What Do the Letters Mean on a Thermostat Terminal?

Thermostat Wiring

Electronic thermostat
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Thermostats are used to control the heating and cooling systems in your home. They tell the units when to turn on and off, according to the temperature that you set them for. Some thermostats do only that, set the temperature while others can control the temperature at preset times. These specialized thermostats are called programmable thermostats.

Programmable thermostats come in many shapes and sizes.

Each has its own unique set of programming features. Some control a few days of the week, while others control many times of the day and many days of the week. The thermostats of today have digital readout screens and many buttons to easily program the thermostat for your individual settings. 

Thermostats are wired to the furnace and air conditioner units in order to turn these units on and off automatically. Between the thermostat and the units is a low voltage set of wires that are connected to the thermostat control terminals and the terminal screws mounted within the furnace and air conditioners control terminal strip.These strips of terminals have markings on the to signify the heating connection, the cooling connection, the fan connection and heat pump connection if you have one.There is also a terminal that supplies the power to run each of these functions. the older dial type thermostat wiring chart is easy enough to understand, and now you can see how easy the newer style is as well.

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.

Some people feel overwhelmed at the sight of terminal wiring connections, but that is only because they don't take the time to learn what each of the connections does and which color wires control them. Hopefully, with the aid of this simple chart, you will be able to make these connections with ease and understand how and why they work.

  • G
    The G terminal controls the fan relay and is responsible for turning the blower fan on and off automatically or manually via the thermostat.
     
  • RC
    The RC terminal is the 24-volt cooling power supply.
     
  • RH
    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.
     
  • Y/O
    The Y/O terminal is used to control the cooling contractor. When the thermostat calls for cooling, power is fed to pull in the cooling contractor and the fan relay. This powers up the condenser and the blower fan, cooling your home.
     
  • W/B
    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.
     
  • Y1
    The Y1 terminal is used for the compressor contact in a single-stage heat pump installation.

As you can see, there are many different types of connections and having the right wire landed on the appropriate terminal makes all the difference. Hopefully, this makes it easy for you to identify the right wire for the right terminal.