Honeywell RTH230B outputs 12 volts

I have a Honeywell RTH230B thermostat connected to a gas boiler. The other day the furnace did not fire when the thermostat called for heat. Through the process of elimination I determined that the White-Rodgers 36c03 gas valve was the problem. The valve was replaced and the system is working properly now. While troubleshooting the problem I noticed that when the thermostat was calling for heat the voltage at the gas valve connection was 24 volts which is what I would expect. However, when the thermostat was not calling for heat the voltage at gas valve connection was 12 volts. I would expect 0 volts since the themostat completes the circuit when it activates the relay that controls the heat signal. My real concern is whether the thermostat is defective and more importantly whether this constant 12 volts could be what caused the gas valve to fail. I checked the voltage at the gas valve connection when the gas valve was disconnected and I still saw 12 volts. Any insight would be greatly appreciated.

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