TABLE OF CONTENTS Heat gain to supply air through ductwork typically results in a temperature rise of 3 F to 5 F (1.6 C to 2.7 C). In a system where air is returned to the air-handling unit via the ceiling plenum, heat gain to supply ducts is normally accompanied by a corresponding heat loss from return air, and the cooling effect is not entirely lost to the system. However, the duct heat gain is added to the space-cooling load, increasing the required supply air volume.
Duct heat gain depends on the temperature difference between the air inside and the air surrounding the duct, on the duct surface area, and on the overall heat transfer coefficient, or U-factor.
The temperature difference across the duct wall may be 40% to 70% greater in a cold-air distribution system than in a conventional design, while the surface area is typically 15% to 40% less. With equal insulation levels, the heat gain for a given cold-air system might be from 15% less than to 40% greater than a conventional design. Normally, increased insulation thickness is specified for cold-air distribution systems, reducing the heat gain to 40% to 80% of that in conventional systems. However, even with this lower heat gain, the supply air temperature rise in the cold-air distribution system will be equivalent to or somewhat greater than that in the conventional design, because of the reduced supply air quantity.
The duct heat transfer coefficient depends on the type, density, and thickness of insulation.
