What You Need to Know Before You Start

Wiring a heat pump to a thermostat is a straightforward task if you follow the correct wire sequence and understand what each terminal does. The job takes about 30 minutes and requires only a screwdriver and wire strippers — no special tools. The key is matching the wires from your old thermostat to the correct terminals on your new one, because heat pumps use different terminal labels than furnaces do.

Heat pumps have an extra terminal called O/B that switches the reversing valve between heating and cooling mode. This is the main difference from a standard furnace setup. If you wire this terminal wrong, your system will try to heat when it should cool, or vice versa. The rest of the wiring — power, fan, and compressor — follows the same logic as any other HVAC system.

If your old thermostat is mechanical (a dial or slider) rather than digital, take a photo of the wire positions before you disconnect anything. If your new thermostat is programmable or smart, you may need to enter your system type in the settings menu after wiring — most ask whether you have a heat pump or a furnace.

Key Takeaways

  • Heat pump thermostats use an O/B terminal to control the reversing valve; this terminal must be set to the correct mode (O for cooling priority or B for heating priority) in your thermostat settings.
  • The standard heat pump wire colors are R (red, 24V power), C (blue, common return), Y (yellow, compressor), G (green, fan), O/B (orange, reversing valve), and W (white, backup heat).
  • Turn off power at the breaker before disconnecting any wires, and take a photo of your old thermostat's wire positions if it is not labeled.
  • After wiring, test heating and cooling modes separately to confirm the reversing valve switches correctly and the fan runs in the right direction.

Identifying the Wires in Your Current Setup

Open your thermostat cover and look at the terminals on the back. Each terminal is labeled with a letter, and a wire runs from that terminal down into the wall to your heat pump unit. Write down which color wire is connected to each terminal, or take a clear photo. The most common wires in a heat pump system are R, C, Y, G, O/B, and W.

R is the 24-volt power wire, almost always red. C is the common return, usually blue. Y is the compressor call, typically yellow. G is the fan, usually green. O/B is the reversing valve, almost always orange. W is the backup electric heat (auxiliary heat), usually white. Not all systems use all six wires — many heat pumps skip W if there is no backup heater.

If your thermostat has wires you do not recognize, or if the colors do not match the standard, check the wiring diagram that came with your heat pump or thermostat. The diagram will show you exactly which wire goes where. If you cannot find the diagram, the model number on your thermostat or heat pump unit can be looked up online to find the manual.

Turning Off Power and Removing the Old Thermostat

Locate the breaker that controls your HVAC system and switch it to the off position. Wait 30 seconds to allow any residual power to drain. Then go back to your thermostat and remove the cover — usually by sliding it up or unclipping it from the base plate.

Loosen the terminal screws on the old thermostat one at a time and pull out each wire. As you remove each wire, when ready label it with a piece of tape and a marker — write the terminal letter (R, C, Y, G, O/B, W) on the tape. This prevents confusion when you install the new thermostat. If the wires are already labeled at the thermostat, photograph those labels before you disconnect anything.

Once all wires are removed, unscrew the old base plate from the wall and set it aside. If the new thermostat comes with a new base plate, install that now. If it uses the old base plate, make sure the holes line up before you proceed.

Connecting Wires to the New Thermostat

Insert each wire into the correct terminal on the new thermostat base plate, matching the letter labels. Start with R (power) and C (common) — these two must be present for the thermostat to power on. Then add Y (compressor), G (fan), O/B (reversing valve), and W (backup heat) in any order. Tighten each terminal screw firmly so the wire cannot be pulled out by hand.

If your new thermostat has a terminal labeled AUX or E instead of W, that is still your backup heat wire — the label varies by manufacturer. Some thermostats also have an L terminal for a humidifier or dehumidifier; if you do not have those devices, leave L empty.

Once all wires are seated and tight, snap the thermostat body onto the base plate or screw it in place, depending on the model. Do not turn the power back on yet — you need to set the system type first.

Setting the System Type and O/B Mode

Turn the power back on at the breaker. The new thermostat will power up and may ask you to select your system type. Choose heat pump from the menu. This tells the thermostat to use the O/B terminal and to expect heating and cooling cycles.

Next, look for a setting called O/B Mode, Reversing Valve, or Heat Pump Mode. You will see two options: O (cooling priority) or B (heating priority). In most climates, choose O, which means the reversing valve defaults to cooling and switches to heating only when the thermostat calls for heat. If you live in a very cold climate where heating is the primary mode, your installer may have set this to B — check your paperwork or the original thermostat settings before changing it.

