Everything You Need to Know About Whole House Air Filters

Why do I need such a big air filter?

There may not be a single product on earth less standardized than the home air filter. It’s the reason why nobody can ever find their size at the hardware store and the reason why we carry more than 66,000 sizes.

One type that can be particularly difficult to locate in the local store is the whole house filter. These are very different than the more common one-inch filters. You may have heard them called whole house air purifiers, which is a misnomer. An air purifier is an addition to your HVAC system, not part of it. Although air filters do indeed clean the air, they are part of your system and not optional. Now that we've clarified that, let’s get back to it.

As the name implies, the concept is defined by the need for only one filter per home. This is the most common setup, but it’s not always true in practice. You’ll need as many whole house filters as you have air handlers, which may vary depending on how your HVAC system is configured.

An air handler (or AHU for Air Handler Unit) is a device used to regulate and circulate air. It literally “handles” your air. Some homes may have more than one air handler to regulate temperatures in different areas separately.

What does it look like?

Any filter three inches or thicker is designed to be a whole house filter. The most common thicknesses are four and five inches, while three- and six-inch varieties do exist. They feature the same construction as the common one-inch pleated filters with one distinction. Whole house filters offer significantly more surface area than one-inch filters because of the deep-v pleats that fill its nearly half-foot thick frame.

With all filters, the “nominal size” of an air filter means the rounded size, where each dimension is rounded up or down to a whole number. There’s a standard undercut on one-inch filters, meaning that all one-inch filters measure ¼” to ½” smaller than their nominal size, but there is not a standard undercut on whole house filters. This means that two 25x20x5 filters may actually be different sizes entirely if they are made by different brands. Know the exact size you need, or you might end up with a whole house filter that doesn’t fit.

With Second Nature, you can select your whole house filter by brand, so you know you’re getting exactly the right size every time.
air conditioning system with whole house filter

How does it differ?

Besides its physical appearance, the primary difference between a whole house filter and a regular one-inch filter is the longevity. Because of its massive surface area, a whole house filter doesn’t need to be changed nearly as often. A replacement is typically required every six months instead of three, and in some cases, a change isn’t needed more often than once a year. Like any filter, the frequency of replacement is determined by what factors contribute to your air quality. Normal living situation with no pets? Six months will do. Lots of pets and live in a polluted area? Probably need to be changed more frequently, like every three to four months. Live in Fiji? Could probably stretch the lifespan of that filter.

The difference in construction and longevity does not fundamentally alter the effectiveness of the filter at catching pollutants. Whole house filters still come in standard filtrating ratings (like MERV 8, 11, and 13) and filter similar types and percentages of particles.

Does it cost more?

Yes. A whole house filter is on average priced somewhere between about $25 and $35, with some pushing into the $40 range. While they’re more expensive than one-inch air filters, you also change them less frequently and usually only have one at a time. Even though a whole house air filter may cost up to four times as much as its smaller cousins, you could end up spending less on air filters per year (depending on your house and living situation).

Where does it go?

A whole house filter does not go in your wall like other air filters. These filters go into the ductwork directly in front of your HVAC unit. Some varieties have a housing or cartridge that they fit in before being installed while others are inserted directly into a slot in the duct.

Should I get a whole house filter?

Well, it’s not really up to you. Your house has what your house has. If you bought a home that uses a whole house air filter, then yes. If you bought one that doesn’t...well, you get the point.

Hopefully, this clears up any questions regarding the mysterious whole house filter you may have. Feel free to reach out to us on Facebook if you have any more.

Alec Lower

Content Writer

Alec is a third-year member of the team at Second Nature. He brings expert knowledge of a myriad of home air filtration topics including HVAC filters, filtration efficiency, and indoor air quality.