How Often to Change a Furnace Filter – The signs.


Changing your furnace filter regularly is essential for maintaining good indoor air quality and ensuring the efficient operation of your HVAC (Heating, Ventilation, and Air Conditioning) system.

Here are signs and indicators that it’s time to change your furnace filter:

  • Time Interval: The frequency of filter changes depends on factors such as filter type, indoor air quality, and system usage. A common guideline is to check and potentially replace the filter every 1 to 3 months. However, you may need to change it more frequently if:
    • You have pets: Pet dander can clog filters quickly.
    • Allergies or respiratory issues: Individuals with allergies or breathing problems may benefit from more frequent filter changes.
    • High system usage: If your HVAC system operates year-round, consider monthly filter checks.
  • Visible Dirt and Dust: Inspect the filter visually. If it’s covered in dirt, dust, and debris, it’s time for a replacement. A dirty filter restricts airflow and diminishes its filtering effectiveness.
  • Reduced Airflow: A significant drop in the airflow from your vents can indicate a clogged filter. Reduced airflow may result in uneven heating, longer heating or cooling cycles, and discomfort in your home.
  • Increased Energy Bills: An overworked HVAC system due to a clogged filter can lead to higher energy consumption and increased utility bills. If you notice a sudden spike in energy costs, check the filter.
  • Visible Mold Growth: If you see mold growth on the filter, it’s crucial to replace it promptly. Mold spores can be harmful to indoor air quality and your health.
  • Unpleasant Odors: If your home develops unusual or musty odors, a dirty or clogged filter may be to blame. Replacing the filter can help alleviate odors caused by trapped contaminants.
  • System Overheating or Short Cycling: A clogged filter can cause your HVAC system to overheat or short cycle (turning on and off frequently). These issues can lead to system malfunctions and costly repairs.
  • Visible Damage: If the filter is damaged or warped, it should be replaced immediately. A damaged filter can allow unfiltered air to bypass the filter, reducing its effectiveness.
  • Age of the Filter: Some filters come with a manufacturer’s recommended lifespan. If you’re unsure about the filter’s age, it’s wise to replace it if you can’t verify its condition.
  • Manufacturer’s Recommendations: Always follow the manufacturer’s recommendations for filter replacement intervals and type. These guidelines are specific to your HVAC system and filter type.

How Long Does a Furnace Filter Usually Last?

Furnace filters can last for 3 months or even longer. There are also those that will not last that long especially if the house attracts dust from outside, has lots of pets, there are many people living in the house among other factors. That is why you should inspect the filter on a monthly basis.

If your furnace filter gets dirty too fast, it means that the furnace runs often, hence more particles are being trapped. Other reasons could be that there are lots of pets shedding far and dander inside the house, there many people living in the house or that the house is attracting a lot of dust from outside.

One-inch furnace filters usually last from 1 to 3 months. A three to or 4-inch filter will last from 6 to 9 months. Thicker (5 to 6-inch) furnace filters can last from 9 to 12 months. Furnace filters with a low MERV rating will last for a longer time than filters with a higher MERV rating.

How to Tell if a Furnace Filter Needs Changing

As I have already mentioned, it is important to inspect you furnace filter at least once per month and just check if it is good working condition or if a replacement is necessary.  But other than that, how can you tell if the filter needs to be changed?

In other words, what are the signs/symptoms of a dirty furnace filter? The following are the signs that your furnace filter is dirty and that it should be changed:

  • The filter is black (it is usually white) or a different color that when it was installed.
  • Energy bills are unusually high. The system is working hard to force air through the dirty filter thereby consuming more power.
  • Reduced air flow. When the filter is clogged/dirty, the amount of air passing through it will reduce and hence the air being circulated back to the house will be low as well.
  • Increased physical problems like fatigue, headaches, asthma and allergies.
  • The HVAC system is running for a longtime.
  • A burning smell in the house (usually caused by an overheating blower motor or even the filter itself).
  • Dusty/unclean indoor air.

Why it is Important to Change a Furnace Filter


Apart from heating and cooling the air in your house, an HVAC system is also designed to clean the air so that the indoor air is free from dust particles, lint, pollen and other impurities. And that is where a furnace filter comes in.

As the air is being pulled from the house to the furnace, it first passes through a filter where all these impurities are removed before being heated or cooled and then supplied to the house. So, why is it important to change the furnace filter?

A filter is basically a restriction with very fine pores that traps solid particles and only allows in air. As is the nature of fine pores, they are easily and gradually clogged by the particles they have trapped.

As the furnace filter traps more particles, it becomes harder for air to pass through it and hence the need to replace it regularly.

