Effect of airflow and heat input rates on duct efficiency

Reducing the airflow and heat input rates of a furnace that is connected to a duct system in thermal contact with unconditioned spaces can significantly reduce thermal distribution efficiency. This is a straightforward theoretical calculation based on the increased residence time of the air in the duct at
the lower flow rate, which results in greater conduction losses. Experimental tests in an instrumented residential-size duct system have confirmed this prediction. This paper compares test results with the heat-loss algorithm in ASHRAE Standard 152P. The paper concludes with a discussion of possible remedies for this loss of efficiency in existing systems and optimal design strategies in new construction.