Leaking ductwork can lose 20 percent or more of the conditioned air in a supply duct run. This loss is duplicated on the return air side, where a large volume of untreated air can be drawn into the duct. Instead of returning cold air to the unit from the conditioned space, the infiltration of air into the duct makes the unit work harder to cool the (humid) warm air. The opposite takes place on the supply side. The net effect is that the ceiling space is being air-conditioned unnecessarily.
Leaking ductwork in a ceiling space between floors is not as bad as leaking duct in the roof ceiling space or an unconditioned zone with an exterior or roof exposure. The average temperature between floors may be 5 or 10° different from the adjacent conditioned areas. The temperature difference will be significantly higher in a roof space, so there will be a more significant impact on the equipment efficiency. As a result, it’s important to first devote limited resources to repairing leaking duct in the roof ceiling space, then in unconditioned spaces, then in the balance of ceiling zones.
An important principle to keep in mind here is that the static pressure in a duct run decreases steadily from the air handler through the main and the branch lines. A leak in a main duct has a much greater impact than the same size penetration in a branch duct because the pressure is greater in the main. Consequently, duct repairs should concentrate on the main lines first, then the branch lines as resources permit.
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Excerpt from Energy Conservation Projects.