The test is similar to a plumber pressure testing water pipes for leaks. A duct blaster combines a small fan and a pressure gauge to pressurize the duct system. The fan is connected to the duct system at the air handler cabinet or a return grille, and all remaining registers and grills are temporarily sealed. The fan is then turned on to force air through the ducts, and the resulting airflow through the fan is measured to determine how much air is being lost to the outdoors. The leakier the house, the harder the fan will work to maintain the pressure.