Does Furnace Filter Size Matter?

One of the most important ways of caring for your furnace or HVAC system is maintaining a properly functioning filter. A clean and well-fitting filter prevents a number of costly issues and will keep your furnace working at its optimum capacity. 

It is important to equip your furnace with the right type of filter. From ensuring the proper fit to choosing a suitable thickness and density for your needs, using the right filter will make a significant difference in the cost and efficiency of your furnace. 

Maintaining your furnace to keep it working as efficiently as possible is not difficult, it simply requires knowing the simple steps to take and where to find the information you need.

We’ve put together this guide to help find the right filter size for your furnace and to learn exactly what to do to keep your furnace in its best shape. 

How to Find Your Furnace Filter Size

Proper filter size is important, so manufacturers make it easy for you to find the information you need.

Most filters will have the size printed directly on the side of the filter in the format of length x width x depth

When determining you filter size, it is important to note that there are the following two types of measurements that could be listed:

  • Actual Size – The actual size uses the exact measurements of the filter vent. This size is usually 0.25-0.5’ smaller than the nominal size. 
  • Nominal Size – The nominal size is the actual size rounded-up to the nearest whole numbers. The nominal size is what is commonly used by most major companies.

Most popular brands will have the nominal size printed on the side of the filter. Some brands that follow this system of measurement include the following:

  • Aprilaire
  • Carrier 
  • Coleman
  • Goodman
  • Honeywell
  • Lennox
  • Toptech
  • Totaline
  • Trion Air Bear
  • Skuttle

Furnace filters of a less common size or from less common brands will often come true to size.

This means that the nominal size of the filter is the same as its actual size.

Filters that are true to size will indicate this by printing “actual size” next to the dimensions on the filter. 

How to Ensure That Your Filter Fits Properly 

Your furnace filter should fit snugly, yet comfortably into your furnace. Having to bend and force your filter into your furnace is a clear indicator that your filter is too large. 

On the other hand, having more than one to two fingers-worth of space around your filter indicates that it is too small.

Always follow your filter sizing guidelines to ensure that you choose the right sized filter for your furnace. 

For HVAC vents located in ceilings, a slightly larger filter size may be used to prevent the filter from falling when the vent is opened.

Uncommonly sized filters such as are often needed in older units may also work well with some foam weatherstripping tape around the edges to increase the size and create a tighter seal. 

When determining whether this would be a good idea for your furnace filter, keep in mind that the main objective is to create a snug fit that will allow all air to pass easily and directly through the filter. 

Why Furnace Filter Size is Important

Choosing the correct size for your filter will allow the filter to do its job properly.

A filter that is either too small or two large, will create gaps around the filter, in turn allowing air to flow around the sides.

When this happens, it can cause a number of problems, including the following:

  • Dust and pollen filled indoor air 
  • Grime and buildup that can lead to a damaged furnace
  • Higher energy bills
  • Irregular heating and furnace malfunction 
  • Costly repairs
  • Permanent damage to the unit

Does Furnace Filter Density Matter?

While the thickness of a filter affects the overall size and surface area, the density of the filter fibers is another factor to take into consideration.

This is indicated by the other number printed on the filter, also known as the filter’s MERV rating

MERV stands for minimum efficiency rating value. This number indicates the size of the individual pores on your air filter.

Image Credit: LakeAir.com

The higher the number of the MERV rating, the smaller the individual pore size and thus the greater the filtration capacity.

MERV ratings generally range from 1-12 and are grouped into the following three categories of filtration:

  1. MERV ratings of 1-4 capture the following particles:
  • Pollen
  • Dust
  • Wood sanding particles
  • Spray paint particles
  • Carpet and fabric fibers
  1. MERV ratings of 5-8 capture the following particles: 
  • Airborne cleaners
  • Hair spray
  • Cement mix
  • Microbiological growth
  • Volatile organic compounds referred to as VOCs
  1. MERV ratings of 9-12 capture the following particles:
  • Vehicle emissions and fumes
  • Dust from lead, flour and dehumidifiers
  • Legionella

Although filters with a higher MERV rating capture more particles from the air, a higher rating is not necessarily the right choice for every furnace.

Filters with a higher MERV rating require more energy to run and consequently result in a higher operation cost.

Most residential furnaces will not require the resistance of a high MERV rating, so it is generally more beneficial to reduce costs by sticking to a lower rating. 

Choosing a filter with too high of a MERV rating may hurt more than your wallet.

When too dense of a filter is used in an HVAC system that doesn’t require that degree of resistance, it may restrict the needed airflow and make your furnace work harder.

This will not only worsen its efficiency but may also lead to further damages. 

Does Furnace Filter Thickness Matter?

The thickness of furnace filters is directly correlated to the capabilities of the furnace. It can range from one to five inches thick with a greater thickness often offering desirable benefits such as the following:

  • Improved energy efficiency saves you money on your monthly bill
  • Additional filtration takes more particles out of the air, in turn leading to improved air quality
  • A greater surface area with the ability to hold more particles results in longer times between routine filter maintenance

Furnace filter thickness is especially important when considering a filter with a higher MERV rating.

Dense filters can capture a wide variety of particles from the air, but the more they can capture, the more is sticking to them.

This means that dense filters tend to become covered in dust and grime and in-turn become clogged much quicker than filters with a lower MERV rating. 

An increased thickness provides greater surface area for all that your filter captures to land on.

This in-turn increases the time it takes for the filter to become clogged, saving money and reducing the risk of damages due to improper air flow. 

Maintaining Your Furnace Filter

Routine maintenance of your furnace filter is an easy way to keep your furnace working at its maximum efficiency.

As a general guideline, it is recommended that filters up to one inch in depth are changed every one to three months while thicker filters that are four to five inches in depth can last for up to a year. 

An awareness of environmental factors in your home and neighborhood that can affect your furnace filter will help you determine how frequently you should change your filters.

Common factors that can affect how frequently furnace filters should be changed include the following:

  • Dust and other particles from building and construction
  • Pets
  • Number of people in the home
  • Indoor air pollutants
  • Age and condition of your furnace
  • Ventilation

Steps can also be taken to clean and maintain the filters to extend the time between filter changes.

Some of these steps include the following Maintaining a clean and ventilated home can reduce the amount of dust and pollen that will pass through the filter.

It is also possible to wipe down vents and registers in order to remove loose buildup and extend the life of your filter. 

Determining When to Change Your Filter

Although the given time frames serve as an excellent starting guideline as to how frequently you should change your furnace filter, there are a few clear indicators that will let you know when your filter has become too clogged.

The first and most obvious comes from simply looking at the filter.

A filter that is ready to be switched out will be gray and covered in buildup, preventing air from easily passing through it.

A restriction of airflow may also be noticeable through an increase in the heating time of your furnace.  

In Summary

A properly fitting filter along with the right thickness and density for your furnace needs makes a difference in both the operation as well as the maintenance of your furnace.

A furnace filter that is either too small or too big for your furnace will allow air to pass by the filter.

But too thick of a density will risk blockages to airflow and can greatly increase the necessary energy and the associated costs. 

Choosing the right type of filter for your furnace along with the routine maintenance mentioned above will keep your costs low and your furnace running great.

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