Does the Height of Air Filter Really Matter?

Learn how size matters when it comes to choosing an optimal performing HVAC system's air filters - from 1 inch filters needing monthly replacement to 4 inch filters lasting up to 6 months.

Does the Height of Air Filter Really Matter?

When it comes to air filters, size does matter. If the filter is not the correct size, whether it is too small or too large, air will flow around the filter rather than through it, and as a result, all the air will not be filtered. Thick air filters may automatically seem like the best option. Thicker means more filtration, right? In most situations, that is the case.

Thicker air filters tend to last longer because they have more square meters to capture and retain air particles. To put it into 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. An incorrectly sized HVAC air filter can compromise your entire system. A filter that is too small or large will not create the seal needed to filter all the air. Gaps will allow dust, dirt and germs to bypass the filter and potentially re-circulate in your home air. At Air Filters Delivered, a common question we hear is how do I know what size air filter I need? Often the nominal size is printed on the side of the filter, but when it isn't, you'll need to know how to measure an air filter.

Watch the following video for step-by-step instructions to find the actual dimensions and get the nominal size of your system. Keep in mind that some HVAC systems require more than one air filter, so make sure you have the correct measurements for all of them. The short answer? A 4-inch filter will last longer and provide better air quality for your home. The 4-inch thick air filter has a definite advantage over the 1-inch in terms of longevity, airflow and maximum filtering potential. Deeper depth can also improve filter life and efficiency; they also make it easier for air to enter and exit the filter.

The combination of a thin air filter with a high MERV rating can also restrict airflow to the point of impairing efficiency and causing excessive wear and tear. But what if the air filter compartment of your oven is only 3 inches thick? In that case, the 1-inch oven filter is the better choice than the 4-inch filter, however a 3-inch thick air filter is the best choice. The tests of K&N have shown that for most two-barrel and four-barrel carburettors, the air flow is greater when the diameter of the air filter is large compared to its height. That said, the 1-inch filter may be too shallow, causing a loose fit that allows unfiltered air to enter the ductwork. Using a 4-inch filter would mean getting a filter with at least a MERV 8 rating, which would remove contaminants up to 3 microns (one unit of measurement), including dust mites and some types of pollen. If improving air quality is a priority for you, filter depth doesn't matter as much as MERV. The shape and size dictate the ability of the air filter element to flow the maximum amount of air into the engine.

Most HVAC technicians recommend changing 1-inch air filters every 30 days, while 4-inch filters can be changed every 3 to 6 months. A 1-inch filter with less surface space will clog up fairly quickly and will need to be replaced much sooner than a coarser filter. In the case of a K&N element, you can use the diameter of the air filter to help determine sufficient area for required airflow. Nominal dimensions refer to rounded numbers used on labels packaging both air filters and wood. Round up to nearest inch to create nominal filter size which is measurement you will use when purchasing a filter. The only exception to K&N rule is when a filter element is attached to long runner such as in cold air intakes of EFI engines.

At same time coarser filter is less restrictive and will allow better flow of purified air.