When an air-source or ground-source heat pump suddenly delivers lukewarm air or refuses to run altogether, the cause is usually found in a predictable set of components. Unlike gas or oil furnaces that generate heat through combustion, heat pumps extract heat from outside air or the ground and move it indoors using a refrigeration cycle. This makes them highly efficient but also dependent on the precise interaction of electrical controls, refrigerant charge, airflow, and mechanical parts. A total loss of heat can be as simple as a thermostat accidentally set to "Cool" or as complex as a failed compressor. Understanding the most common failure points and how to address them can restore comfort quickly and prevent expensive service calls.

How a Heat Pump Moves Heat Into Your Home

In heating mode, a heat pump reverses its normal refrigeration cycle. The outdoor coil becomes the evaporator, absorbing thermal energy from the outside air (or ground). The refrigerant then passes through a reversing valve to the indoor coil, which now acts as a condenser, releasing heat into the home's ductwork. This cycle depends on several key components working correctly: the compressor, fan motors, expansion valve, reversing valve, defrost board, and control thermostat. If any of these fail, the system may blow cold air, cycle on and off repeatedly, or stop working entirely.

Top Reasons a Heat Pump Stops Heating

1. Thermostat Configuration and Power Errors

The thermostat is the command center for the entire system. A surprisingly common reason for no heat is that the thermostat is set to "Cool" instead of "Heat," or the temperature set point is lower than the current room temperature. Modern programmable and smart thermostats introduce additional failure points. They may lose Wi-Fi connectivity, run out of battery power, or have internal relays that fail. Another frequent issue is incorrect configuration for heat pump systems: thermostats must be set to energize the reversing valve in heating or cooling mode depending on the manufacturer. If the thermostat's "O" or "B" terminal is wired or configured incorrectly, the system will cool instead of heat.

Check for a blank screen on the thermostat. If the display is off, replace batteries or check the low-voltage circuit breaker (often located on the furnace or air handler). Ensure the system switch is set to "Heat" and that the temperature setting is raised at least five degrees above the room temperature to trigger a call for heat. If the thermostat is a smart model, verify it is connected to Wi-Fi and update its firmware if necessary.

2. Tripped Breakers, Blown Fuses, and Safety Switches

Heat pumps require substantial electrical power. The outdoor condensing unit typically has its own double-pole circuit breaker in the main panel, plus a fused or non-fused disconnect switch mounted near the unit. A tripped breaker or blown fuse will completely disable the compressor and outdoor fan. Indoor air handlers also have separate breakers for the fan motor and auxiliary electric heat strips. A tripped breaker can be caused by a shorted motor, a failing capacitor, or simply a power surge.

Safety switches are another overlooked power interruption source. Condensate overflow switches installed in the drain pan or on the primary drain line will shut off the system if the drain becomes clogged. This prevents water damage but can be confusing to a homeowner because the thermostat appears normal. Check for standing water in the drain pan and clear any blockages. To restore power, reset the tripped breaker or replace blown fuses. If the breaker immediately trips again, do not reset it repeatedly. Call a licensed electrician or HVAC technician to diagnose the short or ground fault.

3. Airflow Restrictions from Dirty Air Filters

Restricted airflow is one of the most frequent causes of poor heating performance and system shutdowns. A dirty air filter blocks the flow of air across the indoor coil. In heating mode, the indoor coil acts as the condenser. If airflow is reduced, the coil cannot reject heat effectively. This leads to excessively high refrigerant pressures and temperatures. Many heat pumps and air handlers have a high-pressure safety switch that opens and shuts the system down to prevent compressor damage.

Low airflow can also cause the indoor coil to freeze or overheat the heat strips, triggering thermal overloads. Check your air filter at least once a month during peak heating season. Replace disposable filters or clean reusable ones. Use the correct filter size and avoid high-MERV filters (above MERV 11) unless your system is specifically designed to handle the increased static pressure. A dirty filter is an easy fix that restores proper airflow and system function immediately.

4. Frozen Outdoor Coil and Defrost Cycle Failure

All air-source heat pumps accumulate frost on the outdoor coil during cold, humid weather. The system is designed to periodically reverse the cycle and run in cooling mode for a few minutes. This sends hot refrigerant gas from the compressor to the outdoor coil, melting the frost. This is the defrost cycle, controlled by a defrost board, defrost thermostat, and outdoor temperature sensor.

