If your system has six wires, its because it features second-stage heating, second-stage cooling or heat-pump cooling but not all three. The extra wire signals the additional function to come on. If your system has second-stage heating and cooling as well as a heat pump, then the thermostat needs eight wires, not six.
An extra unused wire in the wall can be used as the C wire for your new thermostat. The extra wire will also need to be connected to the C (common) terminal at the heating and cooling system. This will provide power to your new thermostat. Most standard thermostats do not need a C-wire.
What happens if thermostat is wired wrong?
Potential consequences of improper installation could include: Electric shock. Damaging the thermostat unit, the electrical system or even the AC/furnace unit itself.
How do I know if my central heating thermostat is broken?
The most common signs of a broken thermostat are:Thermostat display is off or is non-responsive.Turning on the heat or AC does nothing.The heat or AC will come on, but either stays on nonstop or cuts out before the temperature setting is reached.