air door are powerful enough to provide an air stream to project across the whole doorway opening. Overdoor heaters may have less powerful fans. The discharge nozzle on an air door is optimized to provide a uniform air stream across the whole width of the doorway, which may not be the case with overdoor heaters. Energy savings Air curtains consume electrical energy during their operation, but can be used for net energy savings by reducing the heat transfer (via mass transfer when air mixes across the threshold) between two spaces. However, a closed and well-sealed physical door is much more effective in reducing energy loss. Both technologies are often utilized in tandem; when the solid door is opened the air curtain turns on, minimizing air exchange between inside and outside. An air curtain may pay for itself in a few years by reducing the load on the building's heating or air conditioning system. Usually, there is a mechanism, such as a door switch, to turn the unit on and off as the door opens and closes, so the air curtain operates only while the door is open. Design An authoritative engineering design procedure for calculating the supply air flow and thermal capacity of an air curtain for an HVACR application is explained in the BSRIA Application Guide 2/97 The procedure for a "Building with an Air Tightness Specification" should be followed, i.e. a practical building with some air leakage. Within the BSRIA Application Guide, Section 4.2 explains the design procedure and Section 5.2 gives worked examples for buildings with a range of air tightness specifications. This allows the engineer to calculate the supply air flow rate and thermal capacity of the required air curtain for a particular application. See also References External links Heating, ventilation, and air conditioning