How often should you change a 4 inch furnace filter

The easy answer, but not the one we’re going to give you, is:

You should change your furnace filter every 3 months.

That is what most manufacturers recommend on their packaging. But does a furnace filter need to be changed as often during a “chilly” South Carolina winter as during a sub-zero Minnesota winter?

Obviously not.

In summer, the situation would be reversed. The filter in sweltering SC will get dirtier faster than one in mild MN.

A furnace filter should be changed as often as needed. And climate is just one factor determining how long a furnace filter lasts.

This FAQ guide – How often should you change your furnace filter – gives clear advice related to:

• Climate
• Household factors
• Allergies
• Cleaning a disposable AC/furnace filter

Climate & Filter Change

Here’s a US Hardiness Zone map that comes in very handy for many of our HVAC guides and informational articles. It shows 10 temperature zone based on lowest temperatures reached during an average winter.

Here is our pro recommendation based on climate alone:

In Winter

Zones 1-3: Change your furnace filter every 6-10 weeks

Zones 4-7: Change your furnace filter every 2-3 months

Zones 8-11: Change your once a heating season

In Summer

Zones 1-3: Change your furnace filter once a cooling season

Zones 4-7: Change your furnace filter every 2-3 months

Zones 8-11: Change your furnace filter every 6-10 weeks

The bottom line is clear – the more your furnace or air handler runs, the faster it will become dirty and clogged. That means changing it more often.

But it’s not just climate. There are other important considerations.

Household Factors

Here are a few guidelines for changing a furnace filter based on who lives in your home and related factors.

Number of people: The more people there are in your home, the more often the filter should be changed. People “shed” like pets, right? And dust is mostly made up of shed skin from humans and pets.

• 1-2 people: Slightly less than needed for your climate
• 3-5 people: Average for your climate
• 6+ people: More than average for your climate

Pets: As you probably know, your furnace or air handler filter picks up pet hair. How do pets affect when the filter should be changed.

• No pets: Change your filter according to the climate schedule
• 1-2 pets: Change your filter a week or two earlier
• 3-4 pets: Change your filter two to three weeks earlier

Housecleaning: The more fastidious you are about dusting and vacuuming, the less hair, pet fur and other debris there will be to get trapped by the filter.

Smoking: Smoke from cigarettes, cigars, pipes, etc. all circulate through the furnace and make the filter smell like smoke. Even when nobody is smoking, the air drawn through the filter will send a smoky smell around your home. This necessitates a filter change.

Are there smokers in your household? Perhaps an air purifier designed to capture smoke odors would make your home more pleasant – and extend the life of your furnace filter.

Allergies

While furnace filters aren’t the best defense against allergies, they certainly help to remove dust, pollen, pet dander and other allergens from the air you breathe.

No allergies: If nobody has significant allergies, then let climate and household factors be your guide.

Mild allergies: If members of your household have mild allergies, then consider changing the filter a few weeks earlier than you otherwise would.

Severe allergies: Changing the filter every month is a good idea for those with serious dust/pollen/dander allergies and similar.

There is more to combatting allergens than how often you change your filter. It is also important to use a filter with a higher MERV rating. We discuss filters for allergies in our Best AC and Furnace Filter Guide.

As with smoking, a furnace filter isn’t the most effective method of removing allergens from the air. Consider an air purifier with a True HEPA filter designed for trapping allergens. There’s more information in our Best Air Purifiers for Allergies Reviews and Buying Guide.

An Alternative to Changing your AC/Furnace Filter

Would you like to get more “mileage” from your air handler or furnace filter? You can, if you’re willing to clean it.

We’re not talking about permanent, washable filters. This is a discussion of cleaning your disposable fiberglass or pleated media filter.

Cleaning a spun fiberglass filter: Spun fiberglass filters are cheap, so cleaning one might not be worth your time. In case you’re wondering, they look like this:

Additionally, they don’t do a very good job trapping allergens and dirt in your home. An upgrade to a pleated filter with a MERV rating (most brands), MPR rating (Filtrete equivalent of MERV) or FPR rating (Honeywell equivalent) will improve air quality in your home.

If you clean your home consistently and don’t have heavy-shedding pets, then a spun fiberglass filter might be OK.

