The transient performance of a warm air heating distribution system is examined both experimentally and theoretically. Field tests, laboratory measurements and theoretical analysis indicate that considerably less heat is transferred to the living area through the registers than a steady-state theory would indicate. This discrepancy can be adequately explained by taking into account the heat storage effects of the duct system. The amount of heat transferred to the living area by a forced warm air duct depends upon the length of the blower cycle, the characteristics of the ducts, and the surroundings through which the ducts pass, even if these surroundings are completely interior to the dwelling. The effect of retrofitting the duct system with internal or external insulation is studied. It is shown that the full benefit of the added insulation cannot be obtained unless the burner and/or the blower on-periods of the furnace are increased. This needed increase is less if the insulation is added to the interior of the duct.

With current uninsulated duct systems, typically half of the furnace output that is not transferred via the registers, and not lost to unheated space, is transferred via the duct walls, which does not lead to the intended distribution.