How To Find The Air Filter For Your Furnace

If you haven’t changed the air filter in your furnace because you don’t know where to find it, you’re not alone I promise. Quite a few people who call our office when their furnace stops working go suddenly silent when we ask how long it’s been since they changed their air filter. Then they sheepishly admit that they haven’t because they don’t even know where their filter is.

So we created this quick guide to help you find your filter quickly and easily. Luckily, there are usually only two places it will be so you won’t have to search too hard.

The First Place To Look

Before you take the doors off your furnace to look inside, there is a chance your filter could be inside what’s called a ‘guillotine’ door that has been cut into the return drop, which is the rectangular duct that comes down to the floor on either the right or left side of your furnace.

The rectangular piece of ductwork that runs down next to your furnace is called the return drop.

If you have a guillotine door, you will see what looks to be a hand-made slot with a piece of sheet metal that slides up and down to allow access to the filter. You can see what a typical guillotine door looks like in this picture.

Second Place To Look – Inside The Furnace

If you don’t see a guillotine door cut into the return drop next to your furnace, then it should be inside your furnace. Don’t worry, the hardest part of finding the filter if it’s inside your furnace is just taking the doors off the front of the furnace, and that’s not hard at all.

Your furnace should have two doors on the front, one at the top and one at the bottom. Start by turning the knobs or screws that hold the top door of your furnace in place. Remove the top door, and set it aside. Now remove the bottom door. Depending on the brand of furnace you have, you may just be able to pull the bottom door off after removing the top door.

Look in the bottom section of your furnace. You should see the filter leaning up against one of the sides. It should be sitting on whichever side the return drop is on.

Buying The Right Size Filters

Now that you’ve found your air filter, just take out the old one and look at the size that’s printed on the cardboard frame of the filter. The most common sizes are 14×20, 16×20, 20×20, and 25×20. You will want to buy the correct size for your furnace, so make sure you buy the same size as the filter you removed from your furnace.

Unless you have allergies, asthma, or other respiratory problems we usually recommend buying a pack of inexpensive air filters from Home Depot. We have a short article on our website where you can see what the inexpensive furnace filters at Home Depot look like.

Replacing The Filter

Look at the cardboard frame on the filter, and you should see an arrow that indicates the direction of the airflow so you know how to orient the new air filter.

The air flows through the return drop next to your furnace, and then through the filter before making its way into your furnace. All you need to remember is to make sure the arrow on the filter is pointing towards the inside of your furnace, and not away from the furnace. Then just place the filter in the same spot you found the old air filter and you’re done… well, you’re almost done.

Don’t forget to put the guillotine door (strip of sheetmetal) back in the little slot in your return drop if your filter was inside the drop, or put the doors back on the front of the furnace if your filter was inside the unit.

Put the bottom door back on the furnace first, and then you can put the top door back on and turn the knobs or screws that hold it in place. Now you’re done. That wasn’t so bad was it? Just remember to change your air filter every month from now on, and you’ll avoid a lot of problems with your heating and air conditioning system.