DIAGNOSIS Incorrect wiring configuration in the Nest's software, causing it to activate the wrong HVAC terminal. The most common cause is a misidentified O/B wire setting for heat pumps.
FIX On the thermostat, go to Settings > Equipment. Select 'Heat Pump'. Check the O/B setting. If your system blows cold air in Heat mode, reverse the O/B setting. Change 'O' to 'B' or 'B' to 'O'. Test the heat immediately.
IF THAT DIDN'T WORK Verify the physical wire at the Nest base matches the wire in the Equipment menu. A wire in the O/B terminal must be configured correctly in software. A wire in W1 or W2 is for conventional furnace, not heat pump.
HEADS UP After changing the O/B setting, the thermostat may run a test cycle. Allow it to complete. If the problem persists, the issue may be with the HVAC system's reversing valve, not the thermostat.