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Thermostat Wiring
Thermostats are the control hub for heating, ventilation and cooling systems. They provide access and control from a centralized location making it an important asset of the system. This makes knowledge of their wiring vital for any homeowner.
Before doing any installation, it is important to take note of the thermostat's voltage rating. The thermostats should not exceed the current capacity for the wires. If one is not trained to handle such electrical equipment, it is advisable to get a qualified technician to help in the set-up. This is key for safety.
The majority of HVAC systems are rated at 24 Volts and in such a case they comprise of the following.
- Heating components: heat pump, gas furnace, oil furnace
- Cooling components: air conditioner, heat pump
- Accessories: humidifier, dehumidifier, ventilator plus other air quality equipment
How to wire a thermostat
The methods reviewed below can be used to wire thermostats.
1. Use of standard wiring colours and labels.
Different wire colours have different functions as outlined in the table below. Follow the colours schemes to carry out connections for different functions.
| Label | Function | Wire Colour |
| Y | Cooling | Yellow |
| Y2 | 2 Stage cooling | Light Blue/Other colours |
| w | Heating | White |
| W2 | 2 Stage heating | Brown/Other colours |
| G | Indoor fan/ Air handler | Green |
| C | Common wire | Blue/ Black |
| R | 24V Power | Red |
| RC | 24V Power | Red |
| E | Emergency heating | No universal colour |
| X/ AUX | Auxiliary heating | No universal colour |
| B | Reverse valve in heating | Dark Blue |
| O | Reverse valve in cooling | Orange |
| S1/ S2 | Connects to the outdoor unit | No universal colour |
Sample Wiring Diagram
2. Replacing thermostat wire for wire
This is done when you intend to replace the thermostat without doing any changes to other system components. All wires should not be necessarily used depending on required functions. To do this, proceed as follows:
- Turn off the power supply to the furnace/ air handler.
- Take a picture of the old wiring to work as a reference for connection.
- Detach the wires one by one from the old thermostat connecting it to the new one.
- Turn the circuit ON and make any adjustments to the thermostat.
- If this fails to work, turn the system OFF and contact a certified HVAC expert.
You proceed with the following steps when there is no thermostat in place.
- Turn off the power supply to the furnace/ air handler.
- Uncover the furnace/ air handler.
- Identify positioning of the wires from the thermostat to the furnace. It ranges from 3-8 wires.
- Locate the wiring connection terminals on the furnace or air handler taking note of required wire colours.
- Connect properly coloured wires to the correct terminals.
- Turn the circuit ON and make any adjustments to the thermostat.
A Common wire is an extra wire that creates a continuous 24V power loop between the thermostat and the entire HVAC system. This way, the full potential of thermostats and their applications can be achieved. Basically, this provides the return path that completes the loop avoiding power disruptions.
To identify a C wire, simply detach the thermostat from the wall and look at the wires connecting to it. If there is a wire connected to the terminal labelled C, that is the common wire.
If your system does not have provision for a C-wire, there are several methods that can be used to add one. They include:
1. Use of C wire adapter kit
Some manufacturers including Venstar have an accessory such as Add-a-Wire Adapter that can help in solving C wire problems.
2. Using C wire in place of the existing G wire
3. Addition of C wire to the furnace
Running a new cable from the furnace to the thermostat to install this in a majority of the modern smart thermostat models.
Sample Wiring Diagram
Here are different thermostats that can be wired to your HVAC system.