In a no heat call scenario, if the gas valve coil reads 0 volts and thermostat reads 24 volts, which component is likely open?

In a no heat call scenario, when the gas valve coil reads 0 volts and the thermostat reads 24 volts, the most logical explanation is that the thermostat is not allowing the gas valve to open. The thermostat is responsible for signaling when the heating system should activate based on the temperature settings. If it is reading 24 volts, that indicates it is powered and functioning properly in terms of sensing a demand for heat; however, if the gas valve coil is at 0 volts, it signifies that the thermostat is not sending a signal to open the gas valve, thus preventing gas flow to the burner.

The gas valve requires a certain voltage to operate, and if it is not receiving this voltage due to the thermostat being open or malfunctioning, it will not allow gas to flow. This is a classic cause for no heat issues in heating systems that require gas.

While there are other components in the system such as the high limit switch and blower motor that could also impact heating, in this particular scenario, the direct connection between the thermostat reading 24 volts and the gas valve reading 0 volts clearly points to the thermostat being the component that is open, thereby preventing the heating cycle from initiating.