Can you take a picture of your current thermostat so we can see the wires on the back plate? Basically, a heat pump has a 4-way reversing valve that gets energized to switch modes from cooling to heating or heating to cooling. The wire that energizes the reversing valve is connected to the O/B terminal. The G terminal is for the Blower Fan, the Y terminal is for cooling; actually it turns on the compressor. Just note what color wire is on which terminal on your old thermostat backplate and move to the same on the Nest backplate. As a heat pump it either defaults to heating or cooling. When you test your system with your Nest thermostat, if Cooling is hot air or if Heating blows cold air, you need to reconfigure the nest to change the heat pump O/B wire from O to B function, or B to O function. I hope this helps.
NestPro, Google Pro, Mechanical Engineer and HVAC service company owner.
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