Some thermostats also ask whether you have backup heat (auxiliary heat). If you have a W wire connected, answer yes. If you do not have a W wire, answer no. This setting determines whether the thermostat will call for electric heat strips when the outdoor temperature drops below the heat pump's efficiency threshold.

Testing Heating and Cooling Modes

Set the thermostat to cooling mode and lower the temperature setting below the current room temperature. You should hear the compressor start within 30 seconds, and cool air should begin flowing from the vents within a minute. If nothing happens, check that all wires are tight and that the system type is set to heat pump.

Switch to heating mode and raise the temperature setting above the current room temperature. The compressor should start and warm air should flow from the vents. If the air is cold instead of warm, the reversing valve did not switch — this usually means the O/B terminal is wired incorrectly or the O/B mode setting is wrong. Go back into the thermostat menu and flip the O/B mode from O to B (or vice versa), then test again.

Set the fan to manual (not auto) and confirm that the fan runs continuously. Then set it back to auto so the fan runs only when the compressor is calling for air. If the fan does not run at all, the G wire may be loose or the thermostat may not recognize it — check the terminal screw and the system type setting.

Troubleshooting Common Wiring Problems

If the thermostat powers on but the heat pump does not respond, the most common cause is a missing C (common) wire. Heat pumps require both R and C to complete the 24-volt circuit. If your old thermostat did not have a C wire, you may need to run one from the heat pump unit to the thermostat — this is a job for a technician, as it requires opening the heat pump cabinet.

If the system heats when it should cool or cools when it should heat, the O/B terminal is set to the wrong mode. Flip the O/B mode in the thermostat menu and test again. If flipping the mode does not fix it, the O/B wire may be connected to the wrong terminal — double-check that the orange wire is in the O/B slot, not in Y or another terminal.

If the backup heat (W wire) does not work, confirm that the thermostat is set to use auxiliary heat and that the outdoor temperature is below the threshold where the thermostat calls for it. Most thermostats only use backup heat when the outdoor temperature drops below 35°F and the heat pump cannot keep up. If the temperature is above that threshold, the backup heat will not set up even if it is wired correctly.

When to Call a Professional

If you do not have a C wire and your old thermostat worked without one, do not attempt to run a new wire yourself. This requires access to the heat pump unit and knowledge of low-voltage wiring safety. Contact an HVAC technician to run the C wire for you.

If the thermostat powers on but the heat pump does not respond after you have checked all terminal connections, the problem may be in the heat pump unit itself or in the wiring between the thermostat and the unit. This also requires a technician. Do not assume the thermostat is broken — have the wiring checked first.

If you are uncomfortable working with electrical connections or if your heat pump has a wiring diagram that does not match the standard terminal labels, hire a technician to do the wiring. The cost is usually $100 to $200, and it is worth the peace of mind.

Frequently Asked Questions

Do I need a C wire to run a heat pump thermostat?

Yes, heat pump thermostats require both R (power) and C (common) to complete the 24-volt circuit. If your old thermostat did not have a C wire, you will need to run one from the heat pump unit to the thermostat before the new thermostat will power on. This is a job for an HVAC technician.

What is the difference between O and B mode on the reversing valve?

O mode (cooling priority) defaults the reversing valve to cooling and switches to heating only when you call for heat. B mode (heating priority) does the opposite. Most climates use O mode. Check your original thermostat settings or ask your installer which mode your system was set to before you change it.

Can I wire a heat pump thermostat the same way as a furnace thermostat?

No. Heat pumps have an O/B terminal that furnaces do not use. If you wire a heat pump to a furnace thermostat, the reversing valve will not switch and the system will not heat or cool correctly. Always use a thermostat designed for heat pumps.

Why does my heat pump blow cold air in heating mode?

The reversing valve did not switch, which means the O/B terminal is wired wrong or set to the wrong mode. Check that the orange wire is in the O/B terminal and that the O/B mode in the thermostat menu is set correctly. Flip the mode and test again.

How long does it take to wire a heat pump thermostat?

The wiring itself takes 15 to 30 minutes if all wires are already in place and labeled. If you need to run a new C wire or troubleshoot a problem, add another 30 to 60 minutes. If you are hiring a technician, expect one to two hours including testing.