Testing Air Filters

Keep in mind as you read this the two reasons for having an air filter is to keep small particles that can harm your body out of your lungs and as well the air filter needs to keep HVAC equipment clean so the coil will transfer heat properly and also keep the duct work clean.

 Air filters are tested by procedures set up by ASHRAE (American Society Heating Refrigeration Air Condition Engineers). ASHRAE 52.1 is commonly called a dust spot test and the results are reported in efficiencies, like 30% or 60% (efficiencies of what, we didn’t know). This test has been replaced by ASHRAE 52.2 which reports the results in what is called a MERV rating.  The picture to the right is a typical test duct. 

 Both test procedures require the air filter to be secured in a test duct so all the test air goes through the filter being tested. The ASHRAE 52.2 is a much better test in that it measures the quantity of particles in the air before the air filter and the particles after the air goes through the filter, giving a percentage of a particular range of particles (1-micron to 2-microns). MERV stands for minimum efficiency reporting value.

 In both tests the Independent Testing Company places the filter in a test duct (usually taped in so there is no air by-pass around the air filter) and as the prescribed volume of air is pushed through the filter a charge of dust is metered into the air stream. After the first charge, readings are taken before and after the air filter to check the efficiency. The loading of dust into the air stream is repeated (6 or 7 loadings) until the filter reaches a final resistance (usually called for by the manufacturer). This is called being loaded.

 The charge of dust used for loading is standard for filters of the same category.  HEPA, DOP and Carbon filters are challenged differently. In the ASHRAE 52.2 the minimum efficiency of each category (.3 to 1 micron, 1 to 2 microns or 2 to 3 microns etc.) is charted on a graph.

 In the old ASHRAE 52.1 particle size was not taken into consideration and then all the results of the dust loadings were averaged. For instance if a filter started with an efficiency of 15% on the first loading, 20% on the second and so on until on the 7th loading you reached 45% then you would average all the readings. In this case it would average close to 30% so it is called a 30% filter even though for half it's life it was less than the 30% rating. In the ASHRAE 52.2 the minimum is reported, not an average.

 One thing to note, the air filter gets better as it is loaded with dust. In the ASHRAE 52.1 you have no idea of how it may work in your situation. With the ASHRAE52.2 MERV rating you are able to target a minimum size particle that may fit your situation. A MERVE-7 is the minimum you should consider to reach an indoor air quality (IAQ) for a healthy home.  A goal for the home would be to catch 60% of the 3-micron particles and about 90% of a 5-micron particle. This means that the bigger size particles would be captured at close to 100%. The average person can’t see particles smaller than 25 or 30 microns until they accumulate in great quantities.

 Don’t be fooled by manufacturers that report dust holding capacity. Usually this will be in the 95% range. The dust holding capacity of a 10% filter could be 95% and is not filtering any of the particles that make it to your lungs. The test dust charge is made up of a lot of large particles in order to speed up the test. The filter will capture the large particles which account for 95% of the charge by weight but will allow the 5% by weight to get through the filter. This 5% contains over 95% of the particles. These are the particles germs ride piggy-back on that reach your lungs.

 This is a brief description of how air filters are tested. You can go to the ASHRAE web site (ashrae.org) and read in much more detail if you like. If you have other question go back to the home page.  BUY NOW