Why Your Heat Pump Won’t Turn Off and How to Fix It

The article explains common reasons a heat pump won’t turn off, how to troubleshoot safely, and practical fixes a homeowner or technician can use to restore normal operation. It covers thermostat issues, control board faults, refrigerant problems, and airflow or sensor failures, with clear troubleshooting steps and when to call a professional.

Symptom	Likely Cause	Quick Fix
Unit Runs Continuously	Short cycling disabled, stuck relay, thermostat call	Reset thermostat, check wiring, call tech
Indoor Fan Keeps Running	Fan relay stuck, thermostat fan setting	Set fan to AUTO, power cycle unit
Outdoor Unit Runs Without Stopping	Low refrigerant, defrost cycle stuck	Inspect refrigerant and defrost board

How Heat Pump Systems Normally Cycle

Heat pumps use a thermostat signal and internal controllers to cycle the compressor and fans to maintain set temperatures. In heating mode, the outdoor unit runs to transfer heat indoors. In cooling mode, it removes heat from indoors. Normal cycling includes short stop-start periods and longer off-times once the thermostat setpoint is reached.

A heat pump has safety timers, defrost cycles, and minimum run times to protect the compressor. Misbehavior such as continuous operation usually indicates a control, sensing, or mechanical problem rather than normal operation.

Common Causes For A Heat Pump That Won’t Turn Off

Thermostat Malfunctions Or Settings

Incorrect thermostat settings, stuck relays, or failed sensors can keep the heat pump running. A thermostat left in fan-on mode will keep the indoor blower running even when the compressor is off.

Digital thermostats may show correct temperatures but still have internal faults in the output relay or software causing a continuous call for heat or cool. Wi‑Fi thermostats can also freeze on commands during firmware glitches.

Control Board Or Relay Failures

The outdoor unit and indoor air handler contain control boards and relays that switch the compressor and fans. A stuck relay, welded contacts, or failed control board can energize outputs continuously.

Symptoms include the compressor running while the thermostat is calling for off, or the indoor fan running when the thermostat is set to AUTO. Electrical inspection typically identifies stuck relays or burned components.

Refrigerant Issues And System Pressure

Low refrigerant or incorrect charge can cause the system to run longer trying to reach the setpoint. In some cases, control logic will command continuous operation when pressures are out of expected ranges.

Overcharged systems or blocked metering devices can also prevent adequate heat transfer, forcing continuous operation and high energy use until the underlying issue is corrected.

Airflow Restrictions And Dirty Coils

Restricted airflow due to dirty filters, clogged coils, or blocked return vents reduces heat exchange efficiency. The thermostat will keep calling for temperature correction, so the heat pump runs longer or continuously.

Indoor and outdoor coil fouling increases run times and stresses components. Simple maintenance like filter replacement and coil cleaning often resolves continuous run issues caused by airflow restriction.

Defrost Control Or Sensor Problems

In heating mode, the outdoor unit periodically goes into defrost mode. If the defrost control or sensors malfunction, the outdoor unit may remain in an active state or try to defrost continuously, causing erratic run behavior.

Signs include the outdoor fan not running while the compressor is on or frequent defrost cycles. Technician diagnosis can test sensors and replace defrost boards when needed.

Stuck Reversing Valve Or Mechanical Faults

A stuck reversing valve can cause improper operation, but it usually leads to the wrong mode rather than continuous running. Mechanical compressor issues, like internal damage or overheating, can trigger control circuits to run continuously in an attempt to stabilize pressures.

Mechanical faults often require professional assessment and repair to prevent further damage and ensure safe operation.

Step-By-Step Troubleshooting Checklist

Before starting, always turn off power at the thermostat and disconnect switches for safety. If uncomfortable with electrical systems, contact a licensed HVAC technician.

1. Check Thermostat Settings: Confirm mode (HEAT/COOL/AUTO) and fan setting (AUTO). Verify setpoint and schedule programming.
2. Power Cycle Thermostat: Remove thermostat batteries or turn off the breaker for 30 seconds to reset internal electronics.
3. Replace Air Filters: Dirty filters restrict airflow. Replace with recommended MERV level filters and retest operation.
4. Inspect Outdoor Unit: Look for ice, debris, or bent fins. Note whether the outdoor fan runs with the compressor.
5. Listen For Relay Clicks: On startup and shutdown, listen at the air handler and outdoor unit for relay activation. No click may indicate a stuck relay or lost control signal.
6. Check For Error Codes: Many systems flash error codes on control boards. Document flashes and consult manufacturer documentation.
7. Measure Temperatures: Use a thermometer to compare room temperature to thermostat reading, which helps identify sensor drift.
8. Call A Pro If Needed: For refrigerant, control board, or compressor faults, call an EPA-certified HVAC technician.

