How is the fan cycled during heating in a gas furnace that has an integrated furnace control (IFC)?

In a gas furnace with an integrated furnace control (IFC), the fan is typically cycled through a heat relay as part of the control system's designed operation. The IFC manages multiple functions for the furnace, including ignition, burner operation, and fan control.

When the furnace operates, the heat relay receives signals from the IFC to turn the fan on once the heat exchanger reaches a predetermined temperature. This ensures that warm air is circulated throughout the space effectively. Once the desired temperature is achieved, the relay also controls the fan's operation to turn it off, which helps to optimize energy usage and maintain comfortable indoor conditions.

This cycling mechanism through the heat relay is crucial because it automates the process, allowing the furnace to operate efficiently without requiring manual intervention or being dependent solely on the thermostat's direct control. The other methods, such as continuous operation or manual control, would not provide the energy efficiency and automated response needed for optimal comfort in the same way that a heat relay integrated within the furnace control does.