What Is Furnace Filter:Types ,Function, Size, Efficiency,Parts

Furnaces and air conditioners take ambient air inside and then use that air for conditioning or other similar applications. Let us discuss about what is furnace filter , their different types etc.

The furnace filter as the name suggests filters the air coming inside the blower fan of the furnace. Without filtering the air the efficiency of furnace will become very less.

The furnace filter is located mostly on the side of furnace. We will discuss about it in detail in this article.

Furnace filter function

The furnace filter is located at the side or bottom of the furnace. Let us discuss about the function of furnace filter.

The main function of the furnace filter is to filter the air and protect various parts such as blower fan from debris, dust particles, hair and other pollutants.

Other function of a furnace filter is to aid in maintaining indoor air quality index of the room which will directly affect our health.

what is furnace filter
Image: Dirty side of air filter

Image credits: Donar ReiskofferAir filter, opel astra(1)CC BY-SA 3.0

Furnace filter types

Different types of furnace filters are used for different types of applications. Let us discuss about these types.

The different types of furnace filters are discussed in the section below-

  • HEPA filters
  • Fiberglass filters/synthetic filters
  • Polyester filters
  • Pleated filters
  • Electrostatic filters
  • Carbon filters

Furnace filter location

The location of furnace filter place an important role in justifying its application. Let us discuss about where exactly the furnace filter is located.

Furnace filters are mostly located inside the blower compartment, it’s the place where the air will enter the blower compartment Generally it is found on the bottom or side of the furnace behind a metal compartment.

These loose areas are the place from where the air is sucked inside, so to avoid the impurities to go further inside, the filter is placed in between the course of impure air.

Furnace filters working

Most of us think that filters are used for making the air cleaner. But this is not the case, let us study the reason about how furnace filters work.

Furnace filters works by filtering the dust particles using tiny pores. It is made up of fibres and have tiny pores through which the dust particles cannot pass. In reality, these dust particles are filtered out so that the system’s efficiency for cooling or heating increases. The air quality increases but its not the primary reason for filtering the air.

Furnace filters size

The size of an air conditioner or size of the furnace decides the size of filter to be used. Let us read about furnace filters sizes in the next section.

The standard sizes of a furnace filter are given below-

  • 10 X 20 X 1’’
  • 14 X 20 X 1’’
  • 16 X 24 X 1’’
  • 18 X 30 X 1”
  • 20 x 20 x 1
  • 16 x 25 x 1
  • 20 x 25 x 1
  • 20 x 25 x 4
  • 16 x 20 x 1
  • 16 x 25 x 4
  • 20 x 30 x 1
  • 12 x 12 x 1
  • 14 x 14 x 1

If a filter’s size does not match with the size of furnace then two different sizes of filters are attached together to fit inside the space allocated for filter.

Furnace filters direction

The efficiency of the entire filtering process is affected by the direction of furnace filters. Let us discuss about the direction of furnace filters.

The direction of furnace filters is in the direction of air flow. The furnace filters have arrows marked at their sides. These arrows depict the direction in which the air flow should be kept. This typically is away from the supply ducts and towards the blower.

Furnace filter efficiency

The efficiency of furnace filters will allow us to decide which furnace filter will be suitable for us. Let us discuss about furnace filter efficiency.

The efficiency of furnace filter is typically 100 percent as it prevents all the impurities lesser than 0.3 microns to enter the furnace. The rating is based upon how small particles can be prevented by the filter.

Furnace filter frame

A frame is typically used for supporting structures. Let us discuss about  furnace filter frame .

The frame of a furnace filter holds and supports the furnace filter. The material of frame is tough so that it can hold the filter easily.

Furnace filter whistle

Commonly whistles make sound. This sound can have various use cases. Let us discuss what a furnace filter is.

Furnace filters make sound when the filter is clogged by 50%. This gives an indication that the filter needs to be cleaned or needs to be changed. If the filter is not changed then the efficiency and quality of furnace will be affected.

How to install furnace filter?

The filter needs to be changed after a certain interval of time. Let us discuss how to install a furnace filter.

  • Switch off the air conditioner or furnace before installing the filter
  • Remove the old filter if there is any
  • Place the new filter by matching the direction of air flow and the direction in which the arrows marked on the filter are pointing.
  • Close the filter door and switch ON the furnace or conditioner back on.

How often to change furnace filters?

The quality of furnace filter is very important for effective functioning of a filter. Let us discuss how often these filters need to be changed.

The recommended period for a filter to be changed is 90 days. After that either the filter needs to be changed or the filter needs to be cleaned.

How to clean furnace filter?

Furnace filters are cleaned for increasing the overall performance of the furnace. Let us discuss how to change the furnace filter.

The furnace filter is simply cleaned by spraying water on the filter. This is done with the help of a pipe having tiny pores on its outlet. One needs to make sure that the fibers of filter do not get compromised otherwise the filter will not be of any use.

How to check furnace filter?

There are many ways to find out whether furnace filter is up to date or not. Let us discuss some of the easy ways to find out whether a furnace filter is working or not.

  • Higher electricity bills than usual.
  • Allergies
  • Different colour of filter
  • Headaches
  • Rise in physical issues.

Why to change furnace filter?

We cannot run our furnaces forever on a single filter. The filters need to be changed after some time. Let us discuss why is it necessary to change the furnace filter.

Furnace filters need to be changed because if we do not change the filter then the dust particles will remain between the gaps of fibers present in the filter. Once all the gaps are filled, the air flow will be compromised making the efficiency of the furnace lower. Even more dust will start entering as there will be excess dust accumulated in the filter itself.

Furnace filter vs ac filter

Furnaces and Air conditioners have two different applications. Let us compare about the filters that each of them use.

AC FiltersFurnace filters
These filters are used in systems that are used for cooing the air like the conventional air conditioners we use at homes and offices. Furnace filters are used in furnaces that are used for heating purposes.
It is used to filter the air in such a way that it is fit for breathing. These filters also filter the air but they are not meant for making it fit for breathing.
These filters are located behind the vent through which air is sucked in.These filters are located on the side of the furnace.
These filters need to be cleaned after every 1.5 months.These filters need to be cleaned after 3 months and thicker ones usually after 9 months.
They come with a cost range from Rs 500 – Rs 4000.These come with a cost range of Rs 1000- Rs 2000.
Comparison between AC filters and Furnace filters


In this article we discussed about what furnace filters are and what are their functions. We even discussed about different types of furnace filters and their sizes. Even though there are different types of filters, their primary function is same.