How A Blower Door Works
As you can see in the photo the blower door is installed in an exterior doorway. The equipment consists of variable speed fan, fan speed controller, adjustable door fram for sealing the opening and a set of manometers to measure the pressure differential between the inside and outside of the house.
With this we can then calibrate the equipment and set if to the standard pressure of 50 pascals so that we can determine an ACH50 rating. ACH refers to Air Changes Per Hour.
Once our pressure is set the diagnostic measuring equipment is able to measure the volume of air being pushed through the fan at the desired pressure. This is used to calculate the air exchanges per hour at a given pressure and hence our ACH50 value. The actual test only takes about 10 minutes once everything is set up.