dangerously hot. If your filter is up there, change it early in the morning in cooler months, or ask a pro to do it during a service visit. Never go into an attic in the afternoon in summer.

How often should you change an AC filter in Arizona?

Start with once a month during summer and check it. If it still looks fairly clean, stretch to every two months. A few things shorten that:

• Pets that shed
• Recent dust storms (haboobs) and windy spring days
• Nearby construction
• Running the fan all day

Thicker media filters, often 4 inches deep, can last longer between changes. Follow the maker's directions on the box.

What does MERV mean?

MERV is a rating of how small a particle a filter catches. A higher number catches smaller particles, but it also makes it harder for air to pass through. A filter that is too restrictive can strain the blower and reduce cooling in a system not built for it. For most homes a mid-range filter is a fine balance. Check with an HVAC company before moving to a high-MERV filter, especially if your system is older. You can browse Phoenix businesses to find one.

When to call a pro

Call a pro if the new filter is dirty again within a couple of weeks, if airflow is weak even with a clean filter, if ice forms on the indoor unit, or if the system is not cooling. A tune-up before summer also catches problems a filter cannot. Here are local AC companies in the directory, each with a phone number:

Check any contractor's license at roc.az.gov (Arizona Registrar of Contractors) before you hire.

Questions people ask

What happens if you never change your AC filter?

Airflow drops, the system works harder, and rooms cool more slowly. A badly clogged filter can also contribute to a frozen indoor coil and costly repairs. It is the cheapest maintenance you can do.

Can you run the AC without a filter?

You should not. Without a filter, dust builds up on the indoor coil and blower, which hurts cooling and can lead to repairs. If you are out of filters, buy one the same day.

What if my filter is the wrong size?

A filter that is too small lets dust slip around the edges, and one that is too large will not seat. Match the printed size exactly. If the printed size has fractions, such as 15 1/2, use that exact size.

See every AC and heating company in Phoenix or ask the helper a question.