How Duct Leakage Testers Work
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 (Vent Cap Systems). One register or return is left unsealed. The calibrated fan is then connected to that unsealed register. Pressure is monitored in one of the branches of the ductwork while the calibrated fan delivers air into the system. As air is delivered into the ductwork, pressure builds and forces air out of all of the holes in the various ductwork connections or through the seams and joints of the furnace or air-conditioner. The tighter the ductwork system (e.g. fewer holes), the less air you need from the fan to create a change in the ductwork pressure.
A duct leakage test can be performed by either pressurizing or depressurizing the ductwork. Ductwork that is outside the building envelope, meaning in an unconditioned attic or crawlspace, should be pressurized so as to not bring in unwanted contaminates.
Duct tester airtightness measurements are presented in a number of different formats including but not limited to:
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