Duct Heat Loss and Gain
Duct heat loss or gain is another important factor to consider when designing a duct system. This aspect of heat transmission will depend on some or all of the following factors:
• Temperature of the air in the duct
• Ambient temperature
• Air velocity in the duct
• Duct insulation
If the temperature of the air inside the duct is different from the temperature surrounding it (that is, the ambient temperature), then either a loss or gain of heat will occur.
If the ducts are used to convey heat, excessive heat loss from the ducts will reduce the efficiency of the heating system. This will result in a total loss of heating effect if the heat loss occurs in duct- work passing through an unheated area, but it can be considerably reduced with proper duct insulation (see Duct Insulation). Such heat loss can also occur in heated spaces, but here it is a problem of poor air distribution.
The same conditions exist when the ducts carry cool air. When air passes through spaces subject to the cooling effect of air-conditioning, insulating the ducts will effectively reduce the amount of heat gain.
Proper air distribution will also minimize heat gain in spaces that are partially cooled.
Air velocity will also influence the amount of heat gain in the ducts. High air velocities are recommended when the ducts are carrying cool air because their effect is to reduce the amount of heat gain pickup in the ducts; however, they must be maintained consistent with the acoustic requirements of the installation.