If the defrost thermostat fails to sense the ice, the board fails, or the reversing valve does not shift, the outdoor unit becomes a block of ice. A frozen outdoor coil acts as an insulator, preventing the refrigerant from absorbing heat. The system will struggle, short cycle, or trip safety limits and completely stop heating. In some cases, the compressor will run but the indoor blower will blow cold air or no air at all. To resolve this, inspect the outdoor coil for ice buildup. Clear snow and debris from around the unit. If ice is present, switch the thermostat to Emergency Heat mode. This disables the outdoor unit and runs only the backup heat strips or furnace, allowing the outdoor coil to thaw naturally. Running a heat pump on a frozen coil can damage the compressor. Once thawed, test the defrost cycle by temporarily closing the defrost thermostat. If the unit does not enter defrost, the control board or sensor likely needs replacement by a technician.

5. Refrigerant Leaks and Improper Charge

Heat pumps rely on a precise amount of refrigerant to transfer heat effectively. A sealed system should never lose refrigerant, so a low charge always indicates a leak. Common leak points include the evaporator coil, condenser coil, line set fittings, and service valves. Low refrigerant reduces the system's ability to absorb heat from the outside air. Symptoms include low suction pressure, high superheat, low discharge temperature, and ice formation on the outdoor unit or the refrigerant lines entering the home.

A system with a low refrigerant charge may run continuously without raising the indoor temperature, or it may short cycle on the low-pressure safety switch. Some modern heat pumps have electronic expansion valves (EEVs) and sensors that can adapt to charge levels to a degree, but eventually performance degrades sharply. Detecting and repairing refrigerant leaks requires specialized tools like electronic leak detectors, UV dye, and nitrogen pressure testing. Handling refrigerant also requires EPA Section 608 certification in the United States. Contact a qualified HVAC technician to locate the leak, repair it, evacuate the system, and weigh in the factory-specified charge. Do not attempt to recharge the system yourself without proper training and licensing.

6. Failing Capacitors, Fan Motors, and Compressors

The outdoor unit of a heat pump contains two major motors: the condenser fan motor and the compressor motor. Both typically require a start capacitor and a run capacitor to operate. Capacitors store electrical energy and provide the high torque needed to start the motors. When a capacitor fails, you may hear a clicking sound from the contactor or a humming noise from the motor, but the motor will not spin. If the fan is not running, the outdoor coil cannot exchange heat, and the system will soon trip on high pressure and shut down.

If the compressor fails to start, the system may attempt to run but will not produce heat. A failed compressor often results from a stuck valve, broken internal mechanical parts, or electrical winding failure. Signs include a humming compressor with the outdoor fan running, a hot compressor shell, and a tripping overload protector. Replacing a compressor is a major repair, often approaching the cost of a new outdoor unit. If the fan motor is seized or the capacitor is weak, replacement is more affordable. A technician can test capacitance with a multimeter and swap out bad capacitors or fan motors quickly. Regular maintenance catches failing capacitors early before they cause a total system shutdown.

7. Stuck Reversing Valve

The reversing valve is the component that switches the heat pump between heating and cooling modes. It uses a solenoid coil to shift an internal slide mechanism. If the valve gets stuck in the cooling position, the system will blow cold air even when the thermostat calls for heat. Debris in the refrigerant, a weak solenoid, or a pressure differential across the valve can prevent it from shifting.

A stuck reversing valve is difficult to diagnose without gauges and electrical testing. One indicator is that the outdoor unit sounds different or the discharge line temperature is inconsistent. In some cases, a technician can "tap" the valve with a wrench to free it, or energize the solenoid while the compressor is running to force it to shift. If the valve remains stuck, replacement is necessary. This requires recovering the refrigerant, brazing in a new valve, and pulling a deep vacuum. Do not attempt this without an EPA certification and proper brazing skills.

8. Malfunctioning Auxiliary or Emergency Heat

All heat pumps have a balance point: the outdoor temperature where the heat pump alone can no longer keep up with the heat loss of the home. Below this temperature, the system energizes auxiliary (aux) heat strips or a backup gas furnace. Some thermostats also have an Emergency Heat (Em Heat) setting that manually locks out the heat pump and runs only the backup heat. If the auxiliary heat is not working, the home will never reach the desired temperature in very cold weather. The heat pump will run continuously without satisfying the thermostat.

Reasons for no auxiliary heat include a tripped breaker for the electric heat strips, a blown thermal fuse, a failed sequencer relay, or a broken control board. For dual-fuel systems (heat pump with gas backup), the valve or ignition module may fail. To test auxiliary heat, increase the thermostat temperature set point by several degrees above the current room temperature. If the system runs but delivers air that is only slightly warm, the backup heat is not energizing. Check the indoor unit's breaker panel for tripped breakers or blown fuses. The indoor air handler typically has multiple breakers; one for the fan and up to three for the heat strip stages.

