# How to change your air filter (and how often)

Updated: 2026-10-24 · https://pros411.com/en/guides/hvac/how-to-change-air-filter

## Short answer

Turn the system off. Find the filter behind a return grille on a wall or ceiling, or in a slot at the indoor unit. Slide out the old one and read the size on its frame. Slide in a new one of the same size with the airflow arrow pointing toward the unit. Check it every month and replace it when it looks gray, usually every 1 to 3 months.

> Safety first: If the filter is in a ceiling grille, use a sturdy step ladder and have someone hold it.

## How to change an air filter

![Cartoon indoor AC unit coughing dust while its dirty air filter slides out](https://pros411.com/img/guias/how-to-change-air-filter.webp)

1. **A clogged filter chokes the system.** A gray, dusty filter cuts the airflow. The AC works harder, cools less and can even freeze up.
2. **Turn the system off first.** Set the thermostat to "Off" so the fan does not pull dust into the unit while the filter is out.
3. **Slide out the old one and read its size.** The size is printed on the frame, like 16x25x1. Take a photo of it, or bring the old filter to the store.
4. **Arrow toward the unit, slide the new one in.** The arrow on the frame shows which way the air moves. Point it toward the indoor unit, away from the room, and push it all the way in.
5. **Back on, and check it every month.** Set the thermostat back to "Cool". Write the date on the frame and look at it once a month while you run the AC or the heat.

## How often to change it

- Basic 1-inch filters: about every 1 to 3 months. Check them monthly in summer and winter.
- Pets, allergies, smokers or construction nearby: closer to every month.
- Thick 4- or 5-inch media filters: usually every 6 to 12 months. Follow the label.
- Washable filters: rinse them, let them dry completely and put them back.

## Which filter to buy

Buy the exact size printed on the old frame. The MERV rating tells you how fine the filter is: higher numbers catch smaller particles. Many homes do well in the middle of the range. A very fine filter can choke some older systems, so check your owner manual, and if the airflow drops after you switch, go back to the old type.

## What not to do

- Do not run the system without a filter. Dust coats the coil and the blower, and that is a cleaning job for a pro.
- Do not put it in backward. The arrow points toward the unit, away from the grille you pulled it from.
- Do not force a filter that is too big, or use one that leaves gaps. Air sneaks around it.
- Do not vacuum and reuse a disposable filter.

## When to call an HVAC technician

- The airflow is still weak with a new filter.
- A new filter turns gray within a couple of weeks.
- You see ice on the copper lines or the indoor coil.
- You see dust or mold on the coil or inside the unit.

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## Frequently asked questions

### Which way does the arrow go on an air filter?

Toward the indoor unit, in the direction the air moves. At a wall or ceiling grille, the arrow points into the wall or ceiling, away from the room. At a slot in the unit, it points into the unit.

### How often should I change my air filter?

Usually every 1 to 3 months for a basic 1-inch filter. Check it monthly while you run the AC or the heat, and change it sooner with pets or allergies.

### Where is my air filter?

Behind a return grille on a wall or ceiling, or in a slot at the furnace or indoor unit. Some homes have more than one, so check every large grille that pulls air in.

### What happens if I do not change my air filter?

The airflow drops, so the system runs longer, cools less and costs more to run. A badly clogged filter can freeze the AC coil or overheat a furnace.

### Can I run my AC without a filter?

Not for long. Without a filter, dust builds up on the coil and the blower. If you have no spare, leave the old one in until you buy a new one.

### What do the numbers on an air filter mean?

The first numbers are the size in inches: width, height and thickness, like 16x25x1. The MERV rating says how fine it is: higher numbers trap smaller particles.