If you are looking for a better understanding of heat pump thermostat wiring, here is an example of a typical electronic type heat pump control wiring which is located inside your house.
There are many types of electronic thermostat in the market these days, hence please check that the type of thermostat that you are using can be replaced with the newer ones. A new programmable heat pump thermostat can be purchased for less than $50.
Normally, an electronic thermostat in the United States is powered by a 24V AC power supply which comes from a 110V/24V power transformer. If you are not sure, always refer to the operating manual of your thermostat in your house before attempting to do any trouble shooting or replacement work. As always, if you are not trained to handle electrical equipment, please get a qualified technician to do it.
It is always a good idea to take a picture of the current heat pump thermostat wiring before you start removing them.
In heat pump system, there are at least 8 wires that need to be connected to the thermostat for proper operation.
Heat pump thermostat wiring - A typical wire color and terminal diagram
As shown in the diagram, you will need to power up the thermostat and the 24V AC power is connected to the R and C terminals. The color of wire R is usually RED and C is BLACK. C is known as the common terminal. These two connections will ensure that there is power to the thermostat that you are operating.
The Y terminal is where the signal to the cooling air conditioner signal is connected. This terminal will call for the need to cool the room when the set temperature is lower than the room temperature. The G terminal is connected to the indoor fan which circulates the air in the room.
The reversing valve is a device that reverses the flow of the refrigerant in the piping system. In most cases, the reversing valve is energized when running cool mode. However, there are instances where the reversing valve is off when running cool mode.
Therefore, it is important the check the manufacturer's specifications of the heat pump system that you are using before you can do a proper connection to the thermostat.
The O terminal is used when the system that you are using has a reversing valve (or four way valve) which is turned on when running cooling mode. If the reversing valve is turned on when running heat mode, you will need to connect the reversing valve to B terminal. Only one connection is active at any one time, that is either O or B terminal is used but not both.
In some equipment, there is a 2nd stage cooling that helps to increase the cooling capacity of the room. In this case the terminal Y2 is usually used. The color of the wire vary.
Sometimes, there is a 2nd stage heating where additional heating is provided to supplement the primary heating system. This is usually installed in regions where extreme winter occurred. In this case the terminal W2 will be present.
Some thermostat may have a feature called Emergency Heat where when set, it will shut off the heat pump. It will then turn on the strip heat which becomes the primary source of heating. This feature should only be used for a while as the energy cost is usually higher than a heat pump system. The terminal used is E.
Look out for the following features that are incorporated in most modern programmable heat pump thermostat.