Heat Pump Won’t Turn On: Causes, Troubleshooting, and Solutions

The article explains common reasons a heat pump won’t turn on, step-by-step troubleshooting, safety tips, estimated repair costs, and maintenance to prevent future failures. It helps homeowners identify issues such as thermostat problems, power and breaker faults, defrost and control board failures, and compressor or refrigerant concerns. Use this guide to determine when a simple fix is possible and when to call a licensed HVAC technician.

Symptom	Likely Cause	Action
No power at all	Tripped breaker, blown fuse, disconnected power	Reset breaker; check outdoor disconnect; call electrician if needed
System tries then stops	Thermostat, capacitor, or control board issue	Swap thermostat batteries; inspect capacitor; call tech
Fan runs but compressor doesn’t	Compressor start device or capacitor failure	Test capacitor; call technician to replace start components
Outdoor unit frozen	Defrost cycle failure or low refrigerant	Allow defrost; check defrost control; schedule service

How Heat Pumps Work And Why They Sometimes Fail

Heat pumps move heat between the outdoors and indoors using a compressor, refrigerant, and reversing valve for heating and cooling modes. A thermostat commands the system, while electrical components like contactors, relays, capacitors, and control boards coordinate operation. When one component fails, the entire system may not start.

Understanding component roles helps diagnose why a heat pump won’t turn on. For example, a dead thermostat shows no call for heat, while a faulty capacitor prevents the compressor from starting despite power reaching the unit.

Initial Safety Checks Before Troubleshooting

Always prioritize safety when diagnosing HVAC equipment. Turn off power at the breaker and the outdoor disconnect before inspecting the unit. Do not attempt to repair electrical components without proper training. If a homeowner is uncertain, calling a qualified HVAC technician is the safest option.

Basic Troubleshooting Steps Homeowners Can Try

1. Verify Thermostat Settings And Power

Confirm the thermostat is set to heat or auto, set below room temperature to trigger heating, and that batteries are fresh if wireless. For smart thermostats, check Wi‑Fi connectivity and app status. A misconfigured or dead thermostat commonly causes a heat pump not to turn on.

2. Check Circuit Breakers And Outdoor Disconnect

Inspect the main breaker panel for a tripped breaker and reset it if safe. Verify the outdoor unit’s disconnect switch near the condenser is in the ON position. If breakers trip again, this suggests an electrical fault requiring professional diagnosis.

3. Inspect Safety Switches And Airflow

Ensure filter is clean and supply vents are open. A clogged filter or blocked return can trigger safety devices that prevent operation. For units with a float switch in a condensate pan, a filled pan can shut the system down; clearing the drain or replacing the switch may restore function.

4. Listen For Sounds At Startup

Turn the system to heat and listen at the outdoor unit. A click followed by fan motor running but no compressor hum indicates the compressor is not starting. Repeated clicking may indicate a failed contactor or low-voltage control issue.

Common Mechanical And Electrical Causes

Thermostat Failures

Thermostat wiring faults, dead batteries, or programming errors can prevent a heat call. A handheld multimeter can confirm 24VAC power to the thermostat terminals if the homeowner is comfortable testing. Replacing a simple thermostat or resetting a programmable unit often resolves the problem.

Tripped Breakers And Blown Fuses

Electrical overloads, short circuits, or aging breakers can interrupt power. If a breaker trips repeatedly after reset, persistent faults in the compressor or motor are likely and require a technician to diagnose and repair.

Capacitor And Contactor Problems

Start capacitors provide the torque to spin compressors and fan motors; run capacitors keep them running. A failing capacitor often prevents the compressor from starting while the fan may run. Contactors wear from arcing and can fail to close, preventing power transfer to the compressor.

Control Board And Relay Faults

The control board coordinates signals from the thermostat, pressure switches, and sensors. A faulty board or relay may fail to send the start command. Modern units have diagnostics codes on the control board which technicians can read to pinpoint issues faster.

Compressor And Motor Failures

Compressor failures, motor winding shorts, or locked rotors prevent operation and often present as humming or repeated attempts to start. These are major repairs or replacements and typically require professional service and potential refrigerant recovery.

Refrigerant Leaks And Pressure Switches

Low refrigerant changes pressure readings and may trip safety switches to prevent compressor damage. Pressure switches and sensors can also fail independently, causing the unit to refuse to start until corrected.

