Heat Pump Running but Not Heating: Causes and Practical Fixes

When a heat pump is running but not heating, homeowners face discomfort and potential repair costs. This article explains common causes, diagnostic steps, and practical fixes to restore heating performance. It focuses on actionable guidance for U.S. conditions and highlights when professional service is necessary.

Symptom	Likely Cause	Quick Action
Running But Cold Air	Reversing Valve Stuck, Low Refrigerant	Check mode, inspect airflow, call technician
Short Cycling	Defrost Issues, Thermostat Problems	Reset thermostat, monitor cycles
Frosted Outdoor Unit	Defrost Failure, Low Charge	Manual defrost, schedule service

How Heat Pumps Heat: A Quick Overview

Heat pumps move heat from outdoor air or ground into indoor spaces using refrigeration principles. In heating mode, the system reverses refrigeration flow via a reversing valve so the outdoor coil becomes the evaporator. A functioning compressor, adequate refrigerant, and proper airflow are essential for heat transfer.

Common Reasons A Heat Pump Runs But Does Not Heat

Understanding common failure points helps prioritize diagnostics. Major causes include improper mode selection, thermostat settings, defrost system failures, low refrigerant, compressor issues, airflow restrictions, and electrical faults.

1. Incorrect Thermostat Mode Or Settings

If the thermostat is set to “cool” or to a temperature below room level, the heat pump may run without producing warm air. Thermostat programming errors, a clogged battery-powered unit, or wiring issues can also prevent proper heating calls.

2. Reversing Valve Or Control Failure

The reversing valve switches the refrigerant flow direction for heating. If it fails or its actuator is stuck, the heat pump may operate but remain in cooling mode or an intermediate state, producing cold air while appearing to run normally.

3. Low Refrigerant Charge Or Leaks

Low refrigerant reduces heat transfer capacity and may cause the outdoor coil to freeze. Leaks often cause gradual performance decline and require certified HVAC technicians for leak detection and safe recharge by EPA-certified professionals.

4. Defrost Cycle Failure Or Icing On Outdoor Unit

During cold conditions, frost accumulates on the outdoor coil. The defrost cycle reverses mode briefly to melt ice. If the defrost control or sensors fail, heavy frosting reduces heat extraction and the unit may run but not deliver heat.

5. Compressor Or Electrical Issues

The compressor is the heart of refrigerant circulation. Start capacitors, contactors, and wiring issues can allow the fan or auxiliary heat to run while the compressor fails to start. A non-functioning compressor prevents heating even if the system appears active.

6. Airflow Restrictions: Filters, Ducts, And Fans

Restricted airflow lowers heat output. Dirty filters, closed supply vents, blocked return grilles, or failing indoor blowers cause reduced or cold airflow despite the outdoor unit running. Regular maintenance prevents many airflow problems.

7. Auxiliary Or Emergency Heat Misuse

Heat pumps often use electric resistance or gas backup heat for extreme cold or defrost cycles. If the system is stuck in emergency heat or auxiliary mode due to a control fault, the system may run inefficiently or produce inadequate warmth for the thermostat setting.

Step-by-Step Troubleshooting For Homeowners

These troubleshooting steps help identify common, non-hazardous issues. If any step reveals a suspected refrigerant leak, electrical fault, or compressor problem, contact a licensed HVAC technician.

1. Verify Thermostat Settings: Ensure the thermostat is set to “Heat” or “Auto” and the setpoint is above current room temperature. Replace thermostat batteries and check fan mode is on “Auto.”
2. Inspect Air Filters And Vents: Replace dirty filters and ensure supply and return vents are open and unobstructed. Check indoor blower operation by setting fan to “On.”
3. Check Outdoor Unit Condition: Look for visible frost or ice on the outdoor coil. Remove debris and ensure clear airflow around the unit. Do not chip ice—use a manual defrost or wait for automatic cycle if safe.
4. Listen For Compressor Operation: If the outdoor fan runs but the compressor is silent or humming, this indicates electrical or compressor failure. Note any clicking sounds from contactors or frequent cycling.
5. Test Defrost Cycle: During cold weather, observe whether the unit periodically reverses and melts frost. Continuous icing without defrost cycles suggests defrost control or sensor problems.
6. Check Auxiliary Heat Status: If the system is in emergency heat mode, the heat pump section may be disabled. Reset the thermostat or circuit breakers to clear temporary faults.
7. Reset Breakers And Disconnects: Turn off and on the outdoor unit at the disconnect or breaker to reset controls. If tripped breakers reoccur, do not repeatedly reset—this indicates a deeper electrical problem.

