Understanding MERV air filter ratings for HVAC systems

BY BOB JACKSON, CAFS, NCT PRESIDENT, NORSPEC FILTRATION LTD.

MERV ratings are used to rate the ability of an HVAC filter to remove contaminants from the air as it passes through the filter. Higher MERV ratings mean fewer dust particles and other airborne contaminants pass through the filter; the higher the MERV (Minimum Efficiency Reporting Value), the higher the efficiency of an air filter.

Prior to the MERV rating system, air filters were tested according to a standard known as ASHRAE 52.1. This test provided an average efficiency of the filter over its lifetime, and air filters were identified as a 30 per cent, 65 per cent, 90 per cent, etc. This system was relatively easy to understand. However, it had some major deficiencies in that it did not provide a very good guide for selecting filters for particular contaminants. In order to do that effectively, you need to know two things – one, the particle size of the contaminant you are trying to remove and two, the level of efficiency of the filter on that particle size. To better address those concerns, a new test standard known as ASHRAE 52.2 and the MERV rating system were developed to provide this particle size efficiency data. Obtaining the Data

An air filter’s performance is determined by measuring the particle counts upstream and downstream of the filter being tested. Particle counts are taken over the range of particles six times. One begins with a clean filter and then uses that same filter five additional times.

The particle generator creates particles of a known size in the air stream. The objective is to create particles of sufficient numbers to obtain meaningful counts in all of the measured particle ranges which are the following:

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Range Size (in microns) Group The efficiency of the filter is then measured on

ASHRAE 52.2 Particle Size Ranges

each of the 12 particle size ranges for each of the

six cycles. Efficiency is measured as the percent1 0.30 to 0.40 E1 age of particles captured by the filter. The lowest 2 0.40 to 0.55 E1 of the six readings is then taken to generate the 3 0.55 to 0.70 E1 Composite Minimum Efficiency Curve. 4 0.70 to 1.00 E1 The twelve size ranges are then placed in three 5 1.00 to 1.30 E2 larger groups (E1, E2, E3) and the percentages in 6 1.30 to 1.60 E2 each group are averaged. This average is called 7 1.60 to 2.20 E2 the Particle Size Efficiency, and these are used to 8 2.20 to 3.00 E2 determine the Minimum Efficiency Reporting 9 3.00 to 4.00 E3 Value (MERV). 10 4.00 to 5.50 E3 To use the above table, you move within each 11 5.50 to 7.00 E3 group until you arrive at a true statement. This 12 7.00 to 10.00 E3 will correspond with a MERV number. The lowest MERV of the three groups is the MERV Parameters MERV rating for the filter. MERV Group 1 Group 2 Group 3 Value Av. Eff. % Av. Eff. % Av. Eff. % For more information about MERV (0.30 to 1.00 microns) (1.00 to 3.00 microns) (3.00 to 10.00 microns) ratings, and how to use them to select the proper filter for your s1 n/a n/a E3<20 chool, please contact our office or 2 n/a n/a E3<20 any active member of NAFA. 3 n/a n/a E3<20 4 n/a n/a E3<20 5 n/a n/a 20<35 6 n/a n/a 35<50 7 n/a n/a 50<70 Is Your 8 n/a n/a 70 School District 9 n/a E2<50 85 Going10 n/a 50<65 85 11 n/a 65<80 85 “GREEN?” 12 n/a 80 90 13 E1<75 90 90 PRO-PAC WBU-450 14 75<85 90 90 Water-Bourne Urethane 15 85<95 90 90 Gym Finish 16 95 95 95 is the product you need to meet “GREEN” requirements!

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