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 Many homes are equipped with central forced-air systems that rely on ducts to transport heated or cooled air from the furnace to the rest of the home. If the ducts are leaky, this can lead to poor HVAC performance and comfort problems through loss of heated or cooled air, creating a loss of air pressure in the ducts. 

Air leakage problems can be worst at the HVAC furnace or air handler cabinet, where air pressures are highest. Cabinet seams, holes, and junctions should be sealed to prevent air leakage. 

The only place air should be able to leave the supply duct system and the furnace or air-handling unit is at the supply registers. 

The only place air should be able to enter the return duct system and the furnace or air-handling unit is at the return grilles. 

 The connections with the supply and return plenums are areas that experience difference of pressures (positive or negative). These seams and other cabinet seams should be sealed with mastic, and UL-181 approved tape.