A blower door is a diagnostic piece of equipment (powerful fan with gauges) used by an energy auditor to measure house air exchange and pinpoint air leaks.
The energy auditor will prepare for a blower door test by first explaining the procedure and asking a few simple questions about your normal activities and use of the home. He or she will measure the house and determine the volume of conditioned (area that actually is to be heated or air conditioned) space in cubic feet.
The auditor will then prepare the house by shutting off the gas water heater and furnace, closing all exterior doors and windows, opening all interior doors, and sealing off fireplaces and wood stoves. The blower door is installed in an exterior door opening and the test is ready to begin.
After the test, the house and equipment should be returned to original conditions.
For supporting information and a diagram of a blower door, visit the U.S. Department of Energy web site