Furnace filters, also known as air filters, are necessary components that not only keep your home’s air clean, but also help extend the life of your furnace. However, homeowners have a lot of questions about them, such as how to replace an air filter, which filter they should buy, and more.

While the concept of a furnace filter may seem simple, in practice it can be a little confusing. Having worked with home heating and cooling systems for over a century, we’ve seen many furnaces suffer from common filter mistakes. To help you avoid those mistakes, we’re answering the most commonly asked questions homeowners have about furnace filter maintenance, installation, and utility.

What is a furnace filter?

A furnace filter has a singular purpose: to filter the air that flows through your home when the blower fan is on. 

Homes accumulate dust, dirt and debris throughout the day. When a blower circulates the air in your home, it also circulates the airborne particles, which then end up in your furnace. Your furnace filter stops those particles from going into the ductwork and, consequently, into your furnace. The filter also helps improve your indoor air quality.

1. Will a furnace run without a filter?

Yes, but it will absolutely damage your furnace in the process. Without an air filter, more dust and debris will make it to the core components of your heating system, causing damage to your coils, blower motor, and more. In addition, without an air filter, your furnace won’t be running efficiently, causing your heating bill to soar.

2. Do all furnaces have filters?

Yes, almost all modern gas, electric, and oil furnaces have a filter. However, certain types of wall furnaces or direct vent furnaces in older homes may not, because they vent directly outside.

3. Are furnace filters necessary?

If you want to have an efficient furnace that lives a good life and doesn’t cause your heating bill to skyrocket, air filters are 100% necessary. Filters are important furnace and air conditioner parts as they help keep the rest of the system clean. Dirty furnaces and air conditioners caused by a lack of air filter ultimately have to work harder to get the job done, using up more energy and potentially damaging parts like the heating coils or igniter.

4. Are furnace filters the same as air conditioning filters?

Yes. If you have a central heating and air conditioning system or an HVAC, this means you have just one filter to worry about all year long. However, if you have separate heating and cooling systems, your furnace and air conditioner may require different filters. In terms of function, there is no difference between the two as they’re both designed to serve the same purpose: protect your system from harmful air particles and improve air quality.

5. Are furnace filters universal?

Because furnace and HVAC manufacturers have different size slots for air filters, they are, unfortunately, not universal. This means you will have to find the right size air filter for your furnace as others will be too small or large to effectively clean the air stream of harmful particles.

6. Which furnace filter should I buy?

Furnace filters are sold based on their size and efficiency. To check the size filter that you need, turn the system off and uninstall your current air filter. Then, look for the size labelled on the filter’s frame. This will tell you the size filter you should buy.

7. What is the MERV rating on a furnace filter?

Filters will be labelled with numbers from 1 to 16 based on efficiency. This is the filter’s minimum efficiency reporting value or MERV. The lower the number, the fewer particles the filter can trap. If you want a lower efficiency filter that is less expensive, a MERV 4 is the lowest you should go on the MERV scale.

In our experience, most homeowners choose MERV 7 or MERV 8 filters as they’re a great value in terms of efficiency and price. But if you suffer from allergies, it’s worth it to upgrade to a filter with a MERV rating of 11 or 12 to have the fewest number of potential allergens in your home.

Higher MERV ratings will trap allergens like pet dander, dust mites, mold spores and other small particles, but thicker filters can impede air flow. Check with your HVAC installer to make sure your filter’s MERV ratings aren’t so high that it reduces the efficiency of your furnace.

8. Who sells furnace filters?

If it’s time to change your filter, most big box retail and home improvement stores will carry furnace filters. This includes the likes of Target, Walmart, Lowes, Home Depot, Menards, and others. But if you have your regularly scheduled furnace maintenance appointment coming up, don’t run to the store. Your local heating and cooling expert should replace the filter for you during their visit.

9. When should furnace filters be changed?

Not sure how often you should be changing or checking the furnace filter? If you have a pleated or high-efficiency furnace filter, it should be checked and changed once every three months for maximum efficiency and air quality. During periods of high use (e.g. winter for your furnace or summer for your AC), you should also visually inspect the filter each month in case heavy use caused your filter to clog up. Otherwise, your air filter should be changed on a monthly basis. After reading this, if you aren’t sure when you changed your air filter last, it’s definitely time to replace it with a new one.

