Heated air is less dense than cold air, so houses tested in cold weather appear leakier than they really are (by about 1 percent for each 10°F difference between indoor and outdoor temperature) unless an adjustment for temperature has been made. Otherwise, testing will indicate the amount of less dense air flowing through the blower door, and not the amount of colder, denser air flowing through the holes.
In a depressurized house, air will rush in through any available opening, so combustion appliances need to be shut down during a blower-door test to prevent backdrafting. Here, a smoke puffer indicates that the chimney flue is leaky even with its damper fully closed.
A digital manometer is used to measure the oilfired furnace's draft, or ability to vent combustion gases.
Air leaks around windows and doors typically appear as blue "fingers" on the IR screen, while blue patches indicate conductive losses from problems like thermal bridging, insulation voids, and moisture damage.
Duct system leakage can be estimated using the 'blower-door subtraction method," but a Duct Blaster test is more accurate. After the supply and return registers are sealed with tape, airflow is directed into the supply plenum, measured at a reference pressure of 25 Pa, and compared with accepted leakage rates.