When The Indoor Fan Keeps Running But Compressor Stops

If the indoor fan runs continuously while the outdoor compressor cycles correctly, the thermostat fan setting or fan relay may be the issue. Check thermostat fan setting first; if set to AUTO and fan still runs, the fan relay in the air handler may be welded closed.

Repair typically involves replacing the fan relay or control board. A temporary workaround is switching the breaker off to stop the fan, but professional repair is recommended to avoid system damage.

Electrical And Control Diagnostics

Technicians will use a multimeter to verify 24VAC control signals from the thermostat, check continuity across relays, and verify voltage to the compressor contactor. They also inspect wiring integrity and corrosion at terminals.

Common findings include shorted thermostat wires, corroded connectors, or failed contactor coils. Replacing failed contactors, relays, or control boards usually corrects continuous operation caused by electrical faults.

Refrigerant And Pressure Testing

Refrigerant diagnosis involves measuring suction and discharge pressures and comparing them to expected values for ambient conditions. Low suction pressure may indicate undercharge or restriction, while high discharge pressure suggests overcharge or airflow problems.

Only certified technicians can legally handle refrigerant. If pressure readings are abnormal, a technician will locate leaks, repair, and recharge the system to manufacturer specifications.

Preventive Maintenance To Avoid Continuous Operation

• Replace Filters Regularly: Monthly or as manufacturer recommends to maintain airflow.
• Schedule Biannual Tune-Ups: Professional inspections in spring and fall catch control issues early.
• Keep Outdoor Unit Clear: Maintain 2–3 feet of clearance, remove debris, and trim vegetation.
• Calibrate Thermostats: Verify thermostat readings periodically and replace older thermostats showing drift.
• Clean Coils Annually: Both indoor and outdoor coils should be cleaned to preserve efficiency.

Energy Impact And Safety Concerns

A heat pump that won’t turn off increases energy use and utility bills and raises the risk of component failure due to extended wear. Continuous operation can overheat motors and reduce lifespan of the compressor and fans.

If unusual noises, burning smells, or tripped breakers accompany continuous operation, shut down the system and contact a technician immediately to prevent fire or electrical hazards.

When To Call A Professional

Contact a licensed HVAC technician if troubleshooting steps do not fix the problem or if the issue involves refrigerant, compressor, or control board replacement. Immediate professional attention is warranted for electrical faults, refrigerant leaks, or if the system shows error codes that cannot be resolved with simple resets.

Ask for technicians who are NATE-certified and EPA-certified for refrigerant handling. Request a written estimate and parts warranty before approving extensive repairs.

Cost Expectations For Repairs

Minor thermostat or relay repairs may cost between $100 and $300, depending on parts and service fees. Control board replacements range from $300 to $900. Compressor or refrigerant system repairs can exceed $1,000, especially if major components are replaced.

Preventive maintenance contracts often reduce long-term repair costs by catching issues early and keeping systems operating efficiently.

Key Takeaways And Quick Fix Summary

Key Points: A heat pump that won’t turn off most commonly results from thermostat faults, control relays, airflow restrictions, or refrigerant and defrost control issues. Simple homeowner checks can resolve thermostat, filter, and airflow problems. Electrical or refrigerant issues should be handled by certified technicians.

Problem	Homeowner Action	When To Call A Pro
Thermostat Error	Reset, replace batteries, check wiring	If replacing thermostat or wiring issues
Dirty Filter/Coil	Replace filter, clean visible debris	If coils need professional cleaning
Stuck Relay/Contactor	Power cycle to test	Contact technician for replacement
Refrigerant/Pressure	None (safety/legal)	Always call EPA-certified tech

For persistent issues, document system behavior (times, temperatures, noises) to help the technician diagnose faster. Regular maintenance remains the most effective way to prevent continuous-operation problems and improve heat pump longevity.