Alright, so you've decided to upgrade to a programmable thermostat from your antique rotary dial thermostat, but which wire hooks to what terminal and how in the world do you know what the terminal lettering means. Actually, connecting a set of thermostat wires is fairly easy if you know what the terminal letters stand for and what they control.
The G terminal controls the fan relay and is responsible for turning the blower fan on and off automatically or manually via the thermostat.
The RC terminal is the 24-volt cooling power supply.
The RH terminal is the 24-volt heating power supply.
- Note: The RC and RH terminals are jumpered together in a four-wire heat/cool system and a single-stage heat pump system, but not in a five-wire heat/cool system.
The Y/O terminal is used to control the cooling contactor. When the thermostat calls for cooling, power is fed to pull in the cooling contactor and the fan relay. This powers up the condenser and the blower fan, cooling your home.
The W/B terminal controls the heat relay or valve. When the thermostat calls for heat, power is fed to pull in the heat relay or valve and the fan relay. This powers up the furnace and the blower fan or the boiler, heating your home.
The Y1 terminal is used for the compressor contact in a single-stage heat pump installation.