The thermostat that is used to control the heating and cooling (HVAC) system in your home is a low-voltage device that is easy enough to replace. And replacing one is a very common project, as homeowners swap out older thermostats requiring manual adjustment with newer programmable thermostats that change temperatures automatically at preset times.
But people are often confused by the system by which the low voltage wires are labeled, which can look different in an old thermostat than it does in a new one. Contrary to popular belief, the letters do not correspond to wire colors but rather are used to indicate the various function signals controlled by each wire.
How Thermostats Work
A thermostat is wired to a furnace or air conditioner unit and acts as a switch to turn the unit on and off automatically, based on the thermostat's temperature setting and/or programmed time schedule. Between the thermostat and the heating or cooling equipment is a set of low-voltage wires that are run from the thermostat control terminals to the terminal screws on the furnace or air conditioner control terminal strip.
These strips of terminals have markings on them to signify the heating connection, the cooling connection, the fan connection, and heat pump connection, as applicable. There is also a terminal that supplies the power to run each of these functions.
Most thermostat wires operate on 24-volt power provided by a transformer, and they are generally safe to work on without shutting off the circuit feeding the thermostat wiring. However, if the low voltage wires touch each other during the work, the system's internal fuse will blow. You will likely have to call an HVAC specialist to repair and finish your project.
Only the thermostat wiring is low-voltage. The circuits feeding furnaces, air conditioners, and heat pumps are either 120-volt or 240-volt, and they should never be worked on while the circuit power is on.
Thermostat Terminal Letters
Connecting a set of thermostat wires is fairly easy if you know what the terminal letters stand for and what each terminal controls. Most thermostats follow the standard lettering system shown here, but be aware that the terminals, wiring colors, or the number of wires included in low-voltage cabling for thermostats are not universal. Refer to the thermostat manufacturer's wiring diagram for precise connection information.
- 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 heating/cooling system.)
- Y/O: The Y/O terminal is used to control the cooling condenser. When the thermostat calls for cooling, signals are sent to power 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, a signal is sent to power 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.
Tips for Replacing Thermostats
When installing a new thermostat that is roughly comparable to the old, it is usually an easy enough matter to pay close attention to where each low-voltage wire is connected on the old thermostat, then attach each wire to the terminal that has the same marking on the new thermostat. To make it simpler, you may want to label the wires with small tabs of masking tape before disconnecting the old thermostat.
Sometimes the new thermostat will have different markings, or it may not even use all the wires that were connected to the old thermostat. Some new thermostats require no more than two wires and will work fine if the extra wires are left unattached. Your thermostat may also have extra terminals that don't apply to your particular HVAC system.
Again, it is not a problem if not all the terminal connections are used. Consult the instructions on the new thermostat to determine which connections need to be made.