Pressure Diagnostics and Testing
The blower door
A blower door measures envelope airtightness by pressurizing or depressurizing the house and recording airflow at a reference pressure, usually 50 Pa. The result is CFM50, often normalized as air changes per hour at 50 Pa (ACH50) or as leakage area. Candidates must know that CFM50 is not natural leakage; natural infiltration is much lower and depends on weather. A rough conversion is ACHnatural is about ACH50 divided by 15 to 20, but use the exam's stated method.
The test sequence and traps
Know the standard steps: prepare the house (close windows and doors, open interior doors, turn off combustion appliances or verify they are off, set thermostats), install the door, establish baseline pressures, then test. Traps include testing with the attic hatch open, leaving a fireplace damper open, or failing to account for a baseline pressure difference. Also know that a blower door can be used with a pressure pan or smoke to find leaks, and that zonal pressure testing evaluates the pressure boundary between zones.
Duct and other diagnostics
Know duct blaster testing (duct leakage at 25 Pa), pressure pan readings across registers, and the use of manometers. Understand that duct leakage to outside is worse than leakage to conditioned space. Combustion safety testing (CAZ, worst-case depressurization) belongs here too.
Worked example
A 2,000 square-foot home with 8-foot ceilings has 16,000 cubic feet. A blower door reads 3,200 CFM50. ACH50 = 3,200 x 60 / 16,000 = 12 ACH50. If the exam uses a divide-by-20 rule, natural leakage is roughly 0.6 ACH, which is a moderately tight house.