Depending on how your home's HVAC system is set up, your thermostat may have various quantities of wires. Generally speaking, a thermostat that controls only heat in your home will have two wires, and one that also controls air conditioning must have at least three or four wires. If your system also includes ventilation or a heat pump, the thermostat can have as many as seven wires. If you plan to conduct maintenance to replace your thermostat, you should carefully follow certain safety procedures.
Essential Safety Procedures
Before undertaking any electrical projects, be sure that you fully understand all required steps in the process. Inform others present that you will be working on the building's wiring and ask that they not flip any breakers or switches until you are finished. Turn off the thermostat and associated HVAC systems, as well as the associated circuits at the breaker box. Wear high-voltage rubber gloves if possible when doing work with wires. Additionally, use rubber-handled tools whenever you are able.
HVAC Wiring Basics
The oldest thermostats had just two wires and were used to operate a building's heat. In these cases, the wire terminals are typically marked with an 'R' and a 'W' and usually are 24-volt alternating current wires.
If, however, your HVAC system also includes air conditioning, the wires will likely be labeled differently. In some cases, the thermostat will have both an 'R' terminal for heating ('Rh') and one for cooling ('Rc'), as well as a 'W' and a 'Y' wire. Systems with a fan included might also have a 'G' terminal.
Newer thermostats may have a 'C' terminal, which actually provides power to the controlling mechanism itself. Since older thermostats were often battery-operated, they didn't have a need for this wire.
Systems with two-stage heating or cooling may have additional terminals, such as 'W1' and 'W2' for heating and 'Y1' and 'Y2' for cooling.
Thermostat Color Code
You may also find that the wires in your thermostat are color coded. Most likely, the colors will follow a universal standard as follows. 'R,' 'Rc' and 'Rh' are red power wires. A blue or black wire is usually marked 'C,' while the a white wire is likely marked 'W.' The wire marked 'E' is likely a brown one, and 'Y' and 'Y2' are usually yellow. A wire marked 'G' is typically green, while 'O' is orange and 'B' is blue.
How to Install a 7-Wire Thermostat
To install your 7-wire thermostat, first ensure you have thoroughly read all manufacturer instructions and followed all necessary safety procedures for working with electricity. Next, connect the wires to the appropriate terminals as marked on the unit. For instance, pull the green wire to the terminal marked 'G.' This should be the wire that controls the fan in your HVAC system. After associating all wires with the appropriate terminal based on their colors as outlined, attach the thermostat to the wall. Restore power to the unit and attempt to set the heat or air conditioning. If this proves successful, the thermostat is likely successfully connected.