During a call for heat, which terminals must be closed on the thermostat for it to operate?

To operate a gas or oil heating system during a call for heat, the thermostat needs to create a closed circuit between certain terminals. The correct combination is to close terminals "R" (which represents the power supply from the transformer) and "W" (which represents the heating signal).

When the thermostat is set to call for heat, closing these terminals completes the circuit that signals the heating system to activate. The "R" terminal provides the necessary voltage, while the "W" terminal indicates the request for heating. This action engages the heating system, allowing it to begin the heating cycle.

In this context, the other terminal combinations do not serve the same purpose for initiating heating. Terminals "G" and "Y" typically relate to the operation of fans and cooling systems, respectively, while "C" is generally used for the common wire in a cooling operation, and closing "W" and "C" does not directly activate the heating system in the same manner that closing "R" and "W" does. Therefore, the focused relationship between the terminals in the correct answer is what enables the heating system to respond to the signal for heat.