Do MERV ratings impact air resistance? What is right for my home?

There is a common belief among HVAC contractors that the ‘proper’ MERV rating for home air filters is MERV 7. This is based on a convenient assumption which is, unfortunately, wrong. MERV ratings give less information than you might think.

Here’s what you need to know.

Simply stated, a MERV rating represents the efficiency for a filter’s ability to trap certain particles in a laboratory test at a particular point in time. MERV ratings do not indicate a filter’s air flow resistance, average efficiency during use or expected performance in home environments. MERV ratings can be particularly misleading because tests can be manipulated to falsely increase ratings.

Air flow is critical for proper functioning of air conditioning systems. The greatest factor in air flow reduction has little to do with the filter itself, but how fast air is flowing through it. This is simply a function of filter size. Larger filters allow the same amount of air to pass through more slowly, creating less resistance and better filtration. Because resistance increases exponentially faster than air speed, this can create major problems if the system was built with too small of a filter. When properly sized, almost any high quality filter can be used. More detailed information is below.

Regardless of size, higher MERV rated filters can have lower resistance than lesser rated filters. Recent tests showed that an ACE Hardware MERV 7 filter actually has greater air resistance than leading MERV 10 filters. Our System pre-filter offers less than half the air resistance of both of these filters while removing 90% more particles.

MERV ratings are a poor indicator of true effectiveness. Companies often use different air speeds in the tests to get a higher rating. Our pre-filter for the System is rated MERV 7 but outperforms higher rated filters because it is rated at 500 feet per minute (fpm). It would rate about MERV 10 at 300 fpm. Many filters are rated at even lower air speeds, causing them to perform far worse than their ratings imply. Unfortunately, test air speeds are almost impossible to find out as a consumer.

Finally, ratings in laboratory conditions don’t translate to home environments. ASHRAE, the organization in charge of ratings, actually states that filters may not perform as well as rated because of this. Humidity and cooking vapors reduce electrostatic charges, reducing effectiveness. Electrostatic MERV 13 filters have been shown to perform worse than a MERV 7 within 10 days of installation. The MERV 8 Duo is one of the only filters available that is not subject to electrostatic loss and its average effectiveness during use is better than most filters rated MERV 11.

Our recommendation is to find what works for you and stick with it. If you aren’t concerned about chemicals or odors, you can’t go wrong with using the pre-filter for the SafeHome System which offers superior allergen removal and by far the best air flow we have found.

Proper filter sizing for optimal air quality and air flow.

A properly sized filter should allow air to flow through at 300 feet per minute. At 500 feet per minute, air flow restriction more than doubles and filter efficiency is sacrificed for any filter. HVAC contractors used the range of 300 to 500 feet per minute in calculating a filter size, but we’ve seen far higher speeds in high quality homes by reputable builders and even at 500 feet per minute

The rule of thumb is at least 1.25 square feet of filter area for every ton of air conditioning.

How do I know if my filter is the right size?

The easy way to know is simply listen. If air is howling through and appears to suck most air through the center of the filter rather than across the filter face, air velocity is too high for using any good filter. Some air noise is okay, but it should be below a hushed conversation.

The best way to know is to find out what the intended air flow is from your HVAC contractor. They will provide this in cubic feet per minute (cfm). In general, they go by 400 cfm per ton of air conditioning. Simply divide the air flow in cfm by the total square feet of filter area for that system. A 14×20 filter has 2 square feet, 20×20 and 16×25 filters have 2.78 square feet and a 20×25 has 3.5 square feet.

The closer you are to 300, the better. If it is over 500, too small of a filter was installed and you should consider changing it. This can be relatively inexpensive since new air return grills start at only $35, but costs will vary by home.