Complex sentence: Another form of total duct leakage test is to use a blower door to measure the total amount of leakage of the house, then to seal off all registers and returns, and to measure the total amount of leakage again.
Split into simpler sentences: Another form of total duct leakage test is to use a blower door to measure the total amount of leakage of the house. Then seal off all registers and returns and measure the total amount of leakage again.