5 Inch Furnace Filters: Is Bigger Better?

Replacing the standard one-inch air filter with a 5-inch pleated filter can significantly improve the air quality inside your home. This is because larger air filters have higher MERV ratings, leading to better filtration of contaminants and allergens. Virtually any HVAC professional will tell you that thicker filters work better than common 1-inch oven filters. Thicker filters, known as multimedia filters, are usually 4 to 5 inches thick compared to the ubiquitous 1-inch filters that can even be purchased at grocery stores.

When it comes to the size of the filtered particles, there is no difference between 1 and 5 filters with the same MERV index. However, there is a difference in static pressure between the two. The 5-unit filter has more physical holes for air to pass through, making it easier to pass through. This means that larger filters will make the system less restrictive and will wear down the fan motor less, extending the life of the equipment.

Be aware of the thickness when making your decision, as it will affect the next step you have to take. Most residential HVAC filters are 1 inch thick, but 4-inch filters are also becoming common. These media filters have much stiffer cardboard frames, and some media filters even have plastic “rails” on the top and bottom so you can slide them inside the oven with a very tight seal. Your HVAC professional will install a small cabinet next to the boiler or air controller (on the air intake side) to hold the thicker filter.

I would prefer a reusable filter, but I'm not sure where to find one with the same MERV rating, plus I'm not entirely sure that washing it would properly clean the filter. And the first thing I always checked was the oven filter, because a clogged one-inch filter gets dirty very easily. Some filter brands and retailers use alternative scales, such as Home Depot's Air Filter Performance Rating (FPR) system or the MPR (in 3M Filtrete air filters). If the existing filters fit perfectly, note (or take a picture) of the size printed on the filter frame.

However, with most HVAC systems, you should be able to modernize a media filter cabinet, either under the oven or on the side. Reducing the thickness by one inch shouldn't be a big problem; for example, you should be able to use a 4-inch filter instead of a 5-inch filter. You should expect to replace the filter every three to 12 months of use, depending on the size of the filter. If you can't find Nordic Pure in your size or if you prefer to buy and buy a new filter in a store right away, Honeywell's FPR 9 filters and Filtrete's MPR 1500 filters are similar and are widely available in retail stores. These 4-inch multimedia filters can have between 20 and 30 feet of surface area with their pleated (accordion) filter material.

To put that in perspective, a 1-inch air filter may need to be changed every month, while a 4-inch air filter could last up to six months. You'll often find the exact size in stock at a hardware store, but sometimes you'll need to order the correct filter size online. All of them (and Owen) told us that the MERV filters in this range restrict airflow more than the low MERV filters, as measured by the drop in air pressure created by the filters when installed.

Ben Depauw
Ben Depauw

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