The answer to 'heat pump not heating' is that a common cause is a problem with the reversing valve, which controls the flow of refrigerant to switch the unit from heating to cooling mode. Symptoms may include the unit blowing cool air instead of warm, or the compressor not turning on at all. Solutions can include cleaning or replacing the reversing valve, checking for low refrigerant levels, or addressing other potential issues like a faulty thermostat or compressor.
For most homeowners, troubleshooting and repairing a heat pump that's not heating properly is best left to a professional HVAC technician. Heat pumps contain refrigerant and high-voltage electrical components that require specialized knowledge and tools to service safely. Attempting DIY repairs on a heat pump can be dangerous and may end up causing further damage if not done correctly.