How Often to Change HVAC Filter

Q: How often should I change the filter in my furnace or HVAC system?

A: Your air filter should be changed every 30 to 180 days as needed. If the HVAC system runs frequently, you have shedding pets and your home is large, then you will need to change it on more frequently.

This guide discusses those factors and a few more plus the importance of maintaining good airflow in your heating and air conditioning system.

How Often Should I Change My Air Filter?

Saying you should change your air filter every 30 to 180 days is a wide range. These factors determine when to change your HVAC filter:

  • How much your HVAC system runs
  • Whether you have shedding pets
  • If allergies, asthma or breathing diseases are a problem
  • Whether your filter is a thin fiberglass type or a thicker pleated type
  • The size of your home
  • Whether you use the system’s Continuous Fan mode

Let’s explore each of these factors.

How Much your HVAC System Runs

This is the most important factor by far. The filter collects dust and debris only when the system is running. The more it runs, the faster the filter collect dirt. During a bitter-cold Minnesota winter, a furnace filter will need changing more often than during a mild Missouri winter.

On the other hand, you will have to change the filter more often during a hot, humid Missouri summer than during summer in Minnesota. Remember that a filter gets dirty during AC mode too.

Whether You Have Shedding Pets

Do you find pet hair in your furnace filter? Dog, cat, rabbit and other mammal hair fibers are large compared with the dust and dirt normally trapped by the filter. That means they clog the filter and block airflow pretty fast.

The more pets you have, the more often you will have to change the filter: 1 shedding pet = Maximum filter length of 90 days; 2 pets = 60 days max; 3 or more pets = up to 30 days. If you’ve got 3 shedding dogs in a hot stretch of summer or brutally cold winter, your filter might require changing in less than 30 days.

If Allergies or Asthma is a Concern

The more important clean air is, the more often the filter should be cleaned or changed.

A thick media filter, MERV 11 or higher, or an electronic air cleaner is a better choice when anyone in the household has asthma, severe allergies or comprised breathing for any reason. The air will be cleaner than if a basic fiberglass air filter is used. You might also want to consider an air purifier for your system.

Our Guide to Air Purifiers has complete information on all types including media and electronic air cleaners, how they work, which is best for your purpose, and more.

The Filter Type

Cheap fiberglass filters require changing less often than thicker pleated filters.

Some sites get this exactly backwards, claiming that thicker filters can go longer between changes. That goes against the design of the filters. Thick media filters trap more and smaller particles of dust, so obviously get clogged faster.  

The thin glass fiber filters don’t trap as much dirt and debris, so don’t clog as quickly.

The good news is that some of those filters can be cleaned with a nylon brush and put back into the furnace or air handler rather than replaced. The cleaning should be done outside or in a garage to keep the dust and dirt out of your home.

If you do brush off the dirt rather than replacing the media filter, be sure to change the filter after two or three cleanings. Eventually brush-cleaning it won’t remove enough deeply embedded dirt and debris.

The Size of Your Home

In larger homes, more air flows through the filter than in smaller homes. Since air carries the dust, pet hair and other debris, the filter will become dirty more quickly in a system serving a large home.

If You Use Continuous Fan Mode

Some of you are wondering what we mean by Continuous Fan mode and why a homeowner would use it. There should be a setting on your thermostat, such as options for Auto and Fan. In Auto mode, the fan runs only when the system is heating or air conditioning your home.

In Fan mode it runs all the time until you turn it off. The more the system runs, even if it isn’t heating or air conditioning, the faster your filter will get dirty and need changing.

Is Continuous Fan mode a good idea? Here are the pros and cons.

Fan mode vs. Auto mode does several things. First, it cleans the air by running it through the air filter more often. As a result, the filter will need changing more often. Secondly, it helps balance temperatures in your home. Thirdly, if you have a basement, it can pull cool air up from the basement to help cool the upstairs.

There are downsides too. Running the fan takes electricity, so you’ll see a jump in your electric bill. Secondly, air in the basement is usually more humid than air upstairs. Humidity makes you uncomfortable in summer, so you’ll need to lower the thermostat setting to get rid of the humidity. This is counterproductive, causing the AC to run more frequently.

You might also pull hot air from inside the walls or attic into living spaces with the same result – your AC will run more than it should, resulting in higher electricity costs.

In short, we do not recommend Continuous Fan mode as a standard operating procedure.

How to Know if  Your Air Filter Needs Changing

This is the real question, isn’t it?

The answer to the question, “How often do I really need to change the HVAC filter” is “it depends!”

This question gets to the heart of the issue.

Take these steps to know when to change your air filter.

  1.   Check it 30 days after it is installed
  2.   If the filter has only a fine layer of dirt and debris on it, the filter is OK. In fact, the thin fiberglass type filters don’t filter the air very well until they do have a bit of debris on them. The key is not to let the dirt buildup become too thick.
  3.   If debris is building up on it, check it in another 15 days. Pull it out when the system isn’t running. Hold it up to light. If little or no light comes through, it is time to clean or change the filter.
  4.   If little debris is accumulating at the first 30 days, wait another 30 days to check it.

Signs your HVAC filter needs to be changed immediately:

Wheezing sounds: When the filter is so clogged that air isn’t getting through it, the blower motor will suck air in through tiny gaps in the unit’s door and cabinet. A wheezing sound will result.

You might hear a little wheeze even when the filter is clean. It will be worse when the filter is dirty. Get to know your system and what sounds it makes, so you’ll know what noises indicate a problem.

Musty odor: Do you know that musty, dusty smell old vacuum cleaners produce when they’re running? If you smell that kind of odor coming out of your air grates, it means the filter is quite dirty.

Poor performance: Are some rooms not getting enough heat or air conditioning? Is less air coming out of the grates than is normal?

While these signs could mean there’s a problem with the blower, the more likely issue is a dirty filter. It is blocking airflow into the system. If enough air is not flowing in, there won’t be sufficient outflow of heated or cooled air.

Why It is Important to Change your HVAC Air Filter

There are several reasons to keep your air filter clean. If you don’t, these problems will occur:

  • System damage
  • Mold in ducts
  • Dirty air

Here’s more complete information.

System Damage: A dirty filter causes your HVAC system to work too hard to pull in air. This will cause high heating and AC bills, poor performance and eventually mechanical failure and costly repair or replacement bills.

Mold in Ducts: When airflow is reduced, dust and dirt build up inside the ductwork. The particles absorb moisture. The combination leads to mold in your ducts, and mold spores are blown into your home. This can cause illness for those with allergies, asthma and breathing issues like COPD. It also puts infants, the elderly and pets at risk.

Dirty Air: Eventually a dirty filter might start letting dirt and dust escape, pushing allergens into your home.

These dangers are easily avoided by checking your filter regularly and changing it when necessary.

When in doubt, switch it out! Clean air, lower energy costs and the longevity of your HVAC system are worth the minor cost of a new filter.

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