How, Why and When to Change Your Furnace Filter

How, Why and When to Change Your Furnace Filter
Mark Wilson
HVAC Content Writer

Working with our subject-matter experts, I cover every corner of the HVAC industry for Fire & Ice, educating our customers to help them make the best decisions for their homes.

About This Article

If you want to keep your HVAC systems operating at peak efficiency, make sure to change your furnace filters regularly.

Everyone has to have a furnace filter. And every filter needs to be changed.

Lots of homeowners let this go too long. And when this happens, problems occur. They might not happen right away, but it’s only a matter of time. And even before those obviously bad things (like your system failing entirely!) you’re going to have a less efficient system.

So let’s fix this right now.

We’re going to talk you through choosing a filter, what filter ratings mean, what you can expect if you don’t change your filter on time, and how to take care of the filter change yourself. By the end, you should have a years-long plan for managing your filtration in a way that will benefit your home and family the most.

Choosing a Furnace Filter

Your owner’s manual is a good place to start for filtration requirements and limits, but this manual won’t always provide all the options available to you.

But, I hear you thinking, a filter is a filter, right? Not always. While it’s true that any old filter will capture some particles that travel through your HVAC system, filters are not created equal.

Just as importantly, they’re not equal in multiple ways. Most people think of how many particles a filter captures, but there are other relevant factors to consider as well. These factors include:

  • Size of the filter in relation to your system
  • How much of the filter is uncovered by cardboard, plastic, or other materials
  • How much (or little) a filter allows for airflow in your system (this is a big one)
  • Lastly, its ability to capture small particulates, which relates to your ability to breathe easy when it comes to allergy and flu season

Filter size is something that can be discussed with your HVAC partner. Your options may include store-bought filters or others that your HVAC partner has available to them through various manufacturers.

Filter design is also important. Some filters are designed with a criss-cross pattern of cardboard or other material that limits the surface area of a filter. This can reduce the efficacy of a filter significantly, even if you’ve upgraded it. Design matters, which is why cheaper filters are often cheaper

Proper airflow in your system is vital. We’re going to talk more about this later, but it’s a central reason to clean or replace your filters in a timely manner.

Filter rating can definitely matter, and it’s often what gets the most attention. To discuss it, we need to talk about MERV Rating.

Replacing a furnace filter

MERV Rating

MERV stands for Minimum Efficiency Reporting Value, which is a long-winded way of saying that it’s based on efficiency. The higher your MERV Rating, the more efficient it will be at capturing particles of varying sizes.

Smaller particles tend to be the most dangerous because it’s easier for them to enter your lungs. So a higher MERV Rating, in theory, can help with this.

Higher MERV ratings are generally thought of as “better,” but I put that in quotes because it’s not always true. The density of a filter matters. So a one-inch filter that’s MERV 13 is very different from a four-inch filter that’s MERV 13. The one-inch version will be denser, so it will inhibit airflow.

This is sometimes a problem even with lower MERV-rating filters. If they’re too thin, they’re packed too tightly to allow for proper airflow.

However, as long as airflow is accounted for, higher MERV is indeed usually better. In fact, recent studies have shown that MERV 13 filters are required to reliably stop certain airborne viruses.

RELATED: Can HVAC Protect Against COVID-19 and Other Illnesses?

RELATED: N95 Masks, HEPA Filters & COVID

Why Change Your Filter

We touched on a handful of reasons earlier, but I want to talk about some of the long-term effects of neglecting your filter.

Our service techs will sometimes have to “repair” a system that has fully shut down, and the only immediate problem is the filter. That’s right; a worst-case scenario is a complete system shutdown.

That’s rare, to be clear. But there are a lot of intermediate problems that poor filtration can cause that will ultimately lead to costly repairs or full system replacements.

One example is static pressure. It might sound harmless, but it affects the entirety of your HVAC system. When a filter becomes packed with detritus, the air won’t move through it easily. This creates airflow backups in the system, which in turn makes your system run harder than it would otherwise need to. The downsides of this are myriad.

