What is the first 120 volt load in a call for heat?

In the context of a heating system, the first 120-volt load that activates when there is a call for heat is the induced draft fan. The induced draft fan serves a critical role in safely expelling combustion gases from the furnace to ensure that the system operates efficiently and safely. When the thermostat calls for heat, the control circuit typically initiates the operation of this fan as the first step in the ignition sequence.

The fan must operate before the ignition occurs to create a negative pressure within the combustion chamber. This negative pressure ensures that any flue gases are properly vented and that fresh air is drawn in for combustion, which is essential for maintaining a safe and effective heating operation.

Only after the induced draft fan is confirmed to be running correctly will the system proceed to the next steps, such as igniting the burner. This sequence helps prevent the accumulation of unburned gas and ensures that the air-fuel mixture is optimal for efficient combustion.