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hi i am dan i can help you with this. it shouldnt be a problem is it a boiler you have or a warm air furnace? was there an existing central A/C before?
ok it is very easy to do on the nest. the red wire from the boiler will connect to R on the nest and the white wire will connect to W then the red wire from the A/C will connect to RC on the nest and all the other wires follow the same wiring.
ok is the outdoor unit a heatpump or just A/C
sorry i forgot to mention the B wire from your old tstat will go to C on the nest. trane uses B as the common wire.
acording to your picture B was already conected to the old thermostat. that is the C wire
even though the thermostat has batteries some of them are still powered by the system. the batteries are a backup for when the unit loses power the tstat is still powered so you dont loose programming. the blue wire will conect to C the green to G and yellow to Y. if you have any more questions about this or if this answers all your questions please let me know and if you could rate me on my service by selecting one of the 5 stars at the top of your page i would appreciate it.
did you have any more questions about this?