When To Call A Professional HVAC Technician

Contact a licensed HVAC professional when troubleshooting points to refrigerant leaks, compressor failure, reversing valve issues, failed defrost controls, or persistent electrical faults. Technicians provide leak detection, refrigerant recovery/recharge, compressor diagnostics, and safe electrical repairs.

• Suspected refrigerant leak or low charge
• Compressor not starting or making unusual noises
• Persistent icing that manual defrost does not fix
• Complex electrical control or reversing valve faults
• Unit older than 10–15 years with declining performance

DIY Fixes And Maintenance To Improve Heating Performance

Some maintenance tasks can be performed safely by homeowners and may resolve common issues. Regular maintenance improves efficiency and extends system life.

• Replace Air Filters: Change filters every 1–3 months depending on usage and pets.
• Clear Outdoor Unit: Remove leaves, snow, and debris from around the unit. Maintain at least two feet of clearance for airflow.
• Keep Vents Open: Ensure all supply and return vents are open and unobstructed. Balance airflow by adjusting dampers if necessary.
• Clean Indoor Coils And Registers: Vacuum return grilles and clean registers for better airflow.
• Programmable Thermostat Settings: Use a proper heat pump thermostat configured to prevent short cycling and to manage auxiliary heat usage.

Understanding Costs And Parts Replacement

Repair costs vary by cause. Minor fixes like filters, thermostats, or fan motor capacitors are relatively inexpensive. Refrigerant recharge, defrost control replacement, and reversing valve repairs are mid-range. Compressor replacement or full unit replacement is high-cost and may justify replacement when the unit is older or efficiency is low.

Repair	Typical Cost Range (USD)
Filter Replacement	$10–$50
Thermostat Repair/Replacement	$75–$250
Fan Motor/Capacitor	$150–$500
Refrigerant Leak Repair & Recharge	$200–$800+
Defrost Control Or Sensor	$150–$600
Compressor Replacement	$1,200–$3,500+

Preventive Care To Avoid Future “Running But Not Heating” Issues

Routine preventive maintenance reduces failures and improves efficiency. Annual professional tune-ups and seasonal homeowner checks keep the system ready for cold weather.

• Annual Professional Maintenance: Schedule a full tune-up each year before the heating season. Technicians check refrigerant levels, controls, electrical connections, and perform safety testing.
• Regular Homeowner Inspections: Check filters monthly, clear outdoor debris, and observe the unit during seasonal transitions.
• Upgrade Controls: Consider a heat-pump-compatible smart thermostat to improve performance, optimize auxiliary heat, and reduce cycling.
• Consider System Replacement When Appropriate: When repairs approach 50% of replacement cost or the unit is over 15 years old with reduced efficiency, replacement may be the best long-term option.

Special Considerations For Cold Climates

In colder regions, heat pumps work less efficiently and rely more on auxiliary heat. Modern cold-climate heat pumps provide better low-temperature performance, but homeowners should expect increased defrost cycles and occasional reliance on backup heat below certain temperatures.

Tip: For consistently cold climates, evaluate heat pump models rated for low ambient operation and ensure proper sizing for the heating load to avoid constant auxiliary heat use.

Signs That Indicate Emergency Service Is Needed

Some symptoms require immediate attention to prevent damage or unsafe conditions. Shut down the system and call service if any of the following occur.

• Burning smells or smoke from the system
• Sparking or tripped breakers that won’t reset
• Compressor loud banging or metallic noise
• Visible refrigerant oil or pooling liquid around outdoor unit

Key Takeaways For Homeowners

When A Heat Pump Is Running But Not Heating: first verify thermostat and airflow, inspect outdoor unit for frost, and listen for compressor operation. For refrigerant issues, reversing valve faults, compressor failures, or electrical problems, seek a licensed HVAC technician. Regular maintenance and proper thermostat use significantly reduce the risk of heating failures.

Following systematic troubleshooting helps distinguish simple DIY fixes from issues requiring professional diagnosis. Prioritizing annual service and timely repairs ensures reliable heating performance and energy-efficient operation.