Using Partial Bypass For HEPA Air Filtration

When we hear the term, bypass, we often think of the medical procedure relating to “heart bypass.” Or, to get to route I-70 just take the  Heron road” bypass” off of Jefferson street. These and some other uses for the term bypass may come to mind. However, in this particular blog the term partial bypass is related to one of the methods by which HEPA air filtration systems are attached to an existing heating/cooling system.

Pure Air Systems has been manufacturing HEPA and Carbon based fan powered air filtration systems for 25 years. PAS was one of the first air filtration companies to introduce true, certified, commercial grade HEPA filters into the residential market. They now offer five different systems with over 30 variations and sell them across North America, in Europe and Asia.

One of the challenges in utilizing commercial grade HEPA filters is their resistance to air or pressure drop. It requires a fair amount of power to pull air through the HEPA. For this reason the ECM motors used by PAS range from 1/3 HP up to 1 HP. Also the air flow rates through the systems range from less than 50 cubic feet per minute up to 2000 cubic feet per minute ( CFM ).

The systems were initially designed to attach to the return air side of any forced air, ducted, heating/cooling system or HVAC system. The partial bypass approach allows the use of one of the smaller PAS HEPA systems such as the model 600HS unit ( 600 CFM) to be attached to any  HVAC system up to 5 tons or 2000 CFM. By simply drawing only 600 CFM from return air side of a 1500 CFM system, for example, the 600HS will clean all the air coming through the entire HVAC system once every 3rd pass or about 48 times per day, as long as the HVAC fan is running.

By using the partial bypass approach even if the 600HS unit were to stop running, all the air would just bypass the unit and the HVAC fan system would not be subjected to the higher static pressures of the HEPA filter which could actually stall the HVAC fan if the HEPA filter system was put in-line and the air filter system fan stopped running. This safety feature is one of the benefits of using the PAS HEPA systems for attachment to heating/cooling systems in a partial bypass mode.

For applications where there is no ducted HVAC system, the larger PAS units can be used as stand-alone systems as is done with a number of commercial and institutional applications. For more information on the complete line of HEPA and Carbon based filters from PAS please go to our website at:

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