During the duct blaster test, the HVAC system is sealed off from the rest of the house using temporary sealing tape and the cumulative effects of all the leakage from all gaps and cracks is measured. 

 

The test can also be completed in conjunction with the blower door test to isolate the leakage just to outside the building envelope or without the blower door to measure total duct leakage.

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As air is delivered into the ductwork, pressure builds and forces air out of all of the openings 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 is needed from the fan to create a change in the ductwork pressure.

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A duct leakage test can be performed by either pressurizing or depressurizing the ductwork. Ductwork 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.