The thermostat at my home is set up with only the 24 power and return through the relay. I would like to add a 'smart' thermostat, however the line is fairly well buried in the wall/basement ceiling and will not be fun to rewire to add the 'c' return path. However, I have fairly easy access to all walls via the main ceiling. If I were to add another 24v transformer, could I run an independent line to power the thermostat? My concern is that the input power for the thermostat and the relay will be common, this thwarting my plan. Would the inputs be jumped that I can remove, or is my screwball idea hopeless?
Provide a secondary power source for the thermostat
thermostatthermostat-c-wire
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Best Answer
I think that in most cases you would run into the problem you described where you can't really provide "separate" power to the thermostat. This would obviously depend on the particular thermostat, however.
One smart thermostat manufacturer provides a 2-wire accessory kit. Looks like it completely replaces the 24VAC supply in the furnace.
http://www.smarthome.com/30418/Venstar-ACC0436-Thermostat-Two-Wire-Heat-Only-Kit/p.aspx
Also, definitely check out this Q&A on thermostat wiring.