A basic blower door system includes three components - a calibrated fan, a door panel system, and a device to measure fan flow and building pressure. The blower door fan is temporarily sealed into an exterior doorway using the door panel system. The fan is used to blow air into or out of the building which creates a small pressure difference between inside and outside. This pressure difference forces air through all holes and penetrations in the building enclosure. The tighter the building (e.g. fewer holes), the less air you need from the blower door fan to create a change in building pressure.
The holes or leaks in the wall (or duct-system) can be made visible by a smoke or an infrared device.