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.
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
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
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.
What Will Happen If I Miswire My Thermostat?
Miswiring a thermostat may seem like a minor mistake, but it can lead to a range of issues with your heating and cooling system. If the wires are connected incorrectly or if the wrong terminals are used, it can cause your heat pump to malfunction or even damage the system over time. Below are some common problems you may face if you miswire your thermostat:
1. Thermostat Not Turning On or Off
What Happens: If the thermostat wiring is incorrect, it may prevent the thermostat from receiving power or sending the correct signals to your HVAC system. This could result in your thermostat not turning on at all, or not responding to temperature adjustments.
What to Look For:
- A blank screen on your thermostat even when the system is powered on.
- Inability to change temperature settings or to switch between heating and cooling modes.
- Constantly running system or complete lack of operation.
How It Happens:
- If the R (24V power) wire isn’t correctly connected, the thermostat won’t receive power. Similarly, an incorrect connection to the C wire (common wire) can prevent the thermostat from completing the power circuit.
- Loose or misconnected wires to the terminals can cause the thermostat to stop working altogether.
Solution: Ensure that the R and C wires are connected to their proper terminals, and check for any loose connections.
2. System Constantly Running (Short Cycling)
What Happens: When the thermostat is miswired, the system may turn on and off rapidly, a condition known as short cycling. This occurs because the thermostat is receiving incorrect signals, confusing the system into thinking that it’s reaching the desired temperature when it isn’t.
What to Look For:
- Your HVAC system seems to turn on and off within a short time frame (a few minutes).
- The thermostat displays inconsistent or fluctuating readings that don’t match the actual temperature in the room.
How It Happens:
- A misconnection at the Y, Y2, or G terminals can lead to the system cycling inappropriately. For example, if the Y2 wire (stage 2 cooling) is connected incorrectly, the system may think it needs to activate the second cooling stage too early, leading to unnecessary cycles.
- A faulty G wire connection, which controls the fan, can cause the blower to run without the compressor, leading to short cycling.
Solution: Verify that the wires connected to the Y, Y2, and G terminals are correct and ensure that they’re securely fastened.
3. System Won’t Switch Between Heating and Cooling Modes
What Happens: Miswiring the O/B terminal, which controls the reversing valve, can cause your heat pump to fail when switching between heating and cooling modes. This means that your system may either stay stuck in one mode or fail to switch when needed.
What to Look For:
- The system is stuck in either heating or cooling mode, regardless of the thermostat setting.
- The temperature setting on the thermostat seems to have no effect on the system’s behavior.
How It Happens:
- If the O/B terminal is miswired, the thermostat may fail to send the correct signal to the reversing valve. This valve is responsible for reversing the refrigerant flow between heating and cooling modes.
- In some systems, the O wire is used for cooling mode and the B wire for heating mode, but wiring errors can cause the valve to remain in one position.
Solution: Check the wiring for the O/B terminal and ensure it is connected to the right terminal for your system. Refer to the manufacturer’s wiring guide for clarification if necessary.
4. Auxiliary or Emergency Heat Won’t Engage
What Happens: If your thermostat is wired incorrectly, the auxiliary or emergency heat feature may fail to engage when it’s needed most. This is especially important during colder months when the heat pump can’t keep up with heating demands, and auxiliary heat should kick in.
What to Look For:
- The thermostat fails to activate the auxiliary heat even when it’s cold outside, and the main heat pump can’t keep up.
- Emergency heat isn’t engaged when the heat pump breaks down, leaving the home colder than expected.
How It Happens:
- Miswiring the Aux and E terminals (for auxiliary and emergency heat) is the most common cause. If both wires are connected to the same terminal, the system will treat auxiliary and emergency heat as a single function, leading to inconsistent or nonexistent heat.
- If your heat pump uses separate wiring for auxiliary and emergency heat, both functions need to be wired independently to allow them to function as intended.
Solution: Double-check that the Aux and E wires are connected to their respective terminals, and make sure no wires are “jumped” between the two.
5. Unresponsive or Erratic Temperature Control
What Happens: If your thermostat is miswired, it might cause erratic or unresponsive temperature control, meaning the thermostat may fail to maintain the desired room temperature. This could lead to the home being too hot or too cold, or the system might not react to changes in the thermostat setting.
What to Look For:
- The temperature displayed on the thermostat doesn’t match the room temperature.
- The system runs constantly, even if the thermostat is set to a lower temperature.
- The thermostat displays fluctuating or incorrect temperatures despite no significant changes in the room.
How It Happens:
- Incorrect wiring of the Y, Y2, or G terminals could cause the thermostat to send conflicting signals about the temperature setting.
- A faulty C wire connection could cause erratic behavior due to inconsistent voltage supply.
Solution: Test each wire connection and ensure all terminals are correctly wired. You may also want to test the thermostat with a multimeter to ensure it’s receiving the proper signals.
6. Potential Damage to the HVAC System
What Happens: In some cases, improper wiring can not only cause operational issues but may also damage your HVAC system. For example, if the wires are crossed or placed incorrectly, it can send faulty signals to components like the compressor, reversing valve, or fan motor.
What to Look For:
- Unusual noises coming from the HVAC system, such as buzzing, clicking, or grinding.
- Components (like the compressor or fan) not turning on or operating at full capacity.
- The system feels unusually warm or cool, indicating malfunctioning parts.
How It Happens:
- Connecting the wires to incorrect terminals can result in electrical short circuits or power surges that damage sensitive HVAC components.
- Overloading certain terminals with incorrect wires can cause excessive wear or even burn out components like the reversing valve or blower motor.
Solution: If you suspect damage, immediately turn off the system and inspect all connections. If the issue persists or if you’re unsure about the wiring, it’s best to contact an HVAC technician to inspect and repair your system.
FAQs
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.
