Which terminals should measure 24 volts if the thermostat is calling for heat?

When the thermostat is calling for heat, it sends a signal to the heating system to activate. In a typical gas heating system, this signal results in specific terminals measuring 24 volts to confirm that the system is in heating mode. The terminals that would measure 24 volts in this case are the white wire at the sensor and the gas valve terminal designated as GAS 2.

The white wire often represents the heating call from the thermostat, and it serves to activate components such as the gas valve when there is a need for heat. The GAS 2 terminal likely corresponds to the second stage of heating in a multi-stage system, where additional heat input is necessary to reach the desired temperature.

Understanding this function is crucial for diagnosing heating systems, as measuring the voltage at these terminals helps to confirm that the thermostat's call for heat is effectively reaching the necessary components to initiate the heating process.