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My existing thermostat is an old mercury two wire device, heat only. Looking to upgrade to a better model, but I need to run a c-wire. I don't know where to connect the wire at at the furnace, any help would be appreciated. The valve is a Honeywell VR8200A2132, at the top there is an unused Th and Tr (3rd picture down) Last image looks to be the transformer with no open spots.

Thanks for the assistance

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  • What is the make and model of the furnace? – Tester101 Oct 10 '14 at 2:56
  • The TR terminal on the gas valve is the lead from the transformer, while the TH terminal is the heat call wire from the thermostat. The device in the last image is a fan and limit control. It turns the blower on based on the setting and the temperature of the firebox. Follow the wires from the gas valve, one should lead you back to the transformer (which is likely inside a box where the electrical line feeds in). – Tester101 Oct 10 '14 at 3:31
  • It's a pretty simple circuit. One wire goes from the transformer to the thermostat, the other goes to the gas valve. Then a wire goes from the thermostat to the gas valve. When the temperature in the room drops below the set temperature on the thermostat, the switch closes and the gas valve opens. When the firebox gets hot enough, the fan and limit control sends a signal to turn on the blower. – Tester101 Oct 10 '14 at 3:40
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TR is Common on your unit, Every 24 volt device will have a Common leg as well as the 24 volt hot leg, just as a car battery has pos and neg, common is akin to neg in this case. Common is called this cause every 24 volt circuit terminates upon Common to complete the circuit. The last pic shown is the "time on" "temperature off" fan delay, the 2 lighter colored wires are low voltage, 1 id the heat circuits 24 volt hot leg being W or white, the other being Common or C. The gas valve has the same 2 24 volt leads as well, without completing the circuit nothing will work, every electric circuit requires 2 legs of power be it 1 hot leg and neutral/Common or 2 hot legs as in a 230 volt circuit. Google 40 VA HVAC transformer to see what they look like so you will be able to locate it. Your 2 wire thermostat will have only red and white shown on my diagram

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