How to Wire a Thermostat

Wiring a thermostat isn’t usually complicated, but it can seem daunting to the inexperienced. A thermostat wiring diagram can help you understand how your HVAC system works and how to connect a new thermostat correctly.

Safety and prep work

Before getting started, shut off the circuit breakers to both the heating and air. Switch the thermostat to the on position and go to the unit to make sure power has been cut. This is for your safety and to keep from shorting out the control board or blowing the transformer.

Next, use masking tape to tape the small paper bag just below your work area. It will catch dust and trash as it falls, making cleanup quick and efficient.

Thermostat installation steps

 

1. Remove the front cover of the thermostat and note the heat anticipator setting. It will look like a little tab in the center of the unit, with numbers ranging from .1 to 1.5. The heat anticipator fine tunes the home’s heat source and causes it to run much more efficiently. There is a cooling anticipator too, but it is factory set and non-adjustable.

2. Unscrew the thermostat from the sub-base and see whether the wiring follows standard color code. One trick the pros use is to snap a quick picture with a smartphone. If you run into any issues, you can refer to the photo. If the wires aren’t standard colors, use loops of masking tape to create temporary labels for each wire.

3. Remove the wires from the terminals, being careful to not allow them to fall back into the wall. Most of the time, you can pull a little extra wire from behind the wall to give yourself more to work with. Hold the wires and unscrew the sub-base. Once the sub-base is off, wrap the wires around your extra pen or pencil to keep them from slipping into the wall.

4. Compare the new sub-base with the position of the old one. If the new sub-base won’t cover the same area and old paint will show, use the touch-up paint before moving forward. Use a fan to help the paint dry faster. Alternately, purchase a new wall plate. Some plates come with thermostats and automatically allow you to hide old paint/wallpaper.

5. Position the sub-base where you want it, making sure it is level. This step is important for all units, but especially for mechanical thermostats. Their mercury switches won’t work if they aren’t level. Mark the new holes through the sub-base for wall anchor and screw placement.

6. Drill holes for the wall anchors. Make sure your drill bit is slightly smaller than the wall anchors or they won’t fit snugly. A snug fit is necessary to keep the thermostat attached to the wall. Once the holes are drilled, use the butt end of a screwdriver to push the anchors into place, with about 1/16 of an inch protruding from the wall.

7. Run the wires through the center of the sub-base. Screw it into place, but don’t tighten the screws all the way. Use the level to make final adjustments before tightening the screws down completely, being careful to not allow the sub-base to move in the process.

8. Connect the wires to the proper terminal. Although many people do it, looping the wires around the terminals isn’t strictly necessary. Make sure each attachment is well connected and tight. Bare wire shouldn’t touch anything but the terminal.

9. Attach the thermostat to the sub-base and make sure the heat anticipator is set correctly. Some digital thermostats won’t have an anticipator. Check the manufacturer’s instructions to be sure. Although most thermostats screw into place, some snap on. Attach the front cover and restore power. Check each connection individually to make sure it is working properly. Congratulations, you’ve successfully installed your thermostat.

Thermostat wiring diagram  

To understand which thermostat wire is connected to each terminal, we must first understand each wire’s function. Since thermostats operate on a 24-volt power source, the home’s electricity must flow through a transformer that reduces the current to the correct level. The red wire runs from the transformer to the R terminal. This is a hot wire that is live when the electricity is on.

In some cases, there will be two terminals: RC (cooling) and RH (heating). In a system with a single transformer, such as the one pictured above, the power supply is connected to one of these terminals and a short jumper wire is used to connect the RC and RH terminals to each other.

The white wire connects the heat source to the W terminal, the yellow wire connects the air conditioner to the Y terminal, and the green wire connects the fan to the G terminal.

The optional common wire connects to the C terminal to complete the circuit between transformer and thermostat, providing the thermostat with operating power. In battery-operated models, this black wire and its terminal are unnecessary.

Connecting simple and complex systems

The chart below illustrates the terminals, color codes, and types of connection found in standard HVAC wiring.

Standard HVAC Wiring
Terminal	Color	Type	Description
R or V	Red	Power	Power side of transformer
RH or 4	Red	Heating Power	Heating transformer connection; jumped to RC for single transformer systems
RC	Red	Cooling Power	Air conditioning transformer connection; jumped to RH for single transformer systems
Y	Yellow	Air Conditioner	Sends signal to switch air on
W	White	Heat	Sends signal to switch heat on
G	Green	Fan	Sends signal to switch fan on
C	Black	Common	Provides power to run thermostat; not needed for battery-powered models

Some systems are more complex. Heat pump thermostats may provide controls for multistage heating, auxiliary air conditioning or emergency heat that bypasses the condenser. In addition to the standard connections listed above, multistage systems may use the following wires:

Multistage HVAC Wiring
Terminal	Color	Type	Description
Y2	Varies	Second Stage Cooling	Sends signal to second stage cooling compressor
W2	Varies	Second Stage Heating	Auxiliary heat; may be used as emergency heating
E	Varies	Emergency Heat	Emergency heat; can bypass damaged condenser to keep heat operating in cold weather
0/B	Orange or Blue	Reversing Valve	Switches heat pump between hot/cold air; check manufacturer’s instructions for appropriate connection.

Sometimes water heater controls may be wired into a programmable thermostat when the same source is used to heat the home and for hot water. Stop and ask a certified HVAC professional for advice before proceeding with this type of wiring.

Alternative color codes

Most installers use standard color coding. Older homes and some installers may use alternative colors for each wire. In these cases, it is imperative to identify the mechanism each wire is connected to. Connecting wires to the wrong terminals can blow the transformer when the power is switched on. Replacing a transformer is a professional job that is time consuming and expensive, particularly if it is in the air handler. If you aren’t certain you can correctly identify each connection, ask a certified HVAC Expert for advice.