Where Is My Furnace Filter Located: Air Filter Location

You will know where your furnace air filter is located and how to change it after reading this post.

Most HVAC experts, as well as furnace manufacturers, recommend that you change the air filter in your forced air furnace at least every 3 months. This can vary, depending on the type and quality of the filter that is installed.

Many people are (1) unaware that there is a filter that should be changed, and (2) wouldn’t know where to look for it if they wanted to change it.

Keep reading, and we will give you helpful information on locating and changing your furnace air filter. After that, you’ll find a brief introduction to AC and furnace air filters and why they are important to locate and change.

Where It Is – Air Filter Location

The furnace filter is always located in a place where all of the return air will pass through it. This makes sense, because it is collecting dust and dirt particles from the air after it has circulated through the living space in your home. This is where “new” dust is created, not inside the ductwork.

What you’re looking for: Most furnace cabinets have a small slot, a little more than 1-inch wide and 14 to 25 inches long, that the air filter fits into. Some slots are up to 4 inches wide. You’ll usually only see the side of the air filter frame, which will be paper/cardboard or plastic.

Find the return air duct and you’ll find the filter – right where the return plenum connects to the furnace. Here are some different types of furnaces, and where the filter will probably be located:

On an upflow furnace, it will usually be at the bottom – very near the floor. It may slide into a slot between the furnace and return duct from the side or from the top. But it will be where it is easy to reach and remove.

On a downflow furnace, the filter will be higher up – near the top of the furnace. This is because the return air is coming down, and the filter is located before the air re-enters the furnace. On a downflow furnace, the filter will usually slide out sideways.

Horizontal furnaces are often used in a crawlspace or an attic where there isn’t much headroom. Again, the filter will be to one side of the middle where the return air duct meets the furnace. The filter will usually slide out sideways.

Heat pump systems: Many small homes and manufactured homes have an outdoor heat pump that does both heating and cooling, such as those made by Goodman, Carrier, Rheem and others. With these systems, there will often be one large return air register where the filter is located inside the home. This will usually be in a hallway somewhere near the center of the building. The register will be held in place by screws or perhaps just two knobs that are easily turned to allow access to the filter.

No return air ductwork?

Manufactured homes usually don’t have return ducts at all. So, the return air is sucked into the furnace or air handler through the front of the “door” on the furnace. When you gently pull the door away from the furnace or air handler, the filters will be found in brackets on the inside of the door.

What It Is

furnace filter

It’s good to know what the air filter is and what it does. This makes the whole idea of changing it regularly make sense.

The furnace filter is usually a square or rectangular, thin cardboard box that is usually between 1 and 4 inches thick, with the 1 inch thickness being the most common for residential furnaces. It can have either fiberglass or a pleated paper-looking center. It is installed in a place in your furnace ductwork where all of the air that circulates through your home must pass through.

The scenario is like this: In the heating season, your furnace heats air inside its cabinet by means of burners – usually either natural or liquid propane gas. Assuming the furnace is of the upflow design, there is a blower beneath the burners which blows the heated air upward into the plenum, which is a large box to which the ductwork is connected. This heated air then travels through the ductwork until it comes into your rooms through the various vents or registers – either in the floors, walls or ceilings.

At the same time that it’s blowing air out through the registers, the blower is also drawing “return air” (which is cooler now) back through several return registers, the return air duct, and then it passes through the filter to start the journey all over again as heated air.

There are also downflow furnaces, which push the heated air downward, and horizontal furnaces which push the air sideways. Each type has its place in different types of homes and depending on where the builders chose to install it. But they all do the same thing, and they all use a filter to capture dust, dirt, pet hair and other impurities from the air that circulates through your home whenever the furnace is running.

The same is true of a central A/C unit that uses the same ductwork in the cooling season as was used during the heating season. This is why it is important to change the filter at least every 3 months, because summer or winter, a lot of air is passing through it. If it becomes clogged with too much dust or other impurities, your furnace or A/C unit will not function efficiently, and you will be using more energy to move that air around. Even worse, if the clogged filter isn’t changed, and the HVAC equipment continues to work too hard, it might eventually fail. If it does, you could face a major repair of $500 to $1,500 or the need to replace the system at a cost of $5,000 to $10,000 depending on what equipment fails.

Summing It Up

If you already know where your furnace is located, you are more than half-way to finding the air filter. If there is an owner’s manual in an envelope or plastic bag with it, the location of the filter will be shown – probably in a picture or diagram. But often, just a look around at the furnace will reveal what you are looking for, and you’ll be able to change the filter with a new one quite easily.

Furnace Filter FAQ’s

How do I know what size filter to put in my furnace? 

The best way is to look at the filter that’s already in the furnace. The size is usually printed in large letters on the exposed edge of the filter as it rests in its slot. You can often see this without even removing it. If it isn’t, you can pull it out and the size will be printed on it somewhere easily readable. The size is noted by length, width and thickness – for example: 20 X 25 X 1. Be sure to get another filter the same dimensions.

I went to my local big box store and was confused by all the different types of filters that were the right size for my furnace. What type should I get? 

Any type of the correct length, width and thickness, will work. A furnace filter with a higher MERV (Minimum Efficiency Reporting Value) number, will trap smaller particles in the air. Generally speaking, filters with a MERV 8 or below are average for a home if the occupants don’t have allergies or any other special breathing problems. As expected, the higher the MERV number, the more expensive the filter will be. Your furnace owner’s manual might even recommend a certain type, so be sure to refer to that, or perhaps go online to determine what type is best for you.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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