November 9, 2021

The Right Filter For your furnace

Every furnace needs a filter. That is it. There is no getting around it. Let's discuss why your furnace needs a filter. Let's also talk about the right filter for your furnace. 

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Quick Tips:

1) The filter goes in the return air duct of the HVAC System. 

2) If the size is not listed on the duct, measure the outside of the duct to get the dimensions of the filter (the filter will be slightly smaller than the ductwork)

3) Match the Air Flow arrow on the Filter with the air flow of the HVAC System

4) It is recommended to change your HVAC System filters once every month. If you cannot muster that expense, the filter should be changed at least every three months.

If you want to jump to the filters that we have listed, click here. You will need to find the right size for your furnace and if you read through the article, we will show you how to find that information.

Why does my furnace need a filter?

Your furnace is pretty much a big vacuum (with not as much suck power). Whatever is in the air in your house gets sucked into the furnace.

If the filter is not in place, two things happen to the coils: dust particles collect on the coils and the amount of air flow that needs to pass the coils is reduced. 

It's just a bit of dust, how can it be so bad? 

The build of dust on the coils is more damaging than one might think. The build up could catch on fire in heating mode.

If you furnace has an Air Conditioner on it, the A/C coils are even closer together and the clog of particles can cause your coil to freeze. (The air conditioner needs air to pass by it so it cools the chemicals in the pipes. If it does not cool off it just freezes. I have seen air conditioners frozen from the coil at the furnace through the pipes to the condenser outside. If air isn't moving across it, it will ruin the A/C).

Dust build up can also limit how much air passes the coil causing the furnace to over heat. This triggers a limit switch and turns the furnace off. Without a filter, dust build up on the coils can ignite. Go to Our Links for Filters here.

What is floating in my air?

Have you ever sat by a window in your house on a sunny day with the sun light pouring in? Typical scenery is dust floating in the air and these particles can range in size by quite a bit. Filters do not catch everything but they do catch the things you can see. Otherings floating in your house could be:

Dust particles, animal hair, mold, smoke, smog, people hair, aroma particles, microscopic bugs (this articles will make you want to take a shower) ... really anything that floats in air is in the air. When a door or window is open without a screen the size of particle is unrestricted. From particles that can be seen to particles that cannot, they are in the air. 

The right filter for your furnace

If you are done reading, Go to Our Links for Filters hereThere are a lot of filters on the market. Not all of them are needed for you situation. At minimum a filter needs to filter out mites, pollen, pet dander and dust/lint. There are a few different rating systems to determine what is right for you. The MERV, MPR and FPR.

Filters are inserted into the return air of the system. Sometimes they are called AC filters or AC furnace filters. It should really be called the HVAC System Filter. Air doesn't really care what it passes through and it can pass through all of the standings for those letters with or without a filter. But the air does need to be filtered. Before we get into ratings, let's start with the biggest question:

Where does The Furnace Filter Go?

Okay, you are at the store and you are looking for the filter that fits your needs but where does that filter even go? As stated before, your filter goes in the return air of you furnaceGo to Our Links for Filters here

Now this can be in a few different locations. I have diagrammed a few scenarios below. Look for Filter Slot in the below diagrams.

Up Flow Furnace Configuration

Up Flow Furnace Configuration

Side Flow Furnace Configuration

Side Flow Furnace Configuration

Down Flow Furnace Configuration

Down Flow Furnace Configuration

These are the most standard for residential units. Commercial units can get really complicated. Keep in mind that most commercial units run at a high BTU (British Thermal Unit) and CFM (Cubic Feet Per Minute) and the standard filter from the hardware store should not be put in your commercial unit unless it is rated for your CFM. 

What Size Of Filter Does My Furnace Need? 

The simplest way to determine this is to measure your duct that goes into your furnace. My specific furnace in my house is a down flow. That means my return air is at the top. See the LxWxH pictures below. You measure your return air duct and find what yours is by measuring the duct that comes into the side or end of your furnace. You only need to measure the two most accessible sides. Most duct going into furnaces is a rectangle or a square.

If you do not have a tape measure, use a ruler or a sewing tape. Remember that you do not want the exact measurement of filter. The filter needs to be just smaller than the opening or it will be a bear to get in (and out!). Go to Our Links for Filters here

Filter Slot LxWxH

Filter Slot LxWxH

Pro Tip: Be sure to match the air flow arrow on the filter with the air flow of the furnace unit.

Go to Our Links for Filters here

The MERV Rating

The most confusing is the MERV (Minimum Efficiency Reporting Values) rating. That's right, I said confusing. The MERV rating is a list from 1 to 16 with different micron sizes that pass though each number. Most people have no idea what a micron is or what it looks like. So while the table is useful for people who deal with micron measurements all day, the rest of us need to know what is the size of the mold particle floating in the air. Or the size of a smoke particle. The EPA has an article linked here if you want to read more about it.

The MPR System

The second least confusing is the MPR (Microparticle Performance Rating). This rating was developed by 3M and they break this down a little differently. They have categories of efficiency. E1, E2 and E3. The smallest particles are the E1 and the larger particles are E3. For each filter they sell, they list the percentage of particles stopped at each E level. Yes, larger particles can still pass through a filter. 

The FPR System

The easiest to use comes from The American Society of Heating in 1987 for The Home Depot. They developed a rating system that relates to the most common things in the air in a home and put a number to it. The FPR (Filter Performance Rating) is on the Rheems, HDX and Honeywell filter brands at The Home Depot if you want to purchase in store. We are an affiliate for filters so All of these links will take you to Amazon to see the filters we link to. The categories are simple enough in the table below. 

Be sure to find the right size aside from the particles you want to stop.

Filters by What they Filter

How Often Should I Change my Furnace Filter?

Most manufacturers recommend changing the furnace filter once a month. If you are building a home, require the builder to change the filter before they turn it over to you. Also, be sure the builder is using a filter while they are acclimating the unit for construction purposes...(you'd be surprised at the horror stories). Go to Our Links for Filters here

About the Author TinnersToolBox

Tinner's Tool Box is a resource for HVAC professionals to gain access to information to help them complete their projects. It is also a reference for the everyday person who wants to improve their knowledge on the topic.


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