Which component is identified as open when the thermostat terminals read 0 volts and the gas valve coil terminals read 24 volts?

In the context of HVAC systems and their operation, when the thermostat terminals read 0 volts, it indicates that there is no signal being sent from the thermostat to activate the system. This typically means that the thermostat is not calling for heating. On the other hand, the gas valve coil terminals reading 24 volts signifies that the power supply to the gas valve is present and that the gas valve is capable of operating if it receives a signal to do so.

Given this scenario, the gas valve coil is considered "open" because it is not receiving the necessary signal from the thermostat to close the circuit and activate the gas supply. Therefore, even though there is voltage present at the gas valve coil terminals, it will not function unless it receives a command from the thermostat to engage.

The other components mentioned, such as the thermostat, transformer, and gas line, are not defined as open in this scenario. The thermostat, while it is indicating 0 volts, is not deemed "open" in the sense of circuit status but rather as not sending a signal. The transformer provides power but is not open in functionality, and the gas line is a physical pathway for gas, which does not relate to electrical status. Thus, the correct identification of the component