ASHRAE Standards Made Easy

The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) developed two HVAC industry standards that address the efficiency issue. ASHRAE 52.1 standard measures:

Pressure drop: How the filter affects air flow and energy costs. A low-pressure drop typically translates into higher energy efficiency. A high-pressure drop means reduced air flow to the HVAC unit. This requires more energy to operate the unit.

Arrestance: The amount of synthetic dust a filter is able to capture

Dust-spot efficiency: A measure of the ability of the filter to remove atmospheric dust from test air

Dust-holding capacity: The amount of dust a filter can hold until a specified pressure drop is reached. Higher capacity means a longer filter life. When evaluating dust-holding capacity, it's important to compare dust-holding capacities between filters at the same final pressure drops to make accurate comparisons of projected filter life. The ASHRAE 52.2 standard measures the fractional particle size efficiency (PSE) of an HVAC filter. This indicates the filter's ability to remove airborne particles of differing sizes between 0.3 and 10 microns in diameter.

A MERV (Minimum Efficiency Reporting Rating) is assigned to the filter media depending on the PSE in three different particle size ranges (0.3 to one micrometer, one to three micrometers, and three to 10 micrometers).

MERV is a numerical system of rating filters based on minimum particle size efficiency. A rating of one is least efficient, while a rating of 16 is most efficient. Lower ratings typically reflect a more cost-conscious choice.