10. Where do I change my furnace filter?

Depending on your HVAC system, the air filter can be hard to find. The most common location is in the return air duct or in the blower compartment as this allows the filter to clean the air before it reaches your unit. Other places you may find the air filter are in a slot above or below your HVAC unit, or right behind the return grills in your home (see picture below).

11. How do I install a furnace filter?

Replacing or installing a new air filter is as easy as 1-2-3:

Step 1: Turn off the furnace or HVAC system.

Step 2: Remove the filter by sliding it out of the slot.

Step 3: Replace the filter by sliding a new, clean filter into the slot. If you have a reusable air filter, clean it, let it dry, and slide it back in.

Once finished, you’re free to turn the system back on.

12. Are furnace filters recyclable?

While the materials made to make a furnace filter are recyclable (e.g. paper and cardboard), it is not recommended that you recycle them. Why? Well, your dirty air filter is filled with dust, bacteria, and other particles that we don’t want others to be exposed to. Therefore, your safest option is to place them in the trash.

That being said, there are reusable air filters that are available for use and are environmentally friendly. Their initial cost is a lot higher than their disposable counterparts, however, they can last for several years and won’t end up in a landfill. Instead of replacing these filters, they simply need to be washed regularly.

13. Can furnace filters be washed?

Disposable furnace or air filters cannot be washed and should be thrown away after they’ve been used. However, as mentioned in the previous section, reusable air filters can and should be washed on a regular basis. To wash your reusable filter, take a microfiber cloth and begin removing the trapped dust and debris. A wet cloth can also be effective at cleaning the filter, however, make sure to let the filter completely dry before reinstalling it to avoid mold or mildew build up. 

14. Are reusable filters good? 

There are a lot of pros that come with using a reusable furnace filter. They can be cleaned regularly by washing or vacuuming, rather than buying a brand new filter every time it needs to be replaced. This makes reusable furnace filters more eco-friendly. They cost more upfront but, in the long run, are more budget-friendly than single-use filters.

However, if you struggle with allergies, there are a few things to note about reusable furnace filters. For one, vacuuming might not be enough of a cleaning. You’ll want to wash and then let the filter completely dry. Secondly, if the filter hasn’t fully dried before putting it back in, it can cause mold and mildew that aggravate allergies. Be sure to treat it with extra care, so it does not affect your health.

15. How do I know what size filter to buy?

Furnace filters are not one size fits all. You’ll need to pick your furnace filter size based on the type of HVAC system you have. You can check your manual to find the exact dimensions. Can’t find your manual? Don’t worry. Take a look at the current filter you have in there. It should tell you what the furnace filter size is. 

If there is no filter or manual to reference, you can measure it yourself. Grab some measuring tape and measure the height, width, and depth. Take that to the hardware store, and they’ll be able to find the furnace filter size you need.

16. What are the types of furnace filter material?

There are a few different furnace filter types. One is pleated, and one is non-pleated. Both have their own characteristics:

Pleated 

Pleated furnace filters are made of layers of cotton or plastic that are then pleated. The pleated design gives the filter more surface area to catch debris and dust. Pleated furnace filters can also catch mites, odors, and bacteria, so they are great for anyone with respiratory issues or if you own pets.

Non-pleated

Non-pleated furnace filters are more common. They are made of fiberglass and usually are blue. They are best for catching large pieces of debris but don’t capture finer dust or elements that can cause allergies. If not changed regularly, they can collect build-up affecting your HVAC performance.

17. What is the best brand of air filter?

There are many amazing furnace filter bands to look into for your home. Start by checking out companies like Honeywell, Filtrete, Filter Monster, and Nordic Pure. Remember, you still want to have your filter size on hand, so you don’t get one that is too big or too small.

Take Care of Your Home’s Heat

Furnaces do have filters and they are critical in extending the life of your furnace. For more ways to maximize your furnace’s lifespan, check out our DIY furnace maintenance guide.