Help with wiring my new nest T stat

[Copperline]

hi all want to replace existing thermostat which controls a separate heating system and air conditioning system. My existing thermostat has an Rc Rh W Y G inputs on the thermostat. Heater harness - green wire  is in G input -white wire  in Y input and red is in the RC input (doesn’t make sense to me)?? The other wire harness coming from the air conditioning unit has a red and a white. The red is connected to Rh and the white is connected to the W. The red on ac and heat seems to be backwards but the system has worked for 20 years this way  My question is I have nest thermostat with the following inputs….Y C W G R and *OB. Is this best compatible w my separate heating and ac systems? Please help!! Thank you!

[CoolingWizard · Accepted solution]

@Copperline 

when you say you have a separate heating system from your air-conditioning system they don’t share a common air handler correct?

I see that you have a star OB wire so this tells me you have a heat pump. Now with a heat pump, sometimes you have auxiliary heating strips in the air handler and sometimes they are coupled with a gas furnace with the same air blower, which is known as a dual-fuel system. 
can you tell me more about your separate heating system? What type of heat is it? Is it a water boiler with baseboard heating, or is it a gas furnace with a separate air handler?

AC Cooling Wizard 

[CoolingWizard · Accepted solution]

@Copperline 

when you say you have a separate heating system from your air-conditioning system they don’t share a common air handler correct?

I see that you have a star OB wire so this tells me you have a heat pump. Now with a heat pump, sometimes you have auxiliary heating strips in the air handler and sometimes they are coupled with a gas furnace with the same air blower, which is known as a dual-fuel system. 
can you tell me more about your separate heating system? What type of heat is it? Is it a water boiler with baseboard heating, or is it a gas furnace with a separate air handler?

AC Cooling Wizard 

[Copperline]

Hi my heater is a 5 zone hot water boiler baseboard. The existing old t stat is wired as above. I have 2 separate air handlers wired to 2 of the 5 stats. My real question is on the nest what do the 6 inputs Y C W G R and * OB stand for- I can figure it out from there- l

(how do I get RC and RH out of the existing ports listed above on the nest? )

[CoolingWizard · Accepted solution]

@Copperline 

The thermostat terminal labels are technically simply an industry standard. They don’t stand for anything specifically, however, each one has a defined function. 
R, Rc, Rh; 24V AC Power; Rc Power Cooling, Rh Power Heating

Y Cooling (Air Conditioning); Y turns on the outdoor compressor. 
W Heating; Initiates the heating system

C 24V AC Power Common

G Blower Fan 

*OB Heat Pump Reversing Valve

Since you have 5 separate thermostat one for each zone of heating and one for your air conditioner, each one would require its own nest thermostat if you want to have all of them be smart thermostats.  

AC Cooling Wizard

[Copperline]

Thank you! What do you suggest for the 2 t stats running heat and air (5 wires RH RC W Y G)  and what can i use for the remaining 3 tstats that are just 2 wires for individual zones of heat? 

thanks again!

[CoolingWizard]

@Copperline

if you have two thermostats which runs two independent systems for each, that is you have a separate RC wire and a separate RH wire, then what you have is what we call a dual fuel type system. Number one fuel is electricity, which runs the air conditioner, number two fuel would be the heating system which probably runs a gas boiler. One thermostat can run both you would connect a heating system RH wire and you would connect a cooling system. RC wire. The W wire goes to the heating system, and the RC, Y, C and G go to the cooling system. 

AC Cooling System 

[Patrick_Caezza]

@Copperline 

You have what is referred to as a dual transformer system. All this means is you have separate 24vac (R) wires for heat and cooling.

Only the Nest Learning Thermostat (3rd or 4th Gen) is compatible with a Dual Transformer system.