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A dirty furnace filter gets discarded

Is Your Furnace Filter Dirty?

Sean SVerified

HVAC Owner/Technician

Residential HVAC

292 positive ratings
Clogged filters

A clogged filter can increase your utility bill by at least 15%. The blower fan must work harder to push air through the dirty filter.

An HVAC air filter has two jobs: to protect your furnace and all its components from becoming dirty and to protect you and your dwelling from inhaling the same particles. Keeping them clean will extend the life of your HVAC system and improve the indoor air quality.

A clean air filter catches and removes particles from the indoor air. If the filter becomes clogged, allergens are recirculated throughout your home. When they are dirty, you may even notice more persistent asthma or allergies. Residents may also get more cold or flu-like symptoms.

How to change a dirty furnace filter

Most homeowners can change furnace filters on their own. However, if you have any trouble ask a certified HVAC technician for help.

  1. First, ensure you have the right type of filter and it is not damaged. Filters come in essentially three models: pleated paper, fiberglass, and electrostatic. Electrostatic models can either be disposable or reusable. Most homeowners will use the paper or fiberglass type of furnace filters because of their inexpensive cost. If you want to tell the efficiency of different filters on the market, look at their MERV ratings. These span from 1 to 16, with 16 being the most efficient and 1 being the least. Once you've decided which is right for your furnace, turn the unit off.

  2. Make sure the filter is the right size. If you have the right size, there should be no gaps after it is installed. While removing the dirty filter, inspect it for any numbering that indicates its size. (16” x 25” x 1” or 20” x 20” x 2” are typical dimensions.)

  3. Inspect the filter for the direction it is meant to be installed. If it is installed backwards, the upstream and downstream air flow will be compromised. You want the arrow pointing in the same direction as the air flow – from the return duct and towards the plenum.

  4. Wipe off any dust you can see before installing the new filter. Replace any levers or gaskets you may have removed.

  5. Mark the date you replaced it in a place where you can see it easily. Filters typically need to be replaced seasonally, or every 1-3 months with heavy use.

  6. Install the new filter, then turn the unit on while the door cover is open so you can see it working correctly. Afterward, make sure to close the door cover.

When to ask a technician

There will be problems that will need the service and experience of a professional. These can include a slow-moving or noisy fan, excessive moisture, or a unit that will not turn on. Unless you have the requisite experience, do not chance it without professional help.

What causes furnace filters to get dirty faster?

Furnace filters usually state how long they are designed to be used for. Make sure to read the expiration date and promptly remove a furnace filter if it's been used and is past its prime.

Diagram of typical furnace

Cooler temperatures can cause buildup faster

When the temperature rises or drops, the furnace will cycle more often, pulling more air through. This causes more dust and debris to get caught in the filter, reducing its effectiveness. You should be checking it once a month regardless of how long the filter is supposed to last.

Dirty returns and dirty ducts are another major cause of dirt

Because returns and ducts are the conduits through which air passes, it’s important to clean them periodically. Cleaning the air filters without cleaning the returns and ducts is like laundering clothes and storing them on a dirty rack.

Check your thermostat settings if the fan is running constantly

Your fan setting should be set to “AUTO.” If it is set to “ON,” expect the same results as your furnace blower cycling too often: the furnace pulls air through the filter without automatically turning off, clogging it faster. Plenty of new, high-tech systems have a circulation mode that allows airflow to move slowly throughout home, giving the feeling of a constant temperature.

Return vent dirty or blocked     

The return vents are covers for the openings in your walls, and are connected to your ducts. When your air conditioner is running properly, it sucks the air from your rooms into those return vents, through a series of ductwork and back to your heating and cooling system. Any dust or debris located in around the return vent will be pulled into the furnace filter. The solution is to keep the area around the return vent clean for unimpeded air flow. You can check the air quality yourself to make sure that the flow is not obstructed.

Extra contaminants

For a home with a large number of people living in it, expect a greater number of contaminants to be pulled through the furnace. This is also true if there are pets living in the home. Dirt, pet hair and dander will clog up a filter fast. If this is the case in your home, consider purchasing an extra indoor air filtration system.

A microscopic view of pollen

How often should an old filter be changed?

In an HVAC unit that is used for heating and cooling, you should change the furnace filter every one to three months. Pleated kinds can usually last for three months, while fiberglass filters should probably be changed every month. Models with plastic frames are reusable and can be periodically cleaned with water, a cloth, and time (to air-dry).

There are scenarios where filters will need to be changed every month:

  • If your home has high indoor humidity.

  • If you have pets.

  • A large family.

  • If anyone smokes indoors.

  • If anyone in the home has asthma or allergies.

  • If you have a fireplace or live in an area prone to wildfires.

  • If you live on a farm or have a large garden near your home.

  • If there is any construction going on in or near your home.

Change immediately if any of the following occur:

  • The filter has become damp or wet; this can cause mold and lead to severe health problems.

  • It is damaged at all; this can allow unfiltered air to recirculate into the home.

As seen above, there are many problems that can occur due to neglect. If your heating system is having issues, and you have checked your filters, contacting an HVAC professional for repairs or maintenance can often help save time and money.

A typical air filter

What happens to a furnace with a dirty filter?   

When a filter is dirty and clogged, it makes the furnace and air handler work harder. This, in turn, increases your energy bills, wearing out your furnace faster. Prolonging this state can also affect the heat exchanger and the limit switch, causing them to constantly cycle (turn on and off).

Furnace Failure

It may be hard to believe that a component as small as a dirty filter can cause your heater and air conditioning system to fail. It is, however, the number one cause of furnace failure. Simply changing components on schedule can prevent thousands of dollars in repairs or replacing an entire furnace system.

Frozen evaporator coils

During the cooling season, the condensation that builds up is normally evaporated by the air passing over the coils. If a blocked filter is not changed and air can't pass over the evaporator or cooling coils as intended, they may freeze. This will eventually cause the HVAC system to malfunction and need to be repaired or replaced.

Hot or cold spots in the room

A clogged air filter will decrease the amount of air the blower fan can push through the house. This can cause cold or hot spots in different areas or rooms of the house as air is blocked from circulating. The fan will also continue to run until it meets the desired temperature. Until the dirty air filter has been replaced, your utility bill will increase.

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