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The inside of a heat pump thermostat

Heat Pump Thermostat Wiring

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Tim H.Verified

Lead Building Engineer

Residential and Commercial HVAC

★★★★★
5,194 positive ratings
Pro Tip

Take a picture of the original wiring layout before you do any work. In older homes that do not use modern color code, this can help you identify each wire and its terminal.

Modern thermostats are relatively inexpensive and fairly easy to install. To run your own wiring, you need to know three things:

  • The type of heating used in your home

  • Whether your heat pump is single-stage or multistage

  • Whether you are installing a mechanical, digital, or programmable thermostat

How thermostats work

Understanding how gas fired HVAC systems and heat pumps work is important before beginning the exercise of wiring them. Thermostats essentially function as a temperature-regulated on and off switch. Multistage heat pumps are an exception to this rule because a secondary heater or air conditioner is also wired into the controls.

Many people are under the mistaken impression that bumping the controls up or down a few degrees will change the temperature faster. However, changing the setting only causes the system to work longer to reach the desired temperature.

How heat pumps work

Heat pumps are different from HVAC units, which contain both a furnace and an air conditioning unit. Instead, heat pumps contain a reversing valve. Both air conditioning and heating modes use the compressor, an indoor blower, and an outdoor fan. The reversing valve switches to channel hot or cool air through the air handler, which then distributes air through the house at the correct temperature.

a diagram of a typical heat pump

Reversing valve position

The reversing valve only has two positions: on and off. In most heat pumps, it is on when the air conditioning function is in use, and it is off when the heating function is in use. Some older models reverse these settings, with the valve in the off position for air conditioning or in the on position for heat. Most thermostats cover both options. This distinction is important because it will determine how you wire the thermostat.

Single-stage vs. multistage

Single-stage

Single-stage heat pumps don’t have an option for secondary heating or cooling. The only variable the homeowner can control is the temperature at which the system kicks on or off. Raising or lowering the temperature makes the system work longer, not harder. Single-stage wiring is straightforward and doesn’t usually require extra wires.

Multistage

Multistage heat pumps use the first stage as their primary system, but may also be connected to an emergency heat source, such as a solar panel. Multistage units use additional terminals to wire auxiliary heat or air conditioning independent of the main source. While they aren’t overly complicated to install, homeowners should be able to confidently identify each connection before attempting this type of thermostat wiring.

Heat pump thermostat wiring color code

Most thermostat wiring uses conventional codes for each wire. Single-stage ones use a five-wire system, with one optional sixth.

A diagram of the wiring components of a heat pump thermostat

 

  • Red indicates a live wire. Since it indicates the main power supply, red is also referred to as a “hot” wire. It should be connected to the R terminal.

  • Yellow indicates cooling or air conditioning. It is usually connected to the Y terminal.

  • Green is linked to the indoor blower fan. Connect it to the G terminal.

  • White wires indicates a heat source. It connects to the W terminal.

  • Orange or dark blue power the reversing valve. Most heat pumps engage the reversing valve to run the air conditioner. If this is the case, connect this wire to the O terminal. If the reversing valve should be engaged for heat, connect it to the B terminal instead. Notice the difference in the wiring diagrams above.

  • Black is usually the common wire, but it may be another color. It completes a 24-volt circuit to power mechanical or digital thermostats. It isn’t needed for most programmable thermostats since they use batteries.

Please note that although this code is standard for modern wiring, the wiring in your home may be different. Older homes frequently use alternative color codes, and not all installers follow universal recommendations. For safety reasons, you should be able to clearly identify which wire leads to each component before attempting installation on your own. If you can’t, consult someone familiar with electric installations.

Additional terminals for multistage units

Multistage units use additional terminals to connect auxiliary heat or air. Most of these terminals don’t have a designated universal wire color, but they may have a commonly used color.

Dual power terminals

Some systems will use two terminals to represent separate power sources for cooling and heating. RC indicates the terminal used for the cool air power source and RH indicates one used for the heat power source. If your system uses two transformers, you should have a separate wire for each terminal. However, most heat pumps use a single transformer. In this case, the red “hot” wire is connected to one terminal and a short “jumper” is used to bridge the RC and RH terminals.

Backup heat terminals

Multistage pumps typically contain one of these auxiliary heat terminals.

  • The W2 terminal is used for second-stage heating. The secondary heat source associated with it is used in conjunction with the main heat source to warm the home faster.

  • The E terminal bypasses the condenser to provide emergency heat from a secondary source. The corresponding wire should be fixed directly into the heating relay.

  • The Aux terminal enables auxiliary heating from a secondary source located in the air handler.

Secondary cooling

If your system is equipped with two compressors, it will use a Y2 terminal for second stage air conditioning. The corresponding wire may be light blue, but this isn’t a hard and fast rule.

Tips for effective heat pump thermostat wiring

If you are replacing a heat pump thermostat, use these tips to stay safe and help your wiring job go smoothly.

  1. Make sure you pick up the correct type of thermostat. Most thermostats will work on a conventional or heat pump system, but check the specs to be sure.

  2. Start by turning the power to the furnace or air handler off, preferably by switching the correct circuit breaker off. This reduces the risk of dangerous electrical shock.

  3. Go to the thermostat and take the front section off and look at the wires. Mark all the terminals so that you can place them on the same terminals on the new thermostat. Heat pumps have some additional terminals not used on a conventional system, particularly the O/B terminal which is used to energize the reversing valve.

    • Red goes to R and is considered "hot" (24 volts).

    • Green is used on the G terminal and this is for your blower.

    • Y is for air conditioning and usually a yellow or blue wire is used here.

    • W is for your heat and is typically white.

    • The reversing valve is energized at the O/B terminal and standard color is orange.

    • Sometimes you may see a black wire on the C terminal.

  4. Most new thermostats (unless they incorporate WiFi) do not require a common wire, but you can use it if you want. Keep in mind that these colors are standard colors but there is no guarantee that the person who set up your thermostat used the standard colors. This is why it is so important to label all the thermostat wires and take a picture of the wiring before you take them off the terminals.
  5. Once everything is labeled, take the old thermostat off the wall, feed them through the new thermostat, and mount it to the wall. Put the wires back on the corresponding terminals.

  6. Now it is time to turn the power back on. On digital thermostats, you will need to set the date and time and the system type (heat pump) in the setup menu. The thermostat should come with instructions on how to do this.

  7. Test the system and cycle through heat and AC cycles and make sure everything works.

If you don’t know, ask. It is safer to consult an HVAC Expert if a non-standard wiring scheme has you confused or you need help with your repairs.

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