Heat Pump Thermostat Wiring (Diagrams and Color Code)

Knowing how to wire a heat pump thermostat makes it easy to get your system running.

This page includes a heat pump thermostat color code wiring diagram to assist you as you wire your single stage or 2 stage heat pump thermostat. It works for most brands including popular nest and Honeywell thermostats.

First is the basic wiring diagram.

Then different wiring setups are discussed such as using Emergency and/or Auxiliary heating and 2 stage heating and air conditioning.

Heat Pump Wiring Diagram

The heat pump wiring diagram shows the wiring colors to attach to each terminal. This is the most common wiring setup for a standard heat pump.

Heat Pump Thermostat Wiring

Below is a list of wire colors and what they are used for in standard thermostat wiring for heat pumps. No, the colors aren’t crucial – all the wires inside the colored housings are the same.

But using a standardized list of wiring colors for each task the thermostat performs gives technicians and future users the same information to work with.

Y – Yellow: Cooling – this wire energizes the control for the condensing unit, the outdoor unit. If your heat pump is a 2 stage unit, then your thermostat will also have a Y2 terminal for the second heating/cooling stage.

G – Green: Fan – this is for the blower in the air handler.

R – Red: 24 Volt Power – Your air handler is powered by 110V, but it is converted to 24V for the thermostat.

C – Blue: Common wire for continuous 24 volt power from the thermostat to the air handler.

E – White (optional): Emergency Heat. Most heat pump air handlers have an electric heating strip or strips in them (5kW to 20kW) to provide extra heat when the heat pump isn’t providing any during mechanical failure. If your air handler doesn’t have heating strips, you won’t have this wire or you won’t need to use the white wire or wires.

Aux – White (optional): Auxiliary Heat. This powers heating strips too, if your air handler has them.

But Aux is used in two scenarios. First, when the heat pump is working but can’t keep up with extreme cold temperatures, the heating strips are energized to provide supplemental heat. Secondly, Aux is powered when extra heat is needed immediately, for example, if the thermostat setting is increased significantly, such as from 72F to 76F.

O/B – Orange or Dark Blue: If orange, the Reversing Valve is powered in Cool mode. The reversing valve is the valve that changes the direction of refrigerant flow, so that your system can switch from Cool to Heat mode.

If dark blue, the thermostat is for one of the minority of heat pumps like Rheem or Ruud that power the Reversing Valve in Heat mode.

Extras: Black and Brown wires remain for use if there’s an issue with one of the terminals.

They can also be used if you intend to use an outdoor temperature sensor and wire it all the way back to the thermostat.

How to Wire a Heat Pump Thermostat?

Wire the thermostat using the same wiring colors and terminals on the old thermostat.

If this is a new heat pump installation and new thermostat, then use the wiring diagram and colors above to wire your heat pump thermostat.  

Tip on replacement: If this is a heat pump thermostat replacement, then take a picture of the connected wiring on the old thermostat before removing it. This ensures that the new thermostat will be wired the same – the same wires to the same terminals on the new unit. It’s important because in rare cases, the old thermostat wiring colors and terminal combinations might be different (non-standard).

Single Stage Heat Pump Thermostat Wiring

Heat Pump Thermostat Wiring Single Stage

To wire a single stage heat pump thermostat, connect these wiring colors to these terminals:

Yellow to the Y terminal

Green to the G terminal

Red to the R terminal

Light Blue to C

Orange (or Dark Blue) to the O/B terminal

White to the Aux terminal

White 2 to the E terminal

What if the white wire is jumped between AUX or E on the old thermostat?

This means that a single wire is used for both Aux and Emergency heat. When the wires are jumped, the functions don’t work independently. They are both either ON or OFF.

Wiring tip: Use separate wires for Aux and E if they are provided by your heat pump installer, even if the old thermostat used a jump wire.

2 Stage Heat Pump Thermostat Wiring

Heat Pump Thermostat Wiring 2 Stage

If your heat pump is a 2 stage model, meaning it has low and high Heat and Cool modes, then you’ll need a 2 stage thermostat.

A 2 stage thermostat has both a Y terminal (stage 1 cooling) and a Y2 terminal (stage 2 cooling), and both terminals receive wires. While both wires are typically yellow, wiring colors may vary.

*Old thermostat replacement: Use the same color wires on Y and Y2 on the new thermostat as on the old.

*New installation: Use the thermostat wiring diagram provided in the heat pump Installation Manual – every new heat pump comes with one. Or check our thermostat wiring guide for more tips on how to wire a thermostat for any number of wires. 


How many wires does a heat pump thermostat have?

A heat pump thermostat has 4, 5, 6, 7 or 8 wires, and 5-7 wires are the most commonly used.

See the wiring colors above. Here are the most common options and their functions.

  • 5 wires: 24 volt power, Heat, Cool, Fan, Common
  • 6 wires: 24 volt power, Heat, Cool, Fan, Common, jumped Aux and Emergency
  • 7 wires: 24 volt power, Heat, Cool, Fan, Common, Aux, Emergency
  • 8 wires: 24 volt power, Heat, Cool, Fan, Common, Aux, Emergency and Outdoor temperature sensor

4 wires: Thermostats like nest, ecobee and smart Honeywell thermostats have smart thermostat 4 wire wiring. They require 24 volt power, Heat, Cool and Fan.

Proprietary communicating thermostats also typically use 4 wires – a Common, Power and two Communicating terminals. Nest and Honeywell 4-wire thermostats are not communicating thermostats.

What wire should I use for a heat pump thermostat?

The best choice is 18-8 wire. This means the wire is 18 gauge in thickness, which is pretty thin but adequate for thermostat use. And it has 8 separate wires within the housing.

Even if you don’t use all the wires, you’ll have extras if one of the wires fails and you have to substitute one of the extra wires for it.

Does my heat pump use an O or B wire?

Most heat pumps use the O wire for the reversing valve. Rheem and Ruud are among those that use the B wire and terminal for this purpose.

What voltage is a heat pump thermostat?

24 volt power is used, converted from 110V that powers the air handler.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.
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