How to Switch Heat Pump From AC to Heat

Switching a heat pump from AC to heat is a common homeowner task that involves thermostat settings, equipment checks, and awareness of how heat pumps operate in cold weather. This guide explains step-by-step actions, troubleshooting tips, and maintenance advice to make the transition smooth and efficient.

Mode	What Changes	When To Use
Air Conditioning (AC)	Indoor evaporator cools; outdoor unit condenses heat	Warm weather, cooling needs
Heat Pump Heating	Reversing valve switches refrigerant flow; outdoor unit extracts heat	Cool to moderately cold weather
Emergency/Backup Heat	Resistive or gas backup kicks in; used when heat pump is insufficient	Very cold temperatures or heat pump malfunction

How Heat Pumps Work: Basic Principles

A heat pump moves heat rather than generating it, using refrigerant, a compressor, and a reversing valve to change direction between cooling and heating cycles. In cooling mode the system removes heat from indoors; in heating mode it extracts heat from outdoor air and moves it indoors.

Understanding this mechanism clarifies why the system must switch modes correctly and why some behaviors—like an outdoor unit running in cold weather—are normal. Switching the thermostat to heat triggers the reversing valve and control sequence.

Preparing To Switch From AC To Heat

Before switching, verify the thermostat type, check power to the heat pump, and ensure filters are clean. A functioning thermostat and clean airflow are essential for effective heating and to prevent unnecessary wear.

Confirm the outdoor unit is clear of debris and snow. Obstructed airflow reduces efficiency and can trigger defrost cycles, so clear at least two feet around the unit.

Step-By-Step Thermostat Switching

1. Set Thermostat Mode To Heat

On most digital thermostats choose the mode option and select “Heat” or the heat pump icon. For programmable thermostats, set a target temperature a few degrees higher than the current indoor temperature to prompt the heat pump to start.

2. Confirm Fan Operation

Set the fan to “Auto” for standard operation so it runs only during active heating cycles. Use “On” if continuous circulation is desired to even out temperatures, but expect slightly higher electricity use.

3. Check For Heat Pump Or Emergency Heat

Some thermostats display “Heat” and “Aux/Emerg” or “Emergency Heat.” Do not use Emergency Heat for normal operation; it is a backup electric or gas heat used only when the heat pump cannot operate or during repairs.

Understanding Emergency Heat And Auxiliary Heat

Auxiliary heat supplements the heat pump when temperature drops or during defrost cycles; it engages automatically based on thermostat logic. Emergency heat bypasses the heat pump entirely and relies only on backup heating sources.

Using Emergency Heat increases energy costs because resistive or gas backup is usually less efficient than the heat pump. Reserve Emergency Heat for actual system failure.

What To Expect When Heating Starts

When switched to heat, the thermostat sends a signal to change the reversing valve state and start the compressor. The outdoor unit may run in low-speed or high-speed depending on demand and system design.

The indoor coil will feel warm after a short period. If warm air doesn’t appear within 10–15 minutes, inspect filters, registers, and the thermostat wiring or call a technician.

Defrost Cycles And Cold-Weather Operation

In heating mode, the outdoor coil can accumulate frost. Heat pumps use periodic defrost cycles where the unit briefly switches to cooling operation to melt ice, causing warm air dips indoors. This is normal and controlled by sensors and timers.

Frequent or prolonged defrost cycles may indicate low refrigerant, airflow problems, or sensor issues, and they warrant professional diagnosis.

Optimizing Efficiency After Switching

Set the thermostat to a steady comfortable temperature and use programmable or smart schedules to avoid frequent large setpoint changes. Heat pumps perform best with smaller temperature differentials and consistent operation.

Use ceiling fans on low to improve circulation and minimize thermostat setbacks that cause the backup heat to activate. Adequate insulation and sealed ductwork improve system efficiency.

Common Issues When Switching From AC To Heat

No Heat Or Weak Heat

Possible causes include wrong thermostat configuration, tripped breakers, blown fuses, dirty filters, closed registers, or compressor issues. Check basic components first, then call HVAC support if unresolved.

