Heat Pump Thermostat Wiring Diagram

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 that is located inside your house.

There are many types of smart programmable thermostats on the market, so please check that you can replace the type of thermostat that you are using with a newer one. You can buy a new programmable heat pump thermostat for less than $100.

Getting Started with Heat Pump Thermostat Wiring

Normally, an electronic thermostat in the United States is powered by a 24V AC power source, which comes from a 110V/24V power transformer. If you are not sure, always refer to the operating manual of the thermostat in your house before trying to do any troubleshooting or replacement work. As always, if you are not trained to handle electrical equipment, please get a qualified HVAC technician to do it.

It is always a good idea to take a picture of the current HVAC system thermostat wiring before you start removing them. That way, you know where the wiring colors go for reference.

In the heat pump system, there are at least 8 wires that need to be connected to the thermostat for proper operation. Watch this video from AC Service Tech LLC for a quick overview of each color-coded wire:

Will Any Smart Thermostat Work With a Heat Pump?

Not all smart thermostats are compatible with heat pumps. While you’re shopping, be sure to pick one that specifically supports heat pumps. Look for thermostats that have the necessary wiring terminals, such as the O/B terminal for the reversing valve, and ensure that the thermostat’s software is designed to control heat pump operation efficiently.

Overview of Different Heat Pump Terminals

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 wire, and C is BLACK wire. The C wire is known as the common wire. 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 with the green wire, which circulates the air in the room.

The reversing valve is a device that reverses the flow of the refrigerant in the piping system. Usually, the reversing valve is energized when running cool mode. However, there are models 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 make a proper connection to the thermostat.

The O terminal connects to the orange wire and is used when the system that you are using has a reversing valve (or four-way valve), which activates when running cooling mode. If the reversing valve is turned on when running heat mode, you will need to connect the reversing valve to the 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 of 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 varies. 

Sometimes, there is a 2nd stage of heating where additional heating is provided to supplement the primary heating system. This is usually installed in regions where extreme winter occurs. In this case, the terminal W2 will be present.

Related: What Does Hold Mean on a Thermostat?

Features To Look For in Heat Pump Thermostats

Look out for the following features that are incorporated in most modern programmable heat pump thermostats.

• Low voltage checking prompts you that the incoming power is low.
• Error codes that tell you the reason why your system is not running properly.
• 3-minutes minimum compressor off time to prevent the short cycling of the compressor. Short cycling of the compressor will shorten its lifetime.
• Programmable day and night settings of the set temperature.
• Weekend settings and setback functions for vacations.
• Ability to check the status of the thermostat and control the settings remotely via smartphones or computers. The presence of this feature will raise the cost of the thermostat.

Final Thoughts

Before purchasing a new thermostat for your heat pump, homeowners need to make sure the model they purchase is compatible with heat pumps (and not just a furnace/air conditioner combo). Once you have a compatible model with your heating and cooling system, use the above chart and color coding to properly DIY wire your new thermostat to your heat pump. Thankfully, many manufacturers and installers are moving to a universal contact block that just snaps all wires into the back of the thermostat. In this case, all you have to do is remove a single plug from the old thermostat and insert it into the new one. If you have any issues, get help from a local HVAC technician.