nest Learning thermostat Installation And Configuration Manual page 32

Heat only systems
Heating and cooling systems
The heat call is detected by the controller board when the W wire is shorted to the R wire, which causes the
24V AC voltage to be detected. When the Nest thermostat is taking power, a small voltage is generated
between W and C (typically less than 8 volts) which can cause a false heat call in a small number of
systems. The addition of a 220 Ohm resistor at the furnace provides another current path to the
thermostat, which reduces the voltage input at the detect circuit to prevent false heat calls.
Note that this resistor will draw a couple of watts during call for heat (or cool, if the resistor is added to Y)
and it will become warm. Make sure to mount the resistor so that it is not in contact with any wires or
system components. Between heat or cool calls, the resistor will dissipate less than a watt.
Pull a new C wire
Replace the thermostat wiring bundle to provide an extra wire to be used as common.
Nest Learning Thermostat installation & configuration guide Page 30