Short-term Downsides:

  1. Pockets of hot and cold spots in your home
  2. Lack of airflow to certain vents
  3. Longer run times for your furnace or air conditioner
  4. Higher energy bills as a result of the longer run times
  5. Less comfort in your home

Long-term Downsides:

  1. Spots of high pressure in your ductwork can cause leaks
  2. Making your system work harder and more often means that its lifespan will be significantly reduced over a number of years

The reason this is so insidious is that your system will probably run for months or years with a clogged filter. So if you just forget about it, you’ll get at least some heating and cooling.

But it will be more costly, less comfortable, and your system will either break down or start to have serious problems years before it would otherwise.

All because of a filter. Moral of the story: change your filter on time.

Furnace filter surface

When to Change Your Filter

Any filter will have a recommended replacement schedule on its packaging. This could be as little as every 30-60 days, or as long as a year. It’s recommended that you stick to this schedule.

However, this schedule is just a recommendation, not an inviolable rule. Several factors can (and should) affect how often you change your filter. These factors include:

  1. Pets. If you have multiple pets in your home, your filter will clog more quickly.
  2. Allergy and illness considerations. If it’s allergy season, or your family includes someone who is immunocompromised, you may want to consider replacing more often.
  3. Smokers. If you are in a smoking household, filters will collect particulates from this more quickly than some other homes.
  4. How often you run your system. This is one we forget about, but your filter is only working when the blower fan is running. This could be for air conditioning or heating. But, for example, if you spend the majority of the summer out of the home, you may be able to leave your filter in for longer than recommended. Conversely, if you’re running the system almost constantly in a particular season, timely replacements are of course recommended.

It’s also important for you to monitor your home. Are you starting to notice strange smells? Does dust seem to accumulate more quickly than usual? These could be signs that your filter isn’t working efficiently and requires cleaning or replacement.

How to Change a Furnace Filter

Below are some common steps for changing your filter. When in doubt, refer to the user’s manual for your furnace or call your HVAC contracting partner.

  1. Locate your filter in the furnace unit. Most of the time it's pretty easy but you may have to remove a panel to find it. Your owner’s manual can help in this regard.
  2. Turn your HVAC unit off before doing any maintenance. You don't want it kicking on while you're changing out the filter or taking off panels.
  3. The furnace filter is always located between the air return duct and air supply (conditioned air) duct.
  4. Determine which way the air flows. It always flows from the return to supply, to be recirculated into your ductwork
  5. Once you've located the filter it should be easy to pull it out and slide the new one. This is where you have to remember the direction of airflow. There will be an arrow on the new filter, and it must point in the direction the air is moving. In some cases, the furnace filter can catch on some rough edges when you're trying to slide it in. Don't get frustrated. Just take your time and wiggle it around and it will slide into place. If it's super complicated, in a tight space, or you're completely unsure let us know. We are happy to take a look and change your filter during your maintenance visit.

Washable Filters

Some HVAC systems have washable, semi-permanent air filters. There's nothing wrong with them as long as you wash them on a regular basis. If you don't like the hassle of washing, you can usually replace them with a paper or fiberglass filter.

System Maintenance & Filters

While changing air filters on furnaces is a pretty straightforward do-it-yourself task, a lot of homeowners forget to perform the task regularly.

So it’s here that I’d like to make a suggestion: tie filter changes to your maintenance visits from an HVAC professional.

For clarity, you don’t need to do this. But if you have a full HVAC system (air conditioner or heat pump, plus a furnace) you should be having them serviced once a year. Usually, it makes the most sense to schedule visits six months apart: one for cooling equipment and one for heating equipment.

Any HVAC company worth their salt will be able to change out your filter as part of the routine maintenance process. So if you have

The drawback is if you have a cheaper filter that will need to change every 30-60 days. This is part of the reason we recommend the four-inch media filters that are designed to promote airflow, usually have higher MERV ratings than entry-level filters, and generally last about six months.

Regardless, your HVAC tune-ups are a great time to handle this.

RELATED: HVAC Tune-ups: Why You Need One & What’s Included

FURTHER READING: How Much Does a Furnace Cost?

If you’re ready to schedule service with Fire & Ice, check our service area below, then fill out the service form or give us a call. We’ll be happy to take care of all your air filtration needs.

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