AIR FILTERS TECHNICAL SECTION - Technical Issues

HVAC Air Filter testing methods

Air filter testing, as with any other industry, is of vital importance. The tests give the consumer the ability to evaluate the air filter and its suitability to their situation. This is why air filter testing should always be performed by independent laboratories that have the ability to adequately provide replicable tests that will give valid data with which to draw conclusions.

With air filter testing, there exist three major concerns:

• What will the efficiency of the air filter be in removing the airborne contamination of interest to the user?

• How much of this dust will it remove before maintenance is required?

• What resistance to airflow will the air filter create?

An additional concern exists in the safety of the equipment in any HVAC system. As such, in the United States and Europe, as well as most other countries, there exists air filter fire safety standards.

EXPLANATION OF EN779 AND ASHREA 52-1

For the purposes of simplicity, we will consider both the ASHRAE 52-1 and the EN779 air filter tests as essentially the same.

In essence, these sets of standards describe the methods used to yield a set of results for a variety of different test dusts. These tests are conducted by placing the sample to be tested into the test rig. A known quantity of dust to be tested for is then placed into the air stream of the rig over a set period of time on the up-stream side of the air filter. The air on the down-stream side of the filter is then measured for the presence of the test dust. The difference in the two measurements over time gives rise to the results that are shown on a test report.

A test report (see below) gives the results of the test both initially and over time. The results can then give rise to an average result for that test dust. As you will notice, the filters effectiveness in stopping the dust increases over time, but subsequently, the resistance to airflow also increases as the dust load in the filter increases. As such, both the initial results and the average results are important (the initial result is also in most cases the minimum result that can be expected from that filter sample).

Figure 1– Sample Test Report

The results of these tests can then be used to classify the air filter sample into a class as shown in the table below.

Figure 2 – Filter Classification as per EN779 (ASHRAE 52.1)

As you will notice, in this table the results of two different test dusts are shown, efficiency and arrestance. These test dusts are of a constant composition. The test dust used to determine efficiency is composed of a median particle size of around 0.65 mm with a few particles above 1.6 um. The efficiency figure of a filter gives an indication as to its performance for sub-micron particles.

Arrestance test dust on the other hand, is composed of Cotton Linters (5%), Mollocco Black (23%), and Air Cleaner Fines (72%) which gives a range of particle sizes from 0-80 mm (see table below) and it is designed to simulate the dust found typically in airconditioning systems. The results of the dust holding capacity of the filter’s arrestance is a good indication of filter life.

ASHRAE 52-2

This new test is almost entirely different from ASHRAE 52.1. This method uses a laboratory-generated, instead of atmospheric dust, for the actual measurement of filter efficiency. It does not measure discoloration efficiency but rather it determines a filter’s performance in 12 different particle size ranges (shown below) using a particle counter. A set of particle size removal efficiency (PSE) curves at incremental dust loadings are developed. From this data, a composite curve representing the minimum efficiency reporting value (MERV) of the filter can be constructed.

Figure 3 – MERV particle size ranges

From these results a MERV rating is then given to the air filter as per the chart below.

Figure 4– MERV particle size ranges

ASHRAE 52-1 OR ASHREA 52-2?

Well, simply put, the 52-1 test is the old standard and 52-2 test is the new standard. Both do however still have relevance. 52-1 is still the most common test results published in Australia and as such is still a useful tool with which to compare filters. 52-2 however gives a better understanding as to how the filter will perform within a given particle size range. A correlation between the two tests can be shown below. This will give you an indication as to where each tested filter will fit in.

Figure 5 – Comparative overview of filter test methods and filter classification used in USA and Europe (see: Filter Test Method Comparison Chart in our tech section)

FIRE RESISTANCE

As mentioned earlier, an air filter’s fire resistance is tested for and classified in many countries including USA, and Europe but unfortunately that is not the case here in Australia. In the interests of “Worlds Best Practice” however, I would strongly suggest that you do not use or recommend any filter that has not been tested and certified to a particular fire classification. It is also noteworthy that the existence of a statement that “this filter is manufactured in the fire retardant resin system” does not mean that it has been either tested or classified to any world standard.

All filters that you use or recommend should be classified as compliant to UL900 Class 1 or Class 2 or the European DIN standard classification. The brochure should state that the filter “conforms to all European Union and U.S. Fire Classification standards [DIN 53438 - F1 and UL 900 – Class 1 (or UL 900 – Class 2)] and are self extinguishing.

So why is it important to use fire rated filters in a HVAC system? Well the simple answer is the protection of the building and it’s content (most importantly, all human occupants) against fire. Think of it as one piece of equipment in a whole list that can help minimise any damage that may be created by a fire. Sure the filter won’t put the fire out, but you certainly don’t want the filter adding fuel to the fire.

SO WHERE DOES AS1324 FIT INTO ALL OF THIS?

AS1324 - Air filters for use in general ventilation and airconditioning, was last updated in 2001 (part 2 in 2003).  It was based on the old European standard EN779-1993 test, but it does have a few fairly major differences.  These differences, on the surface, may seem innocuous, but they do greatly affect the classification of a test filter, and of course the minimum standard that is required for an air filter to meet before it can be sold.

The first and most obvious of these changes is the change in a filters fire resistance.  As mentioned above, according to AS1324, the only requirement that a filter needs to comply with as far as fire retardancy is concerned is "....the flashpoint of any gel or oil coating on the air filter shall be no less than 160 deg. C".  Of course this means that all filters that are not coated with a gel or oil need not be tested at all (which is the vast majority of filters sold in Australia), and even those that are treated do not need to be tested, only the gel or oil that is added to the filter needs to be considered.  As such, there is no real requirement under AS1324 for a filter to be fire tested, the results of which can be easily seen in our demonstration video:  Filter Media Fire Classification Demonstration video (Medium Resolution ~2mb) in the Tech section of our website.

The next issue that needs to be considered is that in relation to a filters efficiency (No. 1 test dust according to AS1324) according to AS1324.2 "an accuracy of ±15% of the observed penetration may be assumed with this test method."  This means that a filter that tests at 45% (F5 rating) could be 30% (the lower G4 rating). But when testing to EN779, which has a variation of ±3%, then a filter that tests at 45% (F5 rating) may be as low as 42% (still an F5 rating).

Finally, there has been no independent NATA certified testing authority that tests for AS1324 for quite some time. This of course means that all reports that state that a filter has been tested to this standard are out-of-date and should not be relied upon. (see the NATA website if you would like to confirm this for yourself - simple type in AS1324 into their "Find a Facility or Lab search tool")

So when it come to filter testing, the only methods that can be relied upon are the American standards ASHRAE 52.2 and 52.1 tests and the European standards EN779 test.

Of course, if you want to ensure that you are purchasing the world best air filters, then you must make sure that it has one of the logos below on the filter