Outdoor Unit Frozen Or In Defrost Mode

In cold weather, a heat pump cycles through defrost to remove ice. If defrost controls malfunction, the unit may remain frozen and not operate normally. Allowing controlled defrost and inspecting the defrost control board, sensors, and reversing valve is essential.

Tip: Never chip away heavy ice with sharp tools; that risks damaging coils and refrigerant lines. Use a hair dryer or wait for warm weather while scheduling service.

When To Call A Professional HVAC Technician

Call a licensed technician when troubleshooting involves electrical testing beyond breaker checks, suspected refrigerant leaks, compressor or control board replacement, repeated breaker trips, or unusual sounds like loud humming or grinding. Professionals have the tools and certification to handle refrigerants and high-voltage systems safely.

Estimated Repair Costs And Timeframes

Repair costs vary widely by cause, regional labor rates, and unit make. Typical ranges: thermostat replacement $75–$300, capacitor or contactor $150–$450, control board $300–$900, compressor replacement $1,200–$3,500+, and refrigerant leak repair $200–$800 depending on severity.

Smaller repairs often take 1–3 hours. Major compressor or system replacements can require 4–8 hours or more and may involve permits and refrigerant reclaiming that extend the timeline.

Preventive Maintenance To Avoid Future Failures

Regular maintenance reduces the chance a heat pump won’t turn on. Annual professional tune-ups, filter changes every 1–3 months, clearing debris around the outdoor unit, verifying proper refrigerant charge, and inspecting electrical connections preserve reliability.

• Replace filters regularly to maintain airflow and efficiency.
• Schedule annual HVAC inspections before heating season for testing and calibration.
• Keep the outdoor unit clear of plants, leaves, and snow to prevent airflow restriction.
• Monitor system performance and unusual noises promptly.

Smart Thermostats And Compatibility Issues

Smart thermostats can improve comfort and efficiency but sometimes fail to start older heat pumps due to C-wire (common) requirements or mismatched control logic. Ensure compatibility and proper wiring; if necessary, add a C-wire adapter or a professional thermostat installer to avoid a heat pump not turning on due to control issues.

DIY Repairs Vs. Professional Service

Homeowners can handle thermostat swaps, filter changes, breaker checks, and clearing simple obstructions. Never attempt to service capacitors, contactors, compressors, refrigerant lines, or internal electrical diagnostics without training due to risk of injury and equipment damage.

For any electrical or refrigerant-related repairs, a licensed HVAC technician with EPA certification and proper tools is the correct choice.

Common Diagnostic Tools Professionals Use

Technicians use thermostats to simulate calls for heat, multimeters for voltage and continuity, amp clamps for motor loads, manifold gauges for refrigerant pressure, and leak detectors. Control board error codes and manufacturer diagnostic tools speed accurate diagnosis.

Frequently Asked Questions

Why Does The Heat Pump Fan Run But The Compressor Doesn’t?

Typically this indicates a start device or capacitor failure, a bad contactor, or compressor internal problems. Fan motors use separate circuits and lower starting torque, so they may run while the compressor cannot start.

Can A Dirty Filter Cause A Heat Pump Not To Turn On?

Yes. Very restricted airflow can trigger safety devices or freeze coils, which might prevent operation. Replacing a clogged filter is a simple first step before calling for repair.

Is It Normal For A Heat Pump To Run Continuously On Cold Days?

In very cold weather, a heat pump may run longer to maintain setpoint, but continuous running with no temperature rise could indicate a malfunction or low refrigerant. Verify proper operation and schedule service if performance is poor.

How Long Can A Heat Pump Sit Off Before Causing Damage?

Short downtime doesn’t harm the unit, but prolonged inactivity—especially in humid climates—can allow corrosion or rodent damage to wiring. Annual checks before seasonal use help ensure reliable startup.

Key Takeaways And Next Steps

If a heat pump won’t turn on, start with simple checks: thermostat power and settings, breakers and disconnects, and filter condition. If basic steps don’t restore operation or if electrical components or refrigerant are suspected, contact a licensed HVAC technician for safe, effective repairs.

Keeping records of maintenance, responses from technicians, and any diagnostic error codes will help speed future repairs and improve system longevity.