First, here is some help, the color of the wire makes no difference. The important thing is that if say BROWN wire is attached to the G terminal in the thermostat, the same BROWN wire must be attached to the G terminal in the HVAC equipment cabinet. Now, most HVAC thermostat cables have the colors that correspond to the terminal letters. Y is for cooling and use a yellow wire. In your thermostat some technician mixed this up a bit. But if the old thermostat is working, the Nest Thermostat will work too. Let’s talk about the terminals and the HVAC industry standards.
The Rh and Rc are input power for the thermostat. Rc is for cooling, and Rh is for heating. When the heating and cooling system are the same equipment, there is a jumper wire between Rh and Rc. You see this on your thermostat. On systems that are separate, say boiler for heat and an AC compressor for cooling, the input power for the Rh will come from the Boiler and not be jumper combined with Rc. Your thermostat is using a Red wire for input power connected to Rh with red jumper wire between Rh and Rc.
G terminal is for the indoor Blower Fan. In AUTO, the Blower Fan turns on when the thermostat calls for either Heating or Cooling. When the switch is moved to ON, the Blower Fan will turn on independent of a call for Heating or Cooling. Your thermostat is using a Blue wire on G terminal.
Y terminal is used to call for Cooling. When thermostat determines it needs to cool, it connects Rc to Y, and that causes the AC compressor and Blower Fan to turn on. Your thermostat is using a Yellow wire on Y terminal.
W terminal is used to call for Heat. When the thermostat calls for Heat it connects Rh to W. Your thermostat used a Brown wire number from Y terminal to the W terminal. (Frankly, this makes no sense.) there is a Brown wire not connected in the background.
O and B, these terminals are for a heat pumps. It controls the 4-way reversing valve of the heat pump. This is how a heat pump switches from Heating mode to Cooling mode and Vice versa. Why two terminals? Some heat pumps are B models, and some are O models. In older thermostats you had to connect it to either O or B depending on what model you had, Newer thermostats are software controlled and the O and B terminals are typically combined on a O/B terminal. Your thermostat is using a White wire on the O terminal.
To better assist you, we need more information. Do you indeed have a Heat Pump system? I believe you have a Heat Pump. Otherwise, a call for Heat (W terminal) will energize the Y terminal which turns on the AC compressor. And if you do not have a heat pump, you system will be trying heat and cool at the same time.
My understanding of your system.
You have an O model Heat Pump, it defaults to Heating Mode:
Red Thermostat cable conductor is incoming AC Power. (Rc and Rh)
Yellow Thermostat cable conductor initializes the AC compressor.
White Thermostat cable conductor is used to energize the Heat Pump reversing valve. To get the system cooling, when Y is energized, O must also be energized. Your heat pump defaults to Heating. When your thermostat calls for Heat, it will energize energize the Y without the O.
The brown wire jumper between Y and W will not be needed.
The Blue thermostat cable conductor is connected to the G.
The only missing wire conductor is the C or common. The HVAC control system uses 24 Volts AC power. This 24 VAC power is supplied to the thermostat and connected to Rc. Since it is AC power, we need the voltage return path, that is the C. Smart thermostats are basically computers and need the 24VAC power. If not, they need batteries. To make this work, you need R and C.
This is a lot of information. Let’s stop here and let you respond.
Ken, The AC Cooling Wizard.
NestPro, Google Pro, Mechanical Engineer and HVAC service company owner.
