The Wikipedia Definition for Duct Leakage Testing. A duct leakage tester is a diagnostic tool designed to measure the air tightness of forced air heating, ventilating and air-conditioning (HVAC) duct work. A duct leakage tester consists of a calibrated fan for measuring an air flow rate and a pressure sensing device to measure the pressure created by the fan flow. The combination of pressure and fan flow measurements are used to determine the duct work air tightness. The air tightness of duct work is useful knowledge when trying to improve energy conservation.
A basic duct leakage testing system includes three components: a calibrated fan, a register sealing system, and a device to measure fan flow and building pressure. Supply registers or return air grills are sealed using adhesive tapes, cardboard, or non-adhesive reusable seals. One register or return is left unsealed, and the calibrated fan is connected to it. Pressure is monitored in one of the branches of the duct work while the calibrated fan delivers air into the system. As air is delivered into the duct work, pressure builds and forces air out of all of the openings in the various duct work connections or through the seams and joints of the furnace or air-conditioner. The tighter the duct work system (e.g. fewer holes), the less air is needed from the fan to create a change in the duct work pressure.
A duct leakage test can be performed by either pressurizing or depressurizing the duct work. Duct work that is outside the building envelope, such as in an unconditioned attic or crawlspace, should be pressurized so as to not bring in unwanted contaminants such as dust.