Central In-duct Indoor Air Cleaners
Intended for installation in an HVAC system.

Advantages of a central air cleaner system over individual stand alone units include:

1. better clean air distribution,
2. less noise,
3. keep central heating/cooling equipment clean and efficient
4. ability to filter outside air before it enters the occupied area,
5. ability to place occupied space under positive pressure to reduce infiltration of contaminants
6. less maintenance than with many portables around the facility,
7. more cost effective than portables for larger areas

Installation methods:

1. Inline, In-duct.   
   

   The most direct way to install air filtration equipment in a central HVAC system is to place the equipment directly in the return duct at a common point where all the air is flowing back to the heating and cooling equipment.

   1. Advantage
      When the air filtration equipment is directly in the duct at a common point where all the air is flowing, the air must pass through the air filtration equipment in order to return to the heating and cooling equipment and therefore, all the circulated air is filtered.
   2. Disadvantage
      High efficiency air filters have very high air resistance.  As a result, placing high efficiency filters, such as HEPA filters, directly in the duct, imposes loading on the existing HVAC air handler which the air handler may not be able to adequately overcome.  For this reason, lower efficiency, lower loading, ASHRAE type efficiency filters are generally used in line, in duct, as they impose less loading on the existing air handler.  The use of high efficiency filters could result in the need for an air handler upgrade and additional cost.  An alternative to ASHRAE filters is the use of electronic air cleaners.  Electronic air cleaners generally impose relatively low loads on the HVAC air handler making them popular alternatives to high efficiency filters. Additionally, electronic air cleaners require routine cleaning rather than filter replacement, which is seen by some as an additional advantage.   However, electronic air cleaners will not provide equivalent efficiency as high efficiency filters such as HEPA filters, as electronic air cleaners usually are rated on a ASHRAE efficiency scale.  NOTE: More information about filter efficiency standards is available.
2. By-Pass   
   

   The By-Pass installation of air filtration equipment into an HVAC system circumvents the HVAC air handler loading issue noted in the inline-induct disadvantages above.  In the By-Pass installation, the air filtration equipment intake is connected to a port on the return duct at a common point where all the air is flowing back to the heating and cooling equipment.  After being diverted into the air cleaning equipment and filtered, the clean filtered air is then exhausted back into the HVAC system return duct downstream (closer to the heating and cooling equipment).  If the air cleaning equipment is properly matched to the air flow in the HVAC return duct, a large portion of the return air will pass through the air cleaner and be filtered.

   1. Advantage
      The By-Pass installation avoids the loading effect of air cleaning equipment on the HVAC air handler and also avoids the expense of an HVAC air handler upgrade.
   2. Disadvantage
      Since air is being diverted from the return duct to the air cleaning equipment, there remains a path for the air to return to the heating and cooling equipment without being filtered.  This is generally not an issue unless the application is critical in nature where the air contaminants must be removed on a single pass through the system.  Although only a portion of the return air is being filtered on each pass around the facility, the air is becoming cleaner on each pass and eventually will reach a steady state value of cleanliness after several passes.  Waiting several passes for the air to reach the desired level of cleanliness is generally not an issue in most applications.

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