Solving Filter Bypass Problems

An open filter rack is often installed in an HVAC system to allow easy access to the filter. The problem is by allowing easy access to the filter, it also creates space around it, which leaves the potential for leaks to occur within the system.

As airflow moves closer to the fan within the HVAC equipment, static pressure builds. On the return side of the fan, this air pressure is negative and draws the airflow that is moving through the system. This fan is always moving airflow, and it doesn’t care where it’s coming from.

Air will always take the path of least resistance. With an open filter rack, a lot of unfiltered air is allowed to directly bypass the filter. Air is pulled around any gap it can find, and will easily pull air from all around the filter. When unfiltered air is pulled around the filter, dirt and dust in the air have a greater potential of being distributed by the HVAC system to the rest of the building.

Dirt and dust will accumulate on the first surface it finds as it bypasses the filter. In a gas furnace, it will find the blower wheel. In a condensing furnace, the secondary heat exchanger is right in its path. Air handlers used with heat pumps allow dirt and dust to accumulate on indoor coils.

How do you correct the problem?

The filter rack needs to be as airtight as possible to prevent any unintended bypass effects or depressurization effects from occurring. If the filter is sized properly and has an acceptable pressure drop, the correction may be as simple as retrofitting an airtight filter access door. If the pressure drop is unacceptable, it may be time to upgrade the filtration system and bring in the proper amount of airflow.

On all new systems we change out, a new custom built 1” filter rack comes with the system installation. The filter rack has gaskets around the door to prevent outside air leakage into the system. If Indoor Air Quality has interest to you, we can install a 5” air sealed filter cabinet for Merv 11 or Merv 16 Filters. Protecting your health and your investment is what RS Andrews is about.

How efficient is the airflow in your HVAC equipment?