How to Change a Furnace Filter

Co-authored by wikiHow Staff

Last Updated: March 30, 2023

Changing a home furnace filter regularly is the fastest and easiest way to make sure your furnace is running smoothly and efficiently, as well as to help purify the air in your home. Check the furnace filter monthly to see when it needs to be changed. Dispose of it and replace it with a new furnace filter of the same size when it gets dirty. This will keep the air flowing through your furnace, minimize the strain on your heating system, and help filter dust and other particles out of the air in your home.

Part 1

Part 1 of 2:

Removing the Old Furnace Filter

1
Turn off your furnace before checking or replacing the filter. Set the furnace’s thermostat to the “off” position. This will prevent the furnace from turning on while you are checking or replacing the filter.
- If the furnace turns on while there is not filter in it, then it can suck up potentially-damaging loose debris.
- Refer to the owner's manual for your furnace for any specific instructions regarding how to replace the filter.
Tip: Check your filter every month to see if it needs replacement. Most disposable types of filters are designed to be replaced every 1-2 months. If you have furry pets or your furnace runs all the time, then your filter will get dirty more quickly.
2
Open or slide the door panel off of your furnace. There will be a door on your furnace that either swings open or slides off. Open or remove the door to provide access to the inside of your furnace.^{[1]
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- If your door is the type that slides off, then you can typically remove it by simply lifting it up until it comes free, and then set it aside.
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3
Locate the filter in your furnace and remove any cover that is over it. The filter is usually located at the air return duct or the entrance to the blower chamber. There is sometimes another cover over the filter that you will have to pull off.^{[2]
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- Look for the vents where the air enters into the furnace system to locate the filter near either the air return duct or the entrance to the blower chamber.
4
Slide out the old filter and hold it up to the light. Use both hands to carefully pull the filter out for inspection. Hold it up to a light source to check how dirty it is and determine if it is time to replace it.^{[3]
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- If there is any resistance when you try to slide the filter out, then there may be a locking mechanism keeping it in place. Check for such a system and slide the lock out of the way to release the filter.
5
Replace the filter with a new one if you can’t see through it. It’s time to replace the filter when no light shines through the filter when you hold it up to a light source. It’s important to change the filter regularly to keep good airflow going through the furnace and keep it from overheating.^{[4]
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- Disposable filters have a cardboard frame. If your filter has a plastic or metal frame, then it is a reusable filter than you can clean instead of replacing it with a whole new filter.

Part 2

Part 2 of 2:

Swapping in a New Furnace Filter

1
Look on the cardboard frame of the old filter to find the dimensions. The filter size will be printed somewhere on the frame of the disposable filter. Note it down so that you can refer to the measurements when you shop for a new filter.^{[5]
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- If your filter does not have the dimensions printed on it, then measure it with a measuring tape, or check the owner’s manual for the manufacturer’s recommendations to ensure you get the right replacement filter.
2
Purchase a replacement filter of the correct size. Head to a home hardware store or home improvement center to find a new filter. Refer to the dimensions of the old filter and buy a filter of the same size.^{[6]
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- The cheapest disposable furnace filters are fiberglass with cardboard frames. This kind of filter will trap large particles of dust and is designed to be replaced every 1-2 months. If you want to upgrade your filter, you can get a slightly more expensive disposable pleated filter, which will trap smaller particles of dust , pollen, and dirt. These filters last longer; from 4 months to 1 year.
Tip: If you or someone in your household has allergies, you might want to invest in a highly efficient particle air (HEPA) filter. These types of filters are antimicrobial and specially designed to trap things like mold, mildew, fungus, yeast, and algae. They are available in both disposable and reusable varieties.
3
Look for an arrow on the filter that shows which way it needs to face. Your new filter will have an arrow that shows which way the air flows through it. Find this arrow to determine which way to slide the filter into your furnace.
- Home furnace filters are made to filter air in a single direction so that they do not collapse or fall apart as they trap dust and other debris.
4
Slide the filter into place and replace any cover that goes over it. Slide the new filter in so that the arrow is facing the furnace’s blower motor. Snap the filter cover back into place if you removed one to take out the filter.^{[7]
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- If you put the filter in backwards, then your furnace won’t have sufficient airflow and it can overheat and shut down. This can also happen with a dirty filter, so make sure to keep checking your filter monthly and replacing it when it is dirty.
5
Close or replace the door panel of the furnace. Swing the furnace’s access door closed or slide it back on if you removed it. Your furnace will now be ready to use for at least another month.^{[8]
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- Remember that if you have furry pets or you run the furnace very regularly, then you will need to replace the filter more frequently. Check your filter every month and any time you notice dust building up in your home.
6
Turn your furnace back on. Set the furnace’s thermostat back to the “on” position now that you have replaced the filter. Your furnace will be protected from loose debris by the filter and is safe to run as normal.
- You might want to make a note of the date you changed the filter to remind yourself to check it again in a month.