A blower door is a large calibrated fan that is temporarily mounted in a house door to measure the "leakiness" of the house and to assist in finding the location of the leaks. Modern blower doors have variable speed fans so that the pressure in the house can be adjusted, and they also have door mounting frames so that the fan can be sealed tightly into the door jamb. In order to measure the leakiness of the house, the blower door measures both the air flow through the fan and the pressure difference between the house inside and outside.