Are AC and Furnace Filters the Same? What’s the Difference?

The terms “AC filter” and “furnace filter” are used interchangeably and sometimes it is hard for some folks to understand whether the two are one and the same thing or they are totally different HVAC parts.

Although AC and furnace filters do the same thing (remove impurities from the air), they are not exactly the same. Furnace filters are located where the return air enters the blower compartment of the furnace while AC filters are located just behind the return air vent.


Other than the difference in location, AC filters and furnace filters are the same. However, a house can have several AC filters but only one furnace filter, unless you have 2 HVAC systems.

Both AC and furnace filters are installed on the HVAC system’s return air duct. That is the ductwork that carries air from the house to the furnace.

The difference is that the AC filter is located inside the room just behind the return air grill. On the other hand, a furnace filter is located in the basement, attic or crawlspace (or wherever your furnace is located) just before the return air ducts connects to the furnace.

In terms of appearance and basically how these filters are made, AC and furnace filters are identical. What you however need to consider is the size of air filter you need, the best MERV rating for you and also how to install it properly.

AC vs Furnace Filter

One of the main reason there is a confusion between AC and furnace filters is that some folks think that AC filters are only used when the HVAC system is cooling the house, and that the furnace filter is used when the furnace is heating the house.

The truth of the matter however is that an AC filter or furnace filter is used to remove particles from the house during both heating and cooling. There are no special filters for cooling and others for heating.

In case you don’t already know it, your air conditioner is paired with the furnace. While the air conditioner cools the house during summer, the furnace provides it with a source of heat during winter.

Note: If you have a heat pump instead of an air conditioner, it will be paired with an air handler (which looks like a furnace). An air handler will also need a “furnace filter” or Ac filter to clean the air being supplied to the house.

One of the most important component of a furnace is the blower.  The blower pulls air from the house to the furnace through the return air duct.

An AC has an inside and outside unit. It is the inside unit (evaporator coil) which is housed together with the furnace.

Even when the furnace is not being used to heat the house (when you need the house cooled), the blower will still be operational pulling hot air from the house and moving it across the evaporator coil where it will lose the heat to the refrigerant (which is inside the coil)

The same also happens for the AC filter just behind the return air vent. When the thermostat calls for heating, air from the house will be cleaned by the AC filter even though the AC will be off at that time.

Note: If your house has an AC filter, it will not have a furnace filter and vice versa. Both AC and furnace filters should be replaced after every 1 to 3 months. A house can have more than one AC filter especially when the house has more than 1 return air duct.

As you can see from the above explanation, the difference between an AC filter and furnace filter is just their location. That is the main reason the 2 terms are so often used interchangeably.

Where the AC Filter is Located

ac filter

As I have already mentioned, AC filters are located just behind the return air vent inside the house. A house can have several return air ducts which means several filters as well.

If you are not sure which is the return air grill and which one is the supply air grill, place the palm of your hard close to the vent when the HVAC system is still running. If you feel your palm being sucked inside the duct, then it is indeed the return air grill.

Here is how to replace an AC filter:

  • Start by turning off the HVAC system
  • Press the tabs on the grill to release it and therefore remove it.
  • Simply pull out the old filter. If it was the correct size then you don’t need any tools to remove it.
  • Put in the new filter. It is critical that you get this step right. The airflow arrow on the filter should point towards the ductwork (away from the house) otherwise the filter will restrict the flow of air, resulting in high power bills and even reduced lifespan of the furnace/air handler.

Note: If there is no arrow on the filter, the side of the filter with a wire grid/mesh should be the one pointing towards the furnace/air handler. The frame protects the filter material thereby preventing it from bending.

Where a Furnace Filter is Located

There are 2 types of furnaces:

  • Vertical furnaces
  • Horizontal furnaces

In both of these furnaces, the air filter will be located just before where the return air duct connects to the furnace. The air should be cleaned before getting to the heat exchanger.

In vertical furnaces, the return air enters the furnace from the bottom and you will therefore find the filter in a slot/compartment at the bottom of the furnace just before where the return air ducts enters the furnace.

For horizontal furnaces, the return air duct enters the furnace from the side. The furnace filter will therefore be located on the side of the furnace inside a 1 or 4-inch slot.

Here is how to change a furnace filter:

  • Turn off the furnace. Failure to do that will result in the dirt trapped by the filter being sucked inside the furnace.
  • Locate the filter slot. It is usually 1 or 4-inch in size.
  • Remove the slot cover and slide out the old filter.
  • Slide in the new filter. Again, it is important to make sure that the airflow arrow on the filter is pointing towards the furnace and away from the return air duct.
  • Put back the slot cover and turn on the furnace.

That is how you replace a furnace filter. It is important to make sure that you choose an air filter with a good MERV rating and also ensure that it is of the right size. If you are not sure about the size make you can always measure the old filter (assuming it was properly sized).

Note: Furnace/AC filter sizes are always sold in nominal size instead of actual sizes. Nominal sizes are slightly bigger than actual size (rounded off to the nearest whole number).


And basically that is everything regarding furnace and AC filters. I hope by bow you understand how different or similar the two are.