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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