Filter selection HVAC

HVAC FILTER SELECTION AND MERV RATING

 

In 1968, American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) created Standard 52.1 to test air filters using established techniques employed by the U.S. National Bureau of Standards and the Air Filter Institute.  During the course of the past 40 years, this standard has been modified several times to reflect changing air filtration testing requirements. The version in use today, Standard 52.1-1992, evaluates filters based upon three key measurements:   

Arrestance. Designated as a percentage, the arrestance of the filter, or ability of a filter to remove synthetic dust, is calculated based upon the known weight of dust fed into a test unit under controlled conditions and the amount of weight added or captured by the filter. 

Atmospheric Dust Spot Efficiency. Designated as a percentage, two procedures can be used to calculate dust spot efficiency, which is the ability of a filter to remove atmospheric dust from the air. Both the intermittent flow method and the constant flow method rely upon white filter paper as a target and the ratio of light transmission. Sampling of atmospheric air is conducted at 25%, 50%, and 75% of dust loading and at final resistance. Efficiency is determined by an equation that incorporates total airflow through the upstream and downstream targets and opacity of dust spot on the upstream and downstream targets. Traditionally, engineers and designers have used the terms 30%, 60%, and 85% filters to indicate which type of filter to install. These terms refer to the dust spot efficiency of the filters. 

Dust Holding Capacity. Expressed in grams, dust holding capacity measures the amount of dust that a filter can hold. Synthetic test dust is fed into a filter and the final capacity is based upon the amount of dust held by the filter.  

Standard 52.1 does not evaluate particle size and efficiency of the filter for a particular particle. It is not known what type and size of particles will be captured by the filter.  For instance, a 30% filter (based on arrestance) means that 30% of the weight of the dust has been captured. Depending on the size of the dust particles the actual number of particles (and hence the percentage of particles) captured may be substantially less. 

MERV Defined 

To address the issue of particle size, ASHRAE 52.2 was issued in 1999 and modified in 2007 to complement, not replace, Standard 52.1, by enabling engineers/designers to select a filter based upon the actual known particle size of the contaminant desired to filter.  Standard 52.2 indicates the lowest point of filter efficiency (which is normally right at the time of installation1).  Filters  are  evaluated  based upon initial efficiency as a function of one of 12 possible particle size ranges.  Based on this data, a numerical value is assigned to a filter. This numerical value is designated as MERV: Minimum Efficiency Reporting Value. MERV ratings are based upon specific airflow speeds which must be noted on the manufacturer’s data.     

Testing for MERV  

MERV ratings are created using the following testing process:  

1.  Initial resistance versus airflow is measured. 

2.  An optical particle counter measures and counts the particles in 12 equally distributed particle 

size ranges before and after the filter. 

3.  ASHRAE test dust is introduced to the filter in  five predetermined steps (pressure drop across filter increases by 0.04” water gauge (w.g.), 25% of manufacturer’s recommended final pressure drop, 50% of manufacturer’s recommended final pressure drop, 75% of manufacturer’s recommended final pressure drop and 100% of manufacturer’s recommended final pressure drop). 

4.  Efficiency performance curves are developed based upon the optical particle counter data for each of the six readings (one initial, and five loading steps). 

5.  A composite minimum efficiency curve with the lowest efficiency reading for the six readings for 12 particle size ranges is developed. 

6.  Twelve particle size ranges are placed in three larger groups (E1, E2, and E3) and the percentages of each group are averaged. This average is deemed the Particle Size Efficiency which is the value used to determine the MERV rating based upon ASHRAE data. (See the ASHRAE Application Guidelines chart listed on page 4 for additional information.) 

Filter Selection  

Proper filter selection requires an understanding of several factors: the type and size of particle/contaminant to be removed; expected filter service life; and pressure drop. ASHRAE 52.1 and 52.2 should be used in conjunction with one another to make a final filter selection.  To help in the selection process, the following information will be required:  

Physical size of particulate.  The chart on page 4 should be helpful in determining the MERV rating of the filter once the particulate size has been determined. For example, if bacteria ranges from 0.3 microns to 4 microns, the chart indicates a MERV 13 would be the minimum rating to select, and higher MERV-rated filters should be selected with the end users’ level of comfort and economics in mind.