Nest Learning Thermostat_ wiring confirmation needed

MaxGNaidu

Hello there

My HVAC setup is currently 2 different thermostats one for Heating ( the old mercury based 2 wire honeywell thermostat) and another for cooling ( a moderately modern system with a C wire with 28V, I checked voltage with a meter).

between the heating and cooling thermostats, I have the following wires,

C wire (AC unit)
Rc wire (AC unit) (shorted to R)
Y wire (AC unit)
G wire (AC unit)
W wire (Heating unit)
R wire (Heating unit)


so based on the above wires and the rsearch I did, look slike I am on a straight forward and easy way to upgrade to the nest thermostat. My undertstanding is

1. I remove the old thermostats, bring the wires together.
2. install Nest base, hook up Y to Y1, G to G, W to W1, C to C and Rc to Rc and R to Rh.

the above connections are per my understanding of the wiring based on my research by googling and watching youtube videos (:

AC/Cooling Thermostat wires



Heating thermostat wires



Google Nest base wires





let me know if I have this correct or am I missing anything. would be glad if someone can go over the wiring and correct me if I am going in the wrong direction.



Thanks & Regards,

SteamBoiler

I have a Nest 3rd gen learning thermostat that I used to replace an old Honeywell Chronotherm mercury thermostat, controlling a 2 pipe radiator system; heat only.

Your wiring plan looks good. I believe when Nest gets both Rc and Rh, that it will default to getting power from Rc/C (you should Google). My wiring was Rh from R-equivalent of boiler and Nest tried to turn the boiler on multiple 3 minute durations just to charge itself, even though I had C connected from the boiler transformer. I saw what appeared to be reliable 26V with a meter, but that configuration didn't work reliably and I had to use an external transfomer and isolation relay, simple enough in the end and very robust.

You will likely have an easier time getting Nest to run off Rc/C from a modern AC unit; and Rh and W become a switch equivalent to the Nest.

1. Keep your old thermostat handy in case things don't work out. Or get a Honeywell battery powered 5/2 thermostat. You don't want to end up with no heat mucking around with wiring in the dead of winter. I would even suggest waiting till March for this project.
2. Unless the 2 thermostats are physically close it may not be easy to combine wiring at one location.
3. Addendum to above, make sure to locate Nest at a logical place to serve as sensor both for heat and cool. My AC thermostat is on an upstairs landing and Nest is downstairs for heat.
3. After install make sure Voc and Vin of Nest are between 32V and 36V; and Iin=200mA signifying it sees a reliable common wire.
4. Turn off all learning and auto schedule. Run with a fixed temperature to start. Later you can try set backs from say 68F to 64F during the night but be aware that you can't recover from 64F to 68F in one big step. I do it in 1F increments.
5. After install monitor the heat call durations. You should time them right now with the mercury thermostat. With my system, heat gets to the first radiators at 12-15 minutes in and to the last at about 18 minutes in and heat calls are about 20 minutes long once every 90-120 minutes unless it is very cold.
6. Enjoy being able to monitor your system health with heat call duration as proxy.