How Long Can A Filter Remain In A System?
Phil Maybee, CAFS
I have seen debates on this question for as many years as I have been in the filter business. Over the years "experts" have been giving their opinions on filter life for various reasons and trying to defend their position with mostly anecdotal support data. Some make sense and others haven't been fully informed of the science they claim to exist and the potential of it becoming reality.
Facts - filters remove solid and liquid particles from the airstream. As the tortuous path through the filter media is clogged with dirt and debris the porous passages are restricted. IF the fibrous media is capable of retaining the dirt caught without unloading the filter will gain in particle size efficiency. Certain conditions will make filters unload - moisture is the most common as it dissolves contaminants and allows them to run through the media when there is enough moisture.
Facts - there are conditions that can cause filters to become microbial amplification reservoirs. This means that when the conditions are right previously caught microbes begin to grow within the filter. If these grow through the media and reformulate themselves on the downstream side of the filter then it is possible for a sporulation event to occur that releases spores into the system which said filters were designed to protect.
Charlie Wiles of the Indoor Air Quality Association made the comment that NO Filter should be allowed to operate more than six months as there would be microbial contamination that any lab would be able to culture from filter surfaces. This statement was made at a national convention in a presentation to over 1500 people in the IAQ industry. REALITY - any lab can find culturable organisms on a filter that has been in use for one day let alone six months. So we need a better basis for the filter change cycle than culturable microbial material.
Clean rooms use ceiling HEPA filters usually for the duration of a product manufacturing cycle. This can be 10 - 20 years in many cases and verification of HEPA performance is done regularly at either six month or annual intervals. Oak Ridge National Laboratories has HEPA filters that were installed in the 1960s that still operate and are tested regularly to verify performance.
Then there is the NAFA and ASHRAE push to monitor filters and check pressure drop to gain the optimum life cycle cost to make sure that we get the best "bang for the buck" when purchasing filters. Sometimes when we only check pressure drop we can miss failures that may occur that lead to support of folks like Mr. Wiles - and the posting of 8x10 glossy pix of the interior of HVAC systems that have been neglected. Or as we found in a gas turbine intake where the pressure drop values were recorded to determine filter life cycle and yet the media in the filters cracked and filters were open to penetration of any size airborne particle.
Thus filter life cycles are BEST determined by monitoring the performance of not only the filter, but of the system that the filter is designed to protect. What was the design intent of the filtration for the unit and area it serves? Visual inspection of filters should be made at regular intervals to verify that components are working correctly and media is intact, gaskets are still sealed, that there are no odors originating from the filter section. (ASHRAE STD 180) When a filter has gone beyond its useful life there are signs indicating the reason for replacement.
When looking at critical areas in a Hospital evaluate the conditions with baseline sampling for both particulate counts as well as microbial concentration. These should be compared to the outdoor values at the same time frame. This provides a snapshot in time of what "clean acceptable conditions" are for future comparison to design intent evaluation. When monitoring performance based upon pressure drop and airflow - these additional check points can be done annually or semiannually to verify that systems are working without compromise.
In less critical areas pressure drop and airflow are your key indicators. Many facilities now have upgraded filter efficiencies to MERV 13 in accordance with LEED guidelines. We have seen filters operate for over five years without signs of microbial growth, or odors, or physical degradation that would require filter replacement.
Interestingly it is possible now to install UVC systems in the return air ducts that will destroy the DNA and RNA of most viable organisms. When this kind of system is installed there will no longer be the microbial opportunity for filters to become amplification sites. These systems treat the air not the coil surfaces.
Thus to summarize - change the thinking of TIME based maintenance to performance based. Then verify the performance with credible evaluation. In operating in this fashion a facility will be effective and find that they operate with the lowest cost per CFM of air at the cleanliness that the design requires. All too often we find folks that cannot disassociate the length of time from a sense of need.