The effect of a cool-coating on the thermal effectiveness of exposed exterior ducts

Rooftop air conditioning units are predominantly used for air conditioning of small- and medium-sized commercial buildings. The majority of these rooftop units are all-air systems, using ducts to deliver conditioned air. One source of inefficiency in the delivery of the conditioned air through the rooftop ducts is thermal losses by conduction heat gains through exposed ducts. Most existing ducts are made from bare metal sheets. The daytime peak surface temperature of a rooftop bare metal sheet duct can be as much as 35-40 K warmer than that ambient air. Using sheet metals that are painted white (in a new installation) or coating the existing exposed ducts (in a retrofit application) lowers the duct surface temperature, reducing the heat conducted into the ducts and hence improving the delivery effectiveness of the duct system.

To investigate the effect of increasing the solar reflectance of a duct to reduce conduction heat gains through the duct shell, we measured in three duct systems supply air temperatures at the inlet of the duct and close to the point of delivery into the conditioned space. In one system, coating the ducts with a white paint decreased the average conduction heat gain by about 0.64 MJ/day/m² of coated duct surface area, improving the delivery effectiveness by 2%. In the other two systems, the reduction in the conduction heat gain was about 0.34-0.46 MJ/day/m² and the delivery effectiveness improved by about 5-10%.