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What Does the MERV Rating Say About My HVAC Air Filter?

merv ratingMERV stands for Minimum Efficiency Reporting Value and is a third-party system for rating air filters. It’s a convenient way for homeowners to compare different filters and choose what’s best for their particular situation. Essentially, the higher the MERV rating, the greater the ability to filter contaminants. We talked with Keith Hill, Stay Comfy’s resident HVAC expert to learn more.

What’s the Best MERV Rating for My Home Filters?

That’s a tough question to answer because there are so many variables to consider. There are many different types and styles of HVAC equipment, and as many duct system configurations and designs. These are all factors that affect the selection of your filter—both the size and the type. Here are some important factors that go into deciding what filter is best for your HVAC system.

How Filter Size and Thickness Affect Airflow

Filter length, width, and depth all make a big difference in the amount of airflow. As Keith points out, “Length and width are obvious, but thickness is a huge factor. If you took a 1” thick filter and a 4” thick filter out of their frames and stretched them out, you would see that the 4” is roughly four times as long. That means the filter media in the 4” filter has essentially 400% more available area for the air to pass through. That makes it easy for the air to go through – very low restriction to flow." And you’ll want good airflow, which is a major factor in the effectiveness and efficiency of your HVAC system.

How MERV Rating Can Affect Airflow

It turns out that a higher MERV rating isn’t necessarily a good thing, because the higher the MERV rating, the more restrictive the filter media is. And that can cause trouble with your HVAC system. In heating mode, low airflow can cause overheating and will eventually lead to “tripping” the high limit. If this happens over a prolonged period of time, it can shorten the life of your heat exchanger.

Restricted airflow in cooling mode can also be a problem, although it’s a little more complicated. But Keith explains it perfectly: “During cooling, refrigerant inside the A coil or evaporator coil requires airflow as its heat source to evaporate or boil the refrigerant inside and maintain internal system pressure. At the appropriate pressure the refrigerant “boils” at 40°F. Without enough air, the pressure drops (the same as if it’s low on refrigerant) and the air conditioning coil temperature drops to 30° or less and causes the air conditioning coil to freeze up. The frost and ice build-up then further restricts the air, which causes the problem to snowball (pun intended), ultimately leaving you with a very cold and frosty coil with no airflow into the duct system and cool air being delivered into your home. Even worse, the compressor in the outdoor unit can be damaged if this condition persists. The compressor is the most expensive component in your AC system, so it’s best to ensure adequate airflow.”

A General Recommendation

Given all these factors, Keith’s general advice is “to stick with MERV 7 or less in a 1” filter and 12 or less in a 4” or 5” media filter."

If you’re in any doubt as to the best filters to use for you system, give your local HVAC pro a call—they’ll be able to help out. Need to contact an HVAC pro in your area? Just use our convenient dealer locator.

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