Referencing This excellent answer
I have a two wire furnace controller, 24VAC, no common. I measured the current draw of the relay (when the thermostat wires are shorted), it draws 0.2A
Wish to add an Ecobee Lite 3, which draws about 1.9 watts (just 80mA).
Am reasonably comfortable with general electrical/electronics. Have opened the control box, and located the "second" terminal of the 24VAC transformer, where one would tap a common wire.
Question: Is there any issue if I just directly solder a new common wire straight to the transformer and use that (+"R") to power the smart thermostat? That would mean a draw of 280mA on a system rated for 200mA. Is that likely to be an issue? Potential consequences? is the worst case scenario just the ecobee lite turning off / relay not having enough power to hold closed, or could it be something more problematic, like making the transformer fail?