last updated 6 Oct 2019

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Thermostat controls
C - Common
G - Fan
Y - AC Compressor 
W - Heat
R - Electrical hot +
Adding a 24 volt transformer to power a smart thermostat for systems which do not already have 24 volts.

Heat only system:
  Connect the transformer to C and RC (+/- can be on either one)
Heat and Cooling Systems
  You probably need a jumper between RH and RC.

Simple addition of transformer:

Older furnaces require additional components.

See Heat only 2-wire boiler/furnace installations for ecobee thermostats – ecobee Support for more complicated systems.

Installing Smart Thermostats on old heaters or boilers with millivolt thermostats.
These usually only have two wires (usually red and white) at the thermostat.
They require addition of a 24 v AC c-wire (common wire).
Heat only 2-wire boiler/furnace installations for ecobee thermostats | Ecobee Support

Controlling an ancient millivolt heater with a Nest

TH – The 24v hot leg from the thermostat on a call for heat (R+W closing) to the gas valve (TH terminal) to open the solenoid to allow gas to flow. This is assuming the transformer is good and the high limit is closed. TR – The 24v common/return side of the transformer. TH/TR – This is not internally wired to the gas valve. Not using this makes no difference to the circuit. You could run your 24v hot from the transformer directly to your NC high limit in the above example and be just fine. This is nothing more than a convenience terminal. Joining these wires with a marette (Wire nut or terminal multiplier) does the same thing as this terminal. Hope this helps clear up any confusion.

Search for c-wire adapters
TH, TR and TH/TR Gas Valve Terminals - HVAC School
Gas Furnce systems

Fan too loud on wall heater.