Dirty vs. Clean Air Filter

Why it’s important to replace your furnace air filter regularly.

Myth #1 – The main purpose of the furnace air filter is to keep the air clean in the home.

Myth #2 – You should buy the best filter available.

Both of these statements are false, or at minimum, only partially true.

First and foremost the purpose of a furnace filter is to protect the furnace from damage caused by dust, lint, or foreign objects that find a way into the airstream. Without filtering, these particles may cause damage to the furnace or, at a minimum, reduce the overall furnace efficiency; which will cost you more in energy. Filtering prevents dust and other particles from collecting on the heat exchanger causing excess heat inside the furnace; clogging up the air conditioning coil which reduces the air flow to the house and causes the blower to work harder; or collecting on the fan blade or blower motor causing it to overheat or have electrical damage. Repair of any of these items is expensive and can be avoided by ensuring a filter is in place and receives regular maintenance.

A secondary purpose of the air filter for many households is to improve the air quality by filtering out dust and other small particles, such as pollen.  However, the furnace is not designed as an air quality device so you may not be getting the benefits from a premium air filter that you are led to believe you’re getting. More on that topic in another article; this article is focused on the primary purpose of protecting the furnace.


A clogged filter may cause damage to your furnace.

Dirty vs. Clean Air Filter

Dirty vs. Clean Air Filter

So we established that a filter is needed to protect the furnace, however that same filter may actually cause damage if not maintained. When the air filter gets clogged with dust and other particles, the air flow through the filter is restricted and damage to the furnace and AC unit is likely.

  • Blower (fan) motor failure is common. The restricted air flow causes the blower motor to work harder. This reduces the life of the motor and may cause it to fail, i.e. burn-up. The “burn-up” simply means the motor overheats, which often results in permanent damage to the internal electrical windings or other components. Replacing a blower motor is a common repair and can cost $200 – $500 in parts and labor.
  • In summer months, restricted air flow through the AC cooling coils inside the furnace unit may cause the unit to drop below freezing temperature and “ice up”. When ice forms on the fins of the coil inside the furnace compartment, then the outside air unit must work harder; this may damage the compressor. When the ice melts inside the furnace compartment, the water may cause damage to electrical components or over longer periods of time cause metal to rust and deteriorate. In the best case scenario the iced-up coil can be fixed with a service call. If other components are damaged, then significantly higher repair costs can be expected.

If that’s not reason enough to maintain your air filter, consider the extra energy cost due to reduced efficiency when the air flow is restricted.

The good news is that these costly repairs can often be avoided by replacing, or cleaning if reusable type, your air filter on regular basis.  How often depends on the type of filter, the usage of the furnace, and the conditions in the home or business. More on how frequently to maintain your air filter is discussed later, first let’s talk about the different types of filters.


What type of air filter do I need?

While there are many different types and grades of filters available, most are effective at filtering larger particles, which is the primary concern for protecting the furnace. Refer to your furnace owner’s manual for the manufacturer’s recommendations for type and size of filter to use. The original furnace manufacturer is the best source of information on the type of filter to use as each furnace is designed for a given air flow and using the proper filter will help maintain that air flow. If you are not sure what your manufacturer recommends then contact us. We will evaluate your system and suggest a filter that will meet your needs.

As a general rule, for newer model furnaces the pleated filters work well at trapping the airborne particles. However, for an older style furnace, the pleated filters may be too restrictive on the air flow and thus a woven fiberglass filter is a better choice.


How often do I need to maintain the filter?

How often depends on the type of filter, the usage of the furnace, and the conditions in the home or business.

  • A higher efficiency filter will need replaced more often since they are better at trapping particles and tend to clog up faster.
  • During peak operating months the filter may need replaced more frequently than during months of lower usage. Remember to check your filter during summer months as well.
  • The environmental conditions of the home or business may require changing the filters more often. This is often true during a remodeling project when dust is more prevalent, or if you are sensitive to allergens.
  • Pet owners should consider changing filters more often as pet hair will tend to collect on the filter and cause it to clog up.

A general rule is check the filter monthly and replace (or clean if reusable) if needed.



If you need assistance with replacing your furnace filter, please contact us and we’ll be happy to send a service technician to you.

Thank you,

David McGill and James McGill

Article written by Tim McGill,editor @ Tree Branch Publishing.

Posted in Cooling, Furnace, Heating, Preventive Maintenance and tagged , .


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