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
1A clogged filter chokes the system
A gray, dusty filter cuts the airflow. The AC works harder, cools less and can even freeze up.
Read all the steps
- A clogged filter chokes the system. A gray, dusty filter cuts the airflow. The AC works harder, cools less and can even freeze up.
- 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.
- 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.
- 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.
- 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.
- Still not fixed? Call an HVAC contractor Especially if: the airflow is still weak with a new filter.
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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.
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.