To clean a fiberglass filter:

• Take it outside.
• If it is dirty enough to have a layer of debris on it, like a dirty lint filter in a dryer, then use a nylon brush to gently remove the layer or layers.
• Gently tap it on a hard surface, dislodging more dust and debris.

Why is being gentle important? Because these cheap furnace filters don’t have much structure – just a flimsy cardboard frame and maybe some thin wire to keep the fiberglass in place. They are easily damaged. When the frame bends, it is hard to get the filter back into the slots.

Cleaning a MERV filter: A minimum efficiency reporting value, or MERV, filter captures much more dirt and debris than fiberglass. They are discussed on our Best Filters Reviews and Buying Guide.

To clean a MERV (minimum efficiency reporting value) filter:

1. Take it outside.
2. Use a nylon brush to remove the debris layer. It’s OK to use moderate pressure. You’ll easily see whether the force is damaging the pleats. If so, back off a bit.
3. Vacuum the filter using a stiff brush vacuum accessory.
4. Tap the furnace on the ground to see if any more debris comes out of it.

Compressed air works great: If you have an air compressor, it is an excellent tool for cleaning either type of filter, but especially a MERV filter. Use compressed air from the opposite direction as the airflow indicated on the furnace frame. 

In other words, hold the filter with the arrow pointing toward you, and then blow compressed air onto the filter away from you (on the “clean” side of the filter). You should see dust and lint flying off the dirty side of the filter. This video shows how to clean your air filter with compressed air.

Allergy & asthma alert: If you have allergies or asthma, wear a quality dust mask for this job. Or better yet, ask someone without breathing issues to do it. Encourage them to wear the dust mask for protection to their breathing passages and lungs.

Cleaning an AC/Furnace filter will make it last two to three times as long. Eventually, fine dirt particles will clog the filter. They are tough to remove, and it will be time to change your filter. Clues include:

• Cleaning it with a brush or air compressor, and it still looks very dirty
• Your energy bills go up – a clear sign the filter is clogged and making the AC or furnace work too hard
• Your “Dirty Filter” notification light, if your thermostat has one, comes on soon after you clean the filter
• Your furnace or air handler starts “whistling,” a sign it is sucking air in through places it shouldn’t because the filter is too dirty to allow sufficient airflow

The Eye Test

As time goes on, you’ll become familiar with the look of a filter that is due for a change. Clean it, if possible, and put it back to work. Eventually, cleaning it won’t be enough.

If you don’t want the hassle of cleaning a disposable filter, and many homeowners feel that way, then look at the filter every week or two once it has been in the furnace a month or two, depending on how heavily it has been used.

Change it when it looks like it can’t trap any more debris without decreasing airflow to your furnace or air handler, a situation that will raise energy costs and create the greater chance of a mechanical failure.

When in doubt, switch it out.

Furnace and AC filters are inexpensive compared to major furnace or air handler repairs. Failing to change the filter on time can lead to costly mechanical failures and/or soaring energy costs. However, changing it more often than is necessary is a waste of money.

Find the balance using the tips here. 

How Often to Change Furnace Filter Based on Size

The reason most sites say to change your filter every 1-3 months is that more than 75% of filters used are the thinnest type, usually referred to as 1 inch (one inch, 1-inch) filters. And most searches are about how often to change 1 inch filter.

However, HVAC filters range in size from 1 to 6 inches, and they have different longevities. And a lot of homeowners want to know how often to change 4 inch furnace filter and filters of other sizes.

Whether it is a 1 inch furnace filter or AC filter or a 4 inch furnace filter or AC filter, the answer is still, “change your air filter as often as needed.”

1 inch filters: For a 1 inch furnace filter, change it at least every 3 months. But replace it more often if it gets dirty faster than normal due to frequent running of the HVAC system or due to household conditions.

4 inch filters: The answer to how often to change 4 inch furnace filter is similar. Under normal use, change a 4 inch air filter / HVAC filter every 6-8 months. But check it at about 4 or 5 months to see if it is dirty enough to require changing.

Check your filter every month to be sure that you’re changing it when needed. As we’ve said, the filter will get dirty faster during heavy use. So, in mild times of the year, you won’t need to change it as often as when it is extremely hot or cold outside and your HVAC equipment is working hard.

Remember: The more your furnace or air handler blower motor runs, the faster the furnace gets dirty and the sooner the filter needs to be changed. 

Written by Rene Langer

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree from Lone Star College and EPA & R-410A Certifications.

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