Regularly changing the furnace filter ensures that you have constant supply of not only heated but cleaned air. Clean and fresh air ensures that you don’t suffer headaches, asthma or even allergies as the triggers will have been filtered out.

If a furnace filter is not changed, it will eventually get clogged and restrict the flow of air to the furnace forcing the blower to work harder to pull in the air. That means the furnace will have a long runtime, ultimately increasing your energy bills and sometimes the unit can even get damaged.

Factors Affecting the Frequency of Furnace Filter Replacement

Although I have said that the furnace filter should be replaced quarterly, that is not cast on stone. In some instances you will need to do it sooner than that.

The following are the factors that affect how often a furnace filter should be replaced:

1. State of the House

Do you have a newly constructed or a remodeled house? If your answer is in the affirmative, it means that there will be lots of dust accumulated inside the house, some that are so fine that they are hard to notice.

In that case, you will need to replace your furnace filter more often (ideally monthly) for the first few months until all the dust has been filtered out. After that you can go back to the 90-day schedule.

2. The MERV Rating

Furnace filters have a rating (between 1 and 20) called a MERV rating which is an industry standard. This rating basically indicates how effective the filter is in trapping particles.

Furnace filters with the highest MERV rating can remove up to 95% of all particles passing through them but that means that they clog fast and hence need frequent replacement. The opposite is also true.

Check the MERV rating of your furnace filter and take note. (Residential filters have a MERV rating of 8 to 13). The higher the rating the sooner you will need to change it.

3. The Size of the Filter

Furnace filters are not of the same size. Their sizes are clearly printed on them though. The smaller the size the sooner you will need to replace them and vice versa.

The following is the general guidance on when to replace a furnace filter depending on size:

  • 1 to 2-inch furnace filter – 1 to 3 months
  • 3 to 4-inch furnace filter – 6 to 9 months
  • 5 to 6-inch furnace filter – 9 to 12 months

4. Number of People and Pets in the House

As you would expect, the more the people a house has the more they bring dirt from outside. All of that dirt will end up in the furnace filter.

Therefore, a house with about 6 people will needs its furnace filter changed more often than a house with 1 or 2 people. The same also holds for pets.

Pets are always shedding far and dander. The more the pets the more the far and dander being deposited on the furnace filter and hence the need for constant replacement

5. Quality of Indoor Air

If your house has low quality of air by virtue of letting in dirt particles from outside, your furnace will need frequent replacement. Equipment like air cleaners and purifiers help in maintaining indoor air quality but if they are not available then the dirt from outside the house will find its way on the furnace filter.

6. Your Health Condition

If you or a member of the family suffers from asthma or allergies, the furnace filter will need to be replaced regularly (ideally monthly). The filter plays a big role in preventing the introduction of the allergy triggers in the house so it needs to be in proper working condition.

7. Rate of HVAC Usage

If your home’s HVAC system is used all the time (or just heavily used), there will be more air circulation between the house and the furnace. Every time air is circulated is an opportunity for the filter to trap particles.

In that case, you can’t expect the filter to last longer than a house whose HVAC system is moderately used.

How to Change a Furnace Filter

The first thing you will need to do in order to change the furnace filter is locate it. Surprisingly, there are lots of homeowners who can’t tell where their furnace filters are located.

1. Locate the Filter

Furnace filters are located in the return air duct. That is the air duct that carries air from the house to the furnace. It could be just behind the return air vent/grill or very close to where it enters the furnace.

Inside the house, there will be return and supply air grills. The return air grill is big in size and if you put your hand near it when the unit is on you can feel a suction force.

You only need to pull the return air grill’s tabs to remove it and locate the filter. The grill is usually on the ceiling but could also be on the wall or a utility closet.

If the filter is not behind the return air grill, head to where the furnace is located (basement, attic, crawlspace) and check for a slot with a cover (usually 1 inch) where the return air duct connects to the furnace.

If you have a vertical HVAC unit, the return air duct connects to the furnace from the bottom. On the other hand, if you have a horizontal HVAC system, the return air duct connects to the furnace from the left-hand side.

2. Replace the Filter

In the case of a filter being located behind the return air grill, you will only need to pull the tabs to release the grill then pull out the filter.

Note: It is very important to first turn off the HVAC system before removing the filter. Otherwise the dust in the filter will end up being sucked inside the furnace.

If the filter is in a slot next to the furnace, just slide it out and insert the new one, making sure that it is the same size as the old one.

Important: Make sure you check the arrow indicating the direction of air flow. The arrow should always point towards the furnace and never away from it.

Once the filter is replaced, you can put back the grill/cover and turn on the system.

Wrap Up

And basically that is every about changing the furnace filter in your house. I really hope that this guide was helpful.