Furnace Filter Airflow – What You Need To Know?

To keep your HVAC system functioning properly, you should learn everything you can about the regular maintenance required for optimal performance and improved air quality.

In this article, I will tell you everything about how your furnace works and how the filter affects your system. 

The furnace filter is one of the most important parts of a forced-air HVAC system. It protects the system, the efficiency, and the quality of the air you and your household breathe in every day. The purpose of a furnace filter is to protect the blower fan and other components of the system. 

While improved air quality is a side effect, it is important to note that it is not the filter’s primary function. It blocks particles like dust, debris, and other pollutants from getting into the furnace.

Let us dive deeper into how a furnace works, the different types of filters, how to choose them, and how to maintain your system for many years to come.  

How Does A Furnace Work? 

In its most basic terms, a furnace works with forced air. It operates by taking the air in the home, pulling it through a return, sending it through a filter to the actual furnace. 

The furnace heats (or cools the air) and sends it out through your ductwork to all of the rooms in your house.  

If you have forced air, you have vents where you can feel the air coming through.  Most modern HVAC systems can heat or cool the air depending on the temperature setting on your thermostat settings and the type of system you have.  


So how do you keep this system running at optimal levels while decreasing your energy costs, and improving the quality of air that your family breathes?  

The answer is maintenance. One of the most important parts of maintenance, without a technician servicing it periodically, is making sure your filters are in working condition.

Now we will learn about what filters are and how they work. 

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Types of Filters

There are several different types of filters on the market. Here are the different types available. 

Disposable Pleated 

These filters are made of cotton and polyester paper. It is affordable and catches smaller types of particles like dust mites.

It is also environmentally friendly and recyclable. Disposable pleated filters should be changed every month. 

Fiberglass Disposable

This is the most commonly used filter and is also the cheapest. It only catches the largest of pollutants and may be an issue if someone in the home is susceptible to certain allergies or asthma.

These filters usually need to be replaced monthly.  

High-Efficiency Pleated 

These filters are more costly and do not fit some in-home furnaces. They are typically used in hospitals and are made of 4” to 5” thick synthetic cotton that traps the smallest of particles.

If anyone in the home has autoimmune or respiratory issues, this is a very effective filter. They are also a little more costly. 

Disposable Electrostatic

Electrostatic filters are made of self-charging, electrostatic cotton fibers or paper and would be a great choice if you have smokers or pets in your home. They catch the smallest of potentially harmful particles.

They can be a little costly if you need custom sizes but they are not overly expensive for standard sizes.  

Permanent Electrostatic 

Made of the same materials as the disposable electrostatic filters, this option only differs in the fact that it is permanent. 

Permanent means that you do not have to buy a new one to replace it. This filter is cleaned when it is removed and is machine washable.  

It is initially more expensive than the other filters but since it is permanent and lasts eight to ten years, it pays for itself over time. 

Deciding on the filter that is suitable for your needs depends on a few factors. We will explore them next.   

Filter Sizes

Filter sizes are based on height, depth/thickness, and length. Common thickness ranges from 1 inch to 4 inches. Common height and length range from 14” by 25” and 25” by 25”. 

Furnace filter close-up

The filter size you require is going to depend on your furnace. It obviously has to fit properly.

You can check the size of your filter by removing the old one and looking at the writing on the frame. 

Do Higher MERV Filters Restrict Airflow?  

Minimum Efficiency Reporting Value or, MERV for short, is a term that measures the denseness of a filter. MERV ratings range from one to sixteen.

The higher the MERV rating, the more dense, the filter. Density is synonymous with thickness.  

The thicker filter will undoubtedly catch more pollutants, dust, dander, hair, and the like, but may be so thick that it restricts airflow. This can cause your furnace to work harder and potentially cause functioning problems.  

You will want to check with your furnace manufacturer for a maximum MERV rating. A rating between eight and eleven is normally sufficient for a typical home.   

If you are purchasing your filters from a retail store like Lowe’s or Home Depot, they may have their own rating system. They will be similar to MERV ratings but can vary. 

Always check with your manufacturer to make sure that the retail ratings are comparable to the MERV ratings allowable for your furnace. You never want to exceed the maximum rating for your particular furnace. 

Which Direction Does The Air Flow Through The Filter?

The air is pulled into the ducts from the house through what is referred to as a “return.” The air then passes through the filter and is sent to the furnace.

The furnace heats or cools the filtered air and forces it out through the vents in the rooms. The cycle keeps repeating to maintain the temperature on your thermostat. 

Replacing air filter on furnace

To ensure you are installing your filter in the correct direction, find the arrow marked on the filter.

The arrow must always face the unit and away from the return. The arrow represents the direction the air should flow. 

Regardless of what type of filter you have, they will all be marked with an arrow.

If, for any reason, you cannot locate the arrow, just look at both sides of the filter. The side with the frame will face away from the unit and the side without the frame will face towards the unit.    

How Long Do Air Filters Last?  

As a general rule, you should replace your filters once every three months. However, it can be a little more complicated than that. 

It depends on the thickness of the filter you have. The thinner the filter, the more often it will need to be replaced. 

It may also need to be replaced more often depending on the season.

In the spring, for example, pollen is sometimes higher and can clog the filter faster. In the winter, it can get dirtier more often due to more use. 

It is a good idea to visually inspect your filters to gauge whether or not they need to be replaced. If it looks dirty, clean, or replace it.    

Replacing Your Filter

Now that you have chosen the right filter to fit your furnace and your individual preferences for your household, it is time to actually replace it.  

The first thing you should do is turn off the furnace to prevent it from kicking on while you are replacing the filter. The furnace kicking on when you have the compartment open could get pretty messy.

Have a vacuum and garbage back handy.  

Furnace maintenance

The next step is opening the compartment door and removing the old filter. You may want to start vacuuming up the mess.

I usually vacuum the accessible part of the filter before I take it out. Once you have the filter in hand, gently place it into the garbage bag.  

The last step is installing the new filter.

Find the arrow on the frame. The arrow will let you know that you have the direction of the airflow correct.

Always make sure the arrow is facing towards the furnace.  Slide it in and you are all set.    


I know that we have covered a lot of ground here and it may have seemed overwhelming.

Now that you know everything you need to know about your furnace filter and airflow, it will be easy going forward.  

By properly maintaining your HVAC system, you will increase the life of your investment, save money by running it more efficiently, and, more importantly, improve the quality of the air you and your loved ones breathe.