In a no-heat call, if the gas valve (thermostat side) to common terminals read 0V while the gas valve (transformer side) to common reads 24V, which component is likely open?

The situation described involves a no-heat scenario where there is a voltage read at the transformer side of the gas valve but not at the thermostat side. This implies that the issue likely lies with the gas valve coil itself.

When the gas valve coil is functioning properly, it should allow the voltage from the thermostat to pass through to energize the valve when there is a call for heat. However, if the coil is open, it will prevent the 24V from reaching the gas valve on the thermostat side, resulting in a measurement of 0V. Since the transformer side reads 24V, it indicates that power is available but is not reaching the gas valve coil due to an interruption in the coil circuit.

The other components, such as the thermostat, high limit, and transformer, are either providing power or acting as switches that command the operation of the system. Since the transformer is working and supplying 24V, it's not the problem. The thermostat would not engage the call for heat if it were malfunctioning but would not cause a reading of 0V if it was simply inactive. Similarly, if the high limit were open, it would usually prevent the gas valve from opening, but the valve's inability to receive power indicates the