If you're installing a new thermostat or wiring an old one to a new HVAC system, it's important to understand the basics of this sort of electrical project before you get started. Following a few key safety rules is essential in order to protect yourself and the equipment you're working with, as well. Fortunately, most thermostats, even those of different brands, rely on similar color coding schemes when it comes to their wiring. This can come in very handy when you attempt to determine which wires should match up with which terminals.

Installing a programmable room thermostat.
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How to Install Thermostat Wires

Basic Safety Instructions

Before you begin work on installing your thermostat wires, be sure to read any manufacturer instructions that accompanied your device or the HVAC system. If anything is confusing to you, it's best to ask help from a professional rather than proceed on your own.

For safety reasons, never work on a thermostat's wiring if the circuit breaker is turned on. Be sure to flip the switch to "off" before you get started. In addition, alert others who may be present to the project you plan to undertake. Ask that they refrain from making any changes to the building's electricity or turning anything off or on until you're done working.

To protect yourself, wear high-voltage gloves whenever possible. In addition, if you'll be using tools like pliers, choose ones with rubber handles for the ultimate in protection from electrical shock.

Thermostat Color Coding Schemes

Depending on your thermostat, the color coding of the wires may vary slightly. However, most systems rely on the same thermostat wire colors, which makes installation fairly simple. Generally, the R wire is red and provides power to the system. Some systems may have an Rc wire, which is also red and provides the power for cooling. The Rh wire is red and is the power wire for your heat.

The thermostat C wire can be either blue or back. This is known as the common wire. You may also have a W wire, which is likely white, while the E wire is usually brown. The Y and Y2 wires are most likely yellow ones. If you have a fan relay, it will likely be marked G and be green. The O wire should be orange and is the change-over relay to the cool relay, while the B wire is blue and the change-over relay to the heat relay.

How to Rewire a Thermostat

Once you've determined which thermostat wires correspond to which terminals in your device, you'll need to pull the wires from your HVAC system through the wall plate so that they can reach the thermostat control box. Leave yourself a few inches of wire and strip about 2 inches off the end of each so that they can be connected to the thermostat terminals. You should only complete this step once you have somehow noted the colors of each wire. Marking them with a piece of masking tape on which their proper terminal connection is noted can be helpful. Be sure, however, to remove the tape later.

Once you have completed this step, you can reassemble the thermostat, flip the circuit breaker and power back on your HVAC system. Test the wiring by raising and lowering your heat and air conditioning. If any changes need to be made, shut the system back down and follow all above-outline safety instructions before proceeding.