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

The purpose of fan time/temperature delay switches in gas furnaces is to allow the heat exchanger to warm up before the blower fan turns on. This function is crucial for efficient heating. When a gas furnace is ignited, the heat exchanger needs time to reach a certain temperature to ensure that the air blown through the ducts is heated effectively. If the blower were to operate without this delay, it could circulate cool air initially, leading to discomfort and inefficient heating of the space.

By delaying the operation of the fan, the heat exchanger can reach optimal temperatures, ensuring that the air delivered into the living space is warm and comfortable. Additionally, after the furnace has completed its heating cycle, the switch may keep the fan running for a short period to circulate any remaining heat from the heat exchanger, maximizing efficiency and comfort.