Step-by-Step Troubleshooting Plan

Before calling a technician, perform these checks to resolve or identify the problem quickly.

Step 1: Confirm Thermostat Settings

Ensure the system mode is set to Heat. Raise the temperature setting to at least 5 degrees above the room temperature. If the thermostat is battery-powered, replace the batteries. For smart thermostats, verify that the heating schedule is active and the device is connected to the internet. Look for a "System Is Off" or "Waiting for Equipment" message that indicates a delay or lockout.

Step 2: Inspect the Air Filter

Locate the air filter in the return air duct or the indoor air handler compartment. If it appears dirty or clogged, replace it with a new filter of the correct dimensions. Temporarily removing the filter to see if the system starts is a quick test, but running without a filter can allow debris to damage the blower motor. Install the new filter and wait five minutes for the system to reset.

Step 3: Check Electrical Panels and Disconnects

Go to the main electrical panel and look for tripped breakers (a breaker in the middle or off position rather than fully to the "On" side). Reset it by firmly switching it to "Off" and then back to "On." Locate the disconnect switch next to the outdoor unit. It should be in the "On" or "Closed" position. Some disconnects have fuses inside; check these with a multimeter if you are comfortable doing so.

Step 4: Examine the Outdoor Unit for Ice and Debris

Visually inspect the outdoor unit. If it is encased in ice, turn the thermostat to Emergency Heat to allow the coil to thaw. Remove any snow, leaves, or debris that are blocking airflow to the sides or top of the unit. Trim back any vegetation that is within two feet of the cabinet. A heat pump needs free airflow to operate efficiently.

Step 5: Listen for Operating Sounds

With the system calling for heat, go outside and listen to the unit. You should hear the compressor running (a low hum) and the fan spinning. If you hear a loud buzzing, rattling, or clicking without the fan turning, the capacitor or fan motor may be failing. If the unit is completely silent, check the disconnect and breaker. If the indoor unit runs but the outdoor unit does not, the thermostat wiring, contactor coil, or low-voltage transformer could be faulty.

Advanced Diagnostics Requiring a Professional

Several issues are beyond the scope of basic DIY troubleshooting. If you have completed the steps above and the system still does not heat, the problem is likely one of the following:

  • Low refrigerant charge or leak: Requires leak detection, repair, evacuation, and precise charging using gauges and a scale.
  • Failed defrost control board: Testing voltage inputs and outputs to the board requires a multimeter and a wiring diagram.
  • Failed compressor: Checking compressor windings and megohm testing requires specialized electrical knowledge.
  • Reversing valve malfunction: Diagnosing a stuck valve involves pressure measurements and knowing how to operate the solenoid manually.
  • Damaged ductwork or closed dampers: Supply and return duct leaks dramatically reduce system efficiency. Zone damper motors can fail closed.

If you are not comfortable working with high-voltage electricity or pressurized refrigerant systems, call a licensed HVAC professional. Working on heat pumps can involve risks of electrical shock, refrigerant burns, and damage to expensive components if improperly handled.

Preventative Maintenance to Avoid Heat Loss

A well-maintained heat pump is less likely to fail when you need it most. Schedule professional maintenance twice a year: once in the spring for cooling and once in the fall for heating. During a maintenance visit, a technician will typically:

  • Check and clean the evaporator and condenser coils.
  • Measure refrigerant pressures and superheat or subcooling.
  • Inspect electrical connections and tighten terminals.
  • Test all capacitors and replace weak ones.
  • Lubricate fan motors if applicable.
  • Check the defrost cycle operation.
  • Clean the condensate drain and check the safety switch.
  • Inspect and clean the blower assembly and indoor coil.

For homeowners, the most important maintenance tasks are changing the air filter every one to three months, keeping the outdoor unit clean and free of debris, and ensuring the condensate drain is clear. Installing a surge protector for the heat pump can prevent electrical damage from power surges, which are a common cause of control board and compressor failure. Following these steps reduces the likelihood of emergency service calls and keeps the system running at peak efficiency throughout the winter.

Conclusion

Heat pumps are complex systems that rely on electrical, mechanical, and refrigeration cycles to deliver efficient heating. When the system stops producing heat, the cause is often one of eight common problems: thermostat settings, power failures, dirty filters, frozen coils, refrigerant leaks, failed capacitors, a stuck reversing valve, or inoperative backup heat. By following a structured troubleshooting approach and performing regular maintenance, you can resolve many issues quickly or accurately describe the problem to a technician. For deeper problems involving refrigerant or major electrical components, professional service is necessary to restore safe and reliable heat to your home.