Air Recirculation Units


    • Industrial and large Commercial Space Heating
    • Air Replacement Systems (Ventilation)


See Also Make Up Air and Blow-Thru Space Heater

Make Up Air units with the ability to re-use less than 100% outdoor air are called Recirculation or 80/20 units, meaning up to 80% recirculated inside air is mixed with at least 20% outside air. The best units will have variable air volume control with an adjustable mixed air damper controlled by air pressure and temperature.

Most manufacturers make both kinds of units, and they are often confused as being the same kind of equipment. A closer inspection will reveal 2 ducts between the unit and the conditioned space (even this is sometimes difficult to spot in some installations, such as roof-tops where a very short curb is used). Notice in the indoor photograph the second duct coming through the ceiling of this roof-mounted MUA Recirculation unit. The outdoor photo shows the hooded outdoor air intake, and below the unit penetrating the roof are the recirculation air intake duct and the supply duct to the space.



MUA Recirculation units operate similar to standard MUA. They use a draw-thru blower and similar temperature rise. Similar sizes, ratings, and installation options are available.

Advantages over non-recirculation units

  • lower operating cost for spaces that do not require as much outside air
  • can be used as a buildings only heating system

Disadvantages over non-recirculation units

  • more expensive to purchase and install
  • similar operating cost when building is requiring a lot of ventilation air



See Make Up Air






Source: Text Bob Fegan 1/2009; photo by Bob Fegan;

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