Outdoor Unit Not Running

If the outdoor unit does not start in heating mode, verify the thermostat wiring and breaker. If power is present but the unit remains off, the contactor, capacitor, or compressor may be faulty.

Short Cycling Or Excessive Fan Run Time

Short cycling can stem from oversized systems, refrigerant problems, or faulty controls. Excessive fan runtime may indicate thermostat settings or sensor issues. Professional evaluation can identify underlying faults.

Safety And Electrical Considerations

Turn off power at the breaker before inspecting the outdoor unit. Never attempt to service refrigerant lines, compressors, or electrical components unless certified. Working on high-voltage equipment is hazardous and can void warranties.

Regularly test carbon monoxide detectors if a backup gas furnace is part of the system, and ensure vents are clear and properly functioning.

When To Call A Professional

Contact a licensed HVAC technician if there is no heat after basic checks, if frequent defrosts and frost buildup occur, or if unusual noises or smells are present. Technicians can test refrigerant pressures, controls, and reversing valve operation.

Schedule seasonal maintenance to inspect the reversing valve, check refrigerant levels, clean coils, and confirm proper thermostatic control. Annual inspection improves reliability and extends equipment life.

Thermostat Types And Compatibility

Heat pump thermostats differ from standard cooling-only thermostats and include additional wiring for reversing valve and auxiliary heat. Confirm the thermostat is heat pump-compatible (often labeled “HP”).

Smart thermostats usually support heat pumps but require correct wiring and configuration. During installation, follow manufacturer guidance for O/B reversing valve energization and auxiliary heat settings.

Advanced Settings: Reversing Valve And O/B Config

Some systems use O or B terminals to indicate when the reversing valve is energized in cooling or heating. If the heat pump runs the wrong mode when commanded, swap the O/B setting in the thermostat configuration or consult a technician.

Incorrect O/B configuration can cause the unit to attempt cooling when heating is commanded. Correct configuration ensures the reversing valve energizes correctly for heat mode.

Maintenance Checklist Before Winter

• Replace or clean air filters monthly during heavy use.
• Inspect and clear debris from the outdoor unit and maintain clearance.
• Check thermostat batteries and settings for heat pump mode.
• Schedule professional preventive maintenance to inspect electrical components and refrigerant.
• Seal and insulate ducts and check for leaks at registers.

Troubleshooting Quick Guide

1. Verify thermostat set to Heat and fan to Auto.
2. Check circuit breakers and outdoor unit power switch.
3. Replace dirty filters and open closed registers.
4. Listen for compressor and fan operation at the outdoor unit.
5. If no outdoor operation, contact HVAC technician for electrical or compressor issues.

Costs And Energy Considerations

Heat pumps are more efficient than resistive heating, often providing 2–4 times the heat per unit of electricity used. Using the heat pump rather than emergency heat keeps energy costs lower in most climates.

Electricity costs vary regionally; evaluate the heat pump’s coefficient of performance (COP) and local utility rates to estimate running costs. Consider heat pump maintenance to preserve efficiency and reduce long-term expenses.

Upgrades And Additional Features

Variable-speed compressors, zoned thermostats, and dual-fuel setups (heat pump plus gas furnace) can optimize comfort and efficiency. Smart thermostats with heat pump-specific algorithms improve staging and reduce backup heat usage.

Consider adding a dedicated heat strip or improving ductwork if the system struggles to meet demand in very cold climates, but assess long-term energy trade-offs first.

Key Takeaways For Switching Heat Pump From AC To Heat

Switching is primarily a thermostat-driven process that activates the reversing valve and starts the heating sequence. Proper configuration and maintenance ensure reliable operation and efficiency.

Reserve Emergency Heat for system failures, keep outdoor units clear, and schedule annual professional checks. Troubleshoot basic issues with filters, breakers, and thermostat settings before seeking repairs.

Further Resources

For detailed diagnostics and refrigerant-related repairs, refer to manufacturer manuals or contact certified HVAC professionals. Local utility rebate programs may offer incentives for heat pump upgrades and maintenance services.

Helpful keywords to search for more information: “heat pump thermostat settings”, “reversing valve O B configuration”, “heat pump defrost cycle”, and “heat pump emergency heat vs aux heat”.