What is the purpose of fan time/temperature delay switches in gas furnaces?

Fan time/temperature delay switches serve a critical function in gas furnaces by ensuring that the heat exchanger reaches an optimal temperature before engaging the fan. The primary goal is to allow the heat exchanger to warm up enough so that when the blower fan kicks in, it circulates warm air rather than cold air throughout the space. This process enhances comfort, improves energy efficiency, and helps maintain a consistent indoor temperature.

The logic behind this feature is to maximize the efficiency of the heating process. If the fan were to start too early, it could lead to discomfort due to the mixing of cold air with the already heated air. Therefore, the delay allows the system to maximize heat transfer within the exchanger before distributing warm air into the living areas. This design choice contributes to the overall effectiveness of the furnace and improves the heating experience for users.