Space Heating with an Air Turn Over Unit


  • Industrial and Large Commercial Space Heating


A relatively new type of space heating for industrial and large commercial spaces.Built around the concept of the make-up air unit, but different in that it typically uses all indoor re-circulation air, or optionally a small amount of outdoor air make-up, and has indirect combustion (it has a heat exchanger).

Where heating and moving a large volume of air is desirable, but the operating cost of bringing in large amounts of outdoor air like typical make-up air units, or there is concern about a direct fired unit (products of combustion stay in the space). These units are popular in warehouses and light manufacturing when ventilation is small to none. They offer the ability to handle a lot of air volume and minimize stratification and cold spots with a limited number of units. Depending on size and space usage, often one unit will handle over 50,000 sq.ft. of floor space each (King Co claims up to 122,000 sq.ft.). They have no duct work, only air diverters out the top of the unit; all the return air comes into the bottom with the heating section located in the middle.

For new installations, first cost can be less because of the need for fewer units (such as unit heaters) which means fewer roof penetrations, gas and electric hook-ups, and thermostats. Operating cost could be less, as unit can be controlled easier (one thermostat instead of many), destratification reduces hotter temps at ceiling (reduced heat loss) and comfort could be improved when stratification is eliminated. Units are standard to new high efficiency (80 – 90%). Units may optionally include air conditioning and outdoor air intake.

Heating capacity range is 400,000 to 6,000,000 BTUs


Johnson MarCraft Inc.
11800 Dorsett Rd.
St. Louis, MO 63043
Telephone: 1-800-325-1303

Go to their web site at

From their web site:

Air Rotation is simply the most effective, least expensive way to heat and cool a large, open building. Because the air is constantly being displaced by tempered air, wall to wall temperatures rarely vary more than ±2º. The floor to ceiling temperatures are rarely more than 1º or 2º for every ten feet.

By using large prop fans to draw air in at floor level, moving the air up and over a heat exchanger or cooling coil and sending warm or cool air out the sides of the top. Moving a large volume of air at a very slow velocity creates a slow, almost undetectable rotation of the air. Expensive and unsightly ductwork is eliminated.

Johnson MarCraft brought Air-Rotation® to the market over 40 years ago. We maintain our position as an industry leader by offering these exclusive features:

  • Pressurized combustion system – Eliminates the need for induced draft fan and allows for smaller flue.
  • Heat exchanger designed specifically for large volume air movement – Maximum air movement with minimum fan motor horsepower.
  • Auto fan control – Operate two fans continuously or only one to save electricity. Both fans operate on a cell for heat.
  • Separate 120 volt and 480 volt panels – for added safety.
  • Adjustable air discharge louvers – Equipment is customized for any job condition.
  • Direct tube access – Longer equipment with built-in maintenance simplicity.

Single Fan Capacities

  • Heating capacities from 500,00 BTUs to 3,000,000
  • Air volumes from 18,000 cfm to 95,000 cfm

Double Fan Units Capacities

  • Heating capacities from 500,000 BTUs to 6,250,000 BTUs
  • Air volumes from 12,000 cfm to 100,000 cfm

The Air-Rotation® concept was developed in the 1930s and 1940s when founder Nelson Johnson began converting floor-mounted furnaces into large ductless units and installing them to heat and ventilate large open buildings. The Air-Rotation® concept and design was recognized by the United States Patent and Trademark Office as our invention, and we are the exclusive owner of the intellectual property associated with Air-Rotation® technology. Thousands of our units heat, cool and ventilate warehouses, manufacturing facilities, distribution centers, wholesale clubs and convention centers.
4830 Transport Drive
Dallas, TX    75247

Telephone: 214-638-6010

Go to their web site at

From their web site:

The Air Turnover principle eliminates air stratification in large open space buildings. It does this by recirculating the hot air which becomes trapped at the higher levels. The uniform room temperature improves comfort, conserves energy, eliminates thermal barriers, and eliminates the possibility of condensation forming on stored materials.

In the heating mode: The Air Turnover System picks up the air at the floor, where it is the coolest, then heats and returns it to the space above. Innovative low velocity, low horsepower propeller fans gently turn over the room air 1 to 3 times each hour to maintain uniform temperatures from floor to ceiling throughout the building. Cold air from open doors in shipping and receiving areas becomes less of a problem with an Air Turnover unit. Temperature recovery is almost instantaneous after the doors are closed.

In the cooling mode: The Air Turnover System picks up air at the floor then cools and returns it to the space above. A comfort zone is created from the unit discharge down to the floor level. For buildings requiring cooling from floor to ceiling, extensions are available to get the unit discharge to required height. The centrifugal fans quietly move large volumes of air, turning over the air in the comfort zone 3.5 to 5 times each hour.

Source: Text Bob Fegan 1/2009; Johnson-Marcraft web site 3/2003; Applied Air Company web site 12/2003;

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