Which component is primarily responsible for heat loss in ducts?

The component that is primarily responsible for heat loss in ducts is the temperature difference between the inside airflow and the outside environment. When there is a substantial temperature gradient between the warm air flowing through the ducts and the cooler air outside, heat will transfer from the warmer air to the cooler surroundings. This phenomenon is governed by the laws of thermodynamics, specifically heat transfer principles such as conduction, convection, and radiation.

As the temperature differential increases, so does the rate of heat loss. Therefore, if you have ducts transporting hot air during winter months, the significant difference between the heated air and the colder outside air will lead to more heat being lost through the duct walls. In warm climates, the process works similarly, where cooler air from the air conditioning system will absorb heat from the warmer outside environment.

While the material of the duct, its length, and the humidity level do play roles in the overall efficiency and thermal performance of the duct system, they are secondary factors relative to the impact of temperature difference. Duct insulation materials can slow down heat transfer, the duct length can influence resistance, and humidity can affect air density, but the immediate driver of heat loss is the temperature differential.