nest learning thermostats

DaveM

I have a forced air system for AC only, and a gas fired hydronic boiler for baseboard heating two separate systems. one the same wall there is a old heating only thermostat and six inches away is the AC only thermostat . my question is can I install a nest thermostat and connect both systems to it

Jamie Hall

Yes, but it isn't straightforward. We have had many many threads on trying to persuade Nests to work more or less well, if at all; you might search for "Nest" in The Wall's search box.

pecmsg

Why?

DaveM

the heating thermostats for some reason are line voltage 4 zones I ran new low voltage 18/5 wire from four locations to the boiler room. I'm in the process of installing a switching relay and rewiring circulators. in one location upstairs there is a low voltage thermostat for the air handler in the attic and right next to it is the old line voltage thermostat for the upstairs heating zone controlled by a gas fired hydronic boiler in the basement I plan on installing a nest thermostat there and i would like to connect both units to one thermostat

Jamie Hall

Better find out why those old thermostats are line voltage. What are they actually controlling? If they are directly controlling something using line voltage, you are going to need a 24 VAC power supply and a bunch of relays tor the Nests.

DaveM

like i said the line voltage stats are being eliminated I ran 18/5 wire down to boiler room where a taco 504 switching relay will be installed to control my baseboard heating My original question is can I connect a separate air handler for AC to the same nest thermostat using y,g,rc for cooling and w,rh for heating

HVACNUT

For the upstairs, power the thermostat from the air handler. Nest likes to see 24 volts at Rc for some reason. Hopefully it's already wired for 24 volts. If not, maybe there's an used wire you can use for Common. 
Rc, C, G, Y from the air handler. Then just use Rh and W on the heat side to R-W on the relay. 

DaveM

Thanks I’ll see if that works. isn’t the RC & RH internally jumped on the nest also upon set up they ask what type of fuel, also what type of system you have forced air, radiators etc which do you choose?

HVACNUT

DaveM said:

Thanks I’ll see if that works. isn’t the RC & RH internally jumped on the nest also upon set up they ask what type of fuel, also what type of system you have forced air, radiators etc which do you choose?

Yeah it supposed to know. I'm just going by experience. I don't know why it does what it does. I show up at a lot of Nest invaded houses and have rewired many to get power to Rc because it wouldn't operate with power from Rh. Or it would operate, then get wonky. They're also good for internal sub base shorts.