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How do I find, identify, measure and change my furnace filter?

You might be surprised to know this, but many homeowners don't know how to locate or change their furnace or Air Conditioner (AC) filter. This article was created to answer the following questions:

What does my furnace or AC filter do?
How to I locate my furnace filter or AC filter?
How do I change my furnace or AC filter?
What do I need to know when changing furnace filters?

What is the function of the furnace AC Filter, what does it do?

The filter used for a standard residential Heating Ventilation and Air Conditioning (HVAC) systems was originally designed to protect your home's heating and cooling equipment, but not necessarily to clean the air you breathe.

Air in a home is pulled into the return ductwork by your system's blower fan to be heated by the furnace heat exchanger or cooled by the air conditioner's evaporator coil. The filter, which is placed in the return ductwork, filters all the air pulled through the system before it comes in contact with expensive components of the HVAC system.

Even a low efficiency fiberglass panel filter will remove enough particulates to ensure the heat exchanger or AC coil won't become excessively dirty or damaged by larger particles of debris that enter the return.

Filters attract and capture large allergens such as:
- Pollen
- Mold spores
- Dust mite debris


Microscopic allergens such as:
- Smoke
- Pet dander
- Household dust
- Smog

A filter change is a good idea at least every 1-3 months depending on the type of filter. Certain conditions in your home will significantly add particles to the air, which will cause the filter to capture more particles and shorten its effectiveness. Dirty ductwork, pets, construction work, sanding projects, tobacco smoke, burning candles, fireplaces and wood-burning stoves can all potentially shorten the lifespan of your air filter.

How to I locate my furnace filter or air conditioner filter?

If you're in a new home or you've never changed your filter before, then the best place to start looking for your air filter is by checking your mechanical room. A mechanical room is where all your heating and cooling equipment resides. It can be located in a closet, basement, or even the attic. The easiest way to find your "mechanical room" is by turning the fan setting on your thermostat to the "on" position. Listen for the sound of your system's fan and follow it to its origin.

You may not have a thin 1" filter. If this applies to you, then your furnace uses a "media" filter, which fits in a thick air cleaner enclosure.

In most cases, air cleaner cabinets have a filter that's much thicker or wider than the standard 1" slot. Common whole house air cleaners are manufactured by Aprilaire,Honeywell, Bryant, Carrier, Lennox and Goodman. These larger "box-type" media filters are likely to be 4-6" in thickness.

Though not as common, some air filters are found in return air grilles. The grill has a hinged face with an enclosure where the filter resides. These are typically found in the ceiling or in the wall. Honeywell manufactures a deep pleated return grille filter for this application. Typically 1" filter grilles are more common.

How do I tell what size or kind of furnace or air conditioner filter to purchase?

The easiest way to tell what type of filter you have is to look for a part number or size displayed somewhere on the filter. Most manufacturers will put their part number or nominal size on the outside of the filter for easy identification. Note: The make, model number or serial number of your furnace, air conditioning system, or air handler does NOT provide much useful information on the appropriate replacement filter.

Our rule of thumb is as follows:

If you can't find a filter part number or nominal dimensions printed on the filter, then use a tape measure and record the exact size of the filter you are trying to replace.

Find the length, the width, and the thickness. Filters are usually listed at a nominal size, which is essentially a rounded-up number. A manufacturer might print 20" x 25" x 1" on a filter, but in actuality the filter really measures 19 1/2" x 24 1/2" x 7/8" with a tape measure. It is normal for a filter to be undercut up to 1/2" from the nominal size. On our website we list BOTH the nominal and actual size of the filter to eliminate any confusion.

Now that I have a new furnace filter, how do I change it?

Note: Before changing your filter, you should turn the power OFF to the furnace or air handler. When an air filter is changed a tremendous amount of dust can be dislodged, which means it's probably best if you don't have your system fan operating mid-change since all that debris can be directly sucked into your HVAC system.

Most filters are made in such a way that they can only be inserted one way into their slot or air cleaner cabinet. Often, a rigid scrim or wire mesh is inserted on the downstream side of the filter to give the filter the strength and rigidity it needs to do its job properly. The downstream side of the filter is the last surface the air touches and should face the blower of your HVAC system.

There should be an air flow arrow on your air filter that will show you which direction the filter should be inserted.

What do I need to know when changing furnace filters?

The act of changing your air filter should be as simple as sliding the old one out and sliding a new one in its place. You may have to remove a slot cover or air cleaner door on some HVAC systems. Other air cleaners, such as the popular Aprilaire/Space-Gard 2200 and 2400, may need to have their new new filters assembled before replacing. Click here to learn how to replace an Aprilaire/Space-Gard filter.