Space Heating MUA |

Make Up Air Systems


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


Whenever air is power vented from a space, whether it be from heavy industrial processing or kitchen cooking, the laws of physics tell us the air will be replaced. Air pressure always seek to equalize. A building suffering from negative air pressure – air is being vented, but not being replaced fast enough either mechanically or naturally, has several undesirable affects.

Minor signs of negative air pressure are exterior doors that are hard to open and slam shut; persistent odors; or drafty cold exterior walls and windows. More serious factors are processes that have poor quality – such as painting, or severe indoor air quality problems caused by back drafting of flue gasses and other contaminates that were supposed to have been vented. Negative air pressure that causes these problems is measured in fractions of an inch on a manometer.

Make up air units can use 100% outdoor air as their source. These units are generally used when they are not the only heating source and turned on only when needed by the process. Newer units are often used as the ONLY heating system and will have indoor air re-circulation as an option. Generally, they will modulate between 80/20 mixture as demand dictates, controlled either by temperature or air pressure.

There is some controversy over the use of direct fired equipment in certain types of spaces (mainly where indoor air is not mechanically ventilated). Actual installations have typically shown that air quality improves with a MUA unit.   However, be sensitive to this issue – especially for low air turn over spaces where the unit would likely be in the re-circulation mode most of the time.

The recirculating units are much more economical to operate, assuming the space does not demand as much outside air. For more information on recirculating units see Make Up Air Recirculation



Typical ratings are 1 CFH of gas for each 10 CFM of air; so a 10,000 CFM unit would have an input gas rating of about 1,000 CFH or 1,000,000 BTUs. (Max rating; will vary with air temperature and temperature rise setting)

Most all manufacturers/reps will desire elevated gas pressure as they will be able to get more heating capacity from a smaller unit and down size the gas train; however, standard pressure options are available on most units up to about 25,000 CFM (2,500,000 BTUs)

Operating economies can be improved with automatic controls such as indoor/outdoor pressure differential switches that control the mix of return air and time clocks that shut the unit off completely during unoccupied or non-process hours. However, indoor air quality and the reason the unit was installed in the first place, must be considered before limiting a MUA’s use. If dictated by air quality, MUA is just a cost of doing business.   Advances are being made in filtration and source point contamination removal, but MUA (dilution and replacement) still remains as the number one option for improving indoor air quality.



The following is a very limited list of manufacturers of industrial make-up-air systems. MUA systems like other HVAC systems are sold nation wide by local distributors and installers. Systems should be selected based on the local distributor and dealer/installer support network.
Absolute Aire
5496 North Riverview Drive
Kalamazoo, MI 49004
Telephone: 800-804-4000


Applied Air
4830 Transport Drive
Dallas, TX 75247
Telephone: 214-638-6010


Cambridge Engineering, Inc.
P.O. Box 1010 Dept. K
Chesterfield, MO 63006
Telephone: 800-318-0661 or 636-532-2233


Rapid Air Management Systems
1100 Seven Mile Road
Comstock Park, Michigan 49321
Telephone: 616-784-0500 or 800-536-3461


RUPP Air Management Systems
17645 Juniper Path Suite #150
Lakeville, MN 55044
Telephone: 800-291-2452, 952-435-0870




Source: Text Bob Fegan 1/1009; Make Up Air Graphic – Rapid Engineering web site, 8/2001;

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