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If you recently installed your Nest thermostat and find it annoying that the heat and cold settings are switched, don’t worry.

This is a very common issue in heat pump systems that utilize a single wire to control the reversing valve.

Reconfiguring it is very straightforward, so let’s talk more about how you can resolve this issue step-by-step.

Troubleshoot Heat Cold Switching on a Heat Pump

The cold air on heat or hot air on cold problems is very common in systems with a heat pump.

Typically, this is because the thermostat has flipped the O/B wire signals, which sends two different types of signals to the reversing valve.

In a Nest thermostat, you can toggle this setting internally, allowing you to read just the O/B wire signals so the correct mode shows up when the thermostat calls for heat or cool.

Initial Step: Identify If You Have a Heat Pump System

Heat pumps may seem no different from other conventional air conditioner systems, since side-by-side, the outdoor units can look exactly alike. 

Heat Pump 3

However, upon close inspection, they have specific characteristics that you can observe. These include:

  • Any labels on the outdoor unit indicating the manufacturer and model number or energy rating.
  • The outdoor unit elevated a foot or so above the ground
  • A small circuit board on the outdoor unit (the defrost controller), alongside the other components of a regular air conditioner.
  • The reversing valve inside the outdoor unit: a copper tube-like contraption with three tubes on one side and another tube on the other. (Also, with an electrical solenoid attached to it with wires).

Diagnostic Step: Manually Adjust Both Heating and Cooling Modes

Before toggling the O/B wire setting or checking the wiring, it’s important to confirm that the heat pump is actually reversing the system mode. 

Nest Thermostat Blank Screen

In order to carry this out, you will need to adjust the temperature and system modes manually in the thermostat. Here’s how:

  1. Adjust your thermostat to “heat”
  2. Then, set the target temperature a couple of degrees higher than the current indoor temperature
  3. Wait a couple of minutes for the heat pump to kick in and check for the air coming in from the vents. 
  4. Is it warm? If you don’t feel much of a difference, give it several more minutes and check the thermostat to see if the temperature in your home improves.
  5. If the thermostat doesn’t reach the target temperature after 10 or 20 minutes, there might be an issue with the heating or cooling. 
  6. If anything doesn’t change or the temperature seems to have dipped instead, try switching the mode to “cool”
  7. Then, wait a couple of minutes and see what happens to the temperatures. Does it get hot or does it cool down instead?

If the temperatures do change, but the other way around, you can confirm the HVAC equipment is not defective and instead it’s an issue with the O/B wire. 

However, in some rare cases, it could also be incorrect thermostat wiring, so we'll have to check for that as well.

In the following steps, I’ll talk about troubleshooting the thermostat wiring. So if you’re not skilled in troubleshooting electrical components, it’s best to skip this step or get help from a professional, especially if you're not comfortable doing these yourself.

Step 1: Make Sure the Wiring is Correct

Before you start to toggle the O/B wire setting, it’s important to check the thermostat wiring and confirm everything is properly connected. 

  1. First, turn off the power to the thermostat and the HVAC system
  2. Then, pull the thermostat off the wall mount
  3. See if you have an O/B wire. Then, take a photo of the wiring connections.
  4. After that, see if the wires are configured properly. If the system was installed correctly, typically, the yellow wire should be connected to Y and the white wire should be connected to W. (These indicate the cooling and heating systems respectively.)
  5. If you find the wires are swapped, make sure to remove the wires from the terminals and reconnect them as necessary. 
  6. Make sure the connector tabs are fully held down.
  7. After that, plug the thermostat back into the wall mount, ensuring it clicks into place.
  8. Then, wait for the thermostat to restart or reconnect to the wiring. 
  9. Finally, adjust the temperatures manually and see if the correct mode turns on.

However, if the heating and cooling modes are still switched, or the wiring was installed correctly, it’s best to move on to the next step.  

Thermostat Wiring

Step 2: Swap the O/B Wire Setting in the Thermostat

If the wiring is solid but the system modes are still flipped, then it’s time to swap the O/B wire signal in the thermostat’s settings. 

Depending on your model, you can easily carry this out via mobile apps or the thermostat itself. 

Change the O/B Wire Signal Via Google Home

This applies to the Nest Thermostat 2020 Model

Google Home and Nest Thermostat 2 6
  1. First, open the Google Home app and select your thermostat.
  2. Then, head over to “Settings” at the top right (the gear cog icon).
  3. After that, select “Thermostat.”
  4. Then, scroll the available settings until you find “Heat Pump.” Tap on it or check below the label.
  5. The O/B wire option will be displayed. If either the O or B option is highlighted, toggle to the opposite option.
  6. Then exit the settings, give it a couple of minutes, and see if the issue resolves itself.

Toggle the O/B Signal Via the Thermostat

This applies to the Nest Learning Thermostat and Nest Thermostat E

Nest app and E and learning 1 4
  1. Wake up the display and head over to “Settings” on the Quick-View menu.
  2. Select the option for “Equipment”
  3. The wiring diagram will show up. Confirm all the wires are detected, and tap on “Continue.”
  4. Then, select “Heat Pump.”
  5. Here, toggle the setting as necessary. If it’s highlighted to O, select B or vice versa

Step 3: Check the HVAC Equipment

If the above steps didn't fix your issue, it's most likely a stuck or malfunctioning reversal valve. 

If so, I recommend you get help from a professional since they will have access to the parts and expertise to replace the reversing valve and fix any other issues.


Most of the time, if the heating and cooling systems are mixed, it’s definitely an issue with the O/B wire setting on your thermostat. 

So if you have a heat pump system, make sure to toggle this setting and see if the situation improves. 

Otherwise, it might be a wiring issue, which you can check yourself or get help from a professional. 

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