How To Change a Dirty Furnace Filter

Updated: Dec. 18, 2023

Changing a furnace filter regularly will improve your heating in the winter and your air conditioning in the summer.

Next Project

A few minutes






This easy task will keep your furnace and air conditioner running smoothly for many years to come.

Materials Required

  • Furnace filter
If you’re thinking you only have to change your furnace filter once a year, you may be shortening the life of your furnace. Many costly furnace repairs can be avoided with regular filter changing. If you don’t change the filter, lack of airflow will cause the furnace to overheat and shut down. Similarly, a dirty filter can stop an air conditioner because the coils freeze up due to inadequate airflow. Both issues stress the system.

Project step-by-step (4)

Step 1

Check your filter

Check your furnace filter monthly and change it every three months, depending on the type of filter you use. To do this:

  • Turn off the furnace by flipping the furnace switch. It should be on or next to the furnace, and looks like a standard light switch.
  • Find the filter service rack or door. This is most likely on the side with the intake and outtake blower fan. Remove the cover.
  • Slide out the existing filter and hold it up to the light. If you can no longer see light through it, it’s time for a new one.

Dirty filter on floorTMB STUDIO

Step 2

Purchase a new filter

Filters catch dust and debris in the air, protecting the blower motor. Always have at least one new filter on hand.

When buying filters, an inexpensive glass fiber model will do the job. But to reduce airborne dust in your home, go with a standard disposable pleated filter, which costs more.

Better yet, to remove even more small particles, install an inexpensive, electro-statically charged high-performance furnace filter.

All other options, from a four-inch-thick mechanical air filter to an electronic filter plate system, require electrical or ductwork changes by heating/cooling contractors.

New filters on shelfTMB STUDIO

Step 3

Install new filter

  • Once you’ve removed the old furnace filter, slide the new one in place.
  • Find the arrow on the filter edge and point it toward the blower motor. (Putting it in backward decreases the filter’s efficiency.) Pro tip: Mark an arrow on the furnace panel to remind yourself which direction to place the filter.
  • Slide it in, replace the cover and turn on the furnace.

New filter installedTMB STUDIO

Step 4


Is it better to have a dirty air filter or no air filter?
A dirty air filter is better- it offers at least protection against dust and particles that can damage parts of your heating system. But you should still replace it ASAP.

How long do furnace filters last?
One-inch-thick filters typically last one to three months. Thicker filters last longer. It’s important to change your furnace filter whenever it’s clogged and dirty, no matter how long it has been in use.

What are the most common furnace filter sizes?
For residential furnaces, you’ll find three: 16- by 25- by 1-in.; 20- by 25- by 1-in.; and 16- by 20- by 1-in.