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Filter Maintenance: Tips to Avoid Bypass and Coil Fouling

August 2015 Image

By: Robert Martin, CAFS, Kimberly-Clark Professional Filtration  |   08.13.15

Air that bypasses filters can lead to fouling of HVAC system components like coils and fans. Not only can bypass air increase HVAC system operating costs through inefficient operation and increased maintenance, it can also affect IAQ by increasing the amount of airborne contaminants reaching building occupants. 

Fouling reduces airflow through the HVAC system and heat transfer in the coils, which can add up to a significant increase in energy costs. Even a moderate amount of filter bypass can dramatically increase HVAC heat exchanger fouling and lead to decreased heating and cooling performance. 

Filter Installation and Maintenance Tips

Even a filter with a high MERV may not protect the HVAC system and building occupants from airborne particles if it is not installed and maintained correctly, allowing air to bypass the filter instead of flowing through it.  


Did You Know?

The particles most responsible for coil fouling are 1-10 microns.



Bypass occurs when filter media is not properly sealed in the filter frame, when filters are not properly installed and gasketed in filter racks, or when air handler doors and ducts are not properly sealed. Improper filter installation and poor gasketing create gaps around the filters. Even small gaps can have a big effect: a mere 1mm gap in the installation of a MERV 15 filter can reduce its efficiency to MERV 14, and a 10mm gap can decrease performance all the way down to a MERV 8. 

To avoid bypass air and make sure all the air in the system goes through the filter, consider these tips:

  • Before installation and periodically during operation, visually inspect filters and replace ones that are damaged.
  • Install the filter according to the air flow direction indicated on the frame.
  • Make sure that all filter housings have good filter gaskets, preferably with a non-porous gasketing material.
  • Check to be sure that the filters are properly seated in the filter housing or channel.
  • Ensure that the filter fasteners are in place and correctly installed, especially if filters are serviced from the downstream side.
  • Check to ensure that the bank of filter frames is rigid and well reinforced to avoid collapse.
  • Caulk any cracks between filter banks and the duct wall to prevent leaking of unfiltered air.
  • Make sure all air handler entry doors are gasketed and tightly sealed.
  • Monitor filters regularly to make sure they are providing maximum filtration while not overtaxing the supply fan capability or leading to a “blow-out” situation with no air filtration.

If your building suffers from problems relating to bypass air, be sure to pay close attention to proper filter installation and maintenance procedures to avoid the problem.


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