Why Is My Heat Pump Not Cooling: Common Causes and Fixes

A heat pump that fails to cool can be frustrating and costly. This article explains the most likely reasons a heat pump is not cooling, how to diagnose problems, and practical actions homeowners can take before calling a technician. It covers airflow, refrigerant issues, thermostat settings, electrical faults, and maintenance best practices to restore efficient cooling.

Symptom	Likely Cause	First Action
Warm Air From Vents	Reversing Valve/Mode Setting	Check thermostat mode and fan
Weak Airflow	Clogged Filter/Duct Problem	Replace filter and inspect vents
Hissing Or Frost On Unit	Low Refrigerant Leak	Turn off system and call certified tech
Unit Not Running	Tripped Breaker/Capacitor Failure	Check breaker and outdoor disconnect
Short Cycling	Thermostat Or Compressor Issue	Adjust thermostat and observe cycles

How A Heat Pump Cools And Why It May Stop

A heat pump cools by extracting heat from indoor air and moving it outside using a refrigerant cycle. In cooling mode the indoor coil acts as the evaporator while the outdoor coil rejects heat. For efficient cooling, the system needs proper refrigerant charge, unobstructed airflow, correct reversing valve operation, and working electrical components.

When any part of that chain fails, the heat pump will struggle or stop cooling. Common problems include blocked airflow, low refrigerant, electrical failures, and thermostat misconfigurations. Understanding these systems helps diagnose why a heat pump is not cooling and prioritizes safe interventions.

Check Thermostat And Settings First

The simplest causes are thermostat-related. Ensure the thermostat is set to cool (or heat pump auto with correct mode), and the setpoint is lower than indoor temperature.

• Verify thermostat fan is set to auto rather than on, which can affect cycling.
• Replace thermostat batteries if the display or response seems sluggish.
• Check for a heat pump-specific setting like changeover valve or aux/emergency that might be misconfigured.
• If a programmable or smart thermostat was recently installed, confirm wiring (O/B, Y, G, C) matches heat pump requirements.

Inspect Airflow: Filters, Vents, And Ductwork

Poor airflow frequently causes ineffective cooling. Restricted airflow reduces heat transfer over the evaporator coil and can cause the coil to freeze, further reducing capacity.

Key checks for airflow problems include:

• Replace or clean the air filter if dirty; a clogged filter is one of the most common issues.
• Ensure supply and return vents are open and unobstructed by furniture or drapes.
• Inspect the evaporator coil and indoor unit for visible dust buildup or ice—do not force ice removal; instead, switch to fan mode to thaw before further operation.
• Check ductwork for obvious disconnections, crushed runs, or major leaks; these can divert cooled air away from living spaces.

Refrigerant Problems And Leaks

Low refrigerant charge prevents the heat pump from absorbing enough heat indoors and is a primary reason a heat pump is not cooling. Refrigerant does not burn off; low levels typically indicate a leak.

• Signs of refrigerant issues: hissing sounds, frost on the indoor coil or refrigerant lines, and a longer run time without temperature drop.
• Homeowners should not attempt to top off refrigerant. Refrigerant handling requires EPA certification and leak repair before recharge.
• A certified HVAC technician will perform leak detection, repair, and weigh-in the correct refrigerant charge to OEM specifications.

Outdoor Unit Problems: Fan, Condenser, And Reversing Valve

The outdoor unit must expel heat. Obstructions or mechanical failures reduce cooling performance.

• Clear leaves, debris, and vegetation from around the outdoor condenser. Maintain at least 2–3 feet of clearance.
• Ensure the outdoor fan runs when the system is on. If the fan doesn’t start, the motor or capacitor may have failed.
• The reversing valve controls heat pump direction. If the valve is stuck or the control signal is wrong, the system might be in heating mode or not switch properly.
• Unusual noises from the outdoor unit—clanking, humming, or grinding—indicate mechanical issues requiring professional inspection.

Electrical And Component Failures

Electrical faults can prevent the compressor or fans from operating. Common issues include tripped breakers, blown fuses, faulty contactors, or failed capacitors.

• Check the service disconnect at the outdoor unit and the circuit breaker panel for tripped breakers. Reset only if allowed and safe.
• Burnt smells or visible burn marks on electrical components indicate a serious problem—do not run the system and call a technician.
• Capacitor failures often result in the compressor humming but not starting, or fans failing to spin. These require trained electricians or HVAC techs to replace safely.

Defrost Cycle And Frost Build-Up Issues

In cold weather, heat pumps enter a defrost cycle to melt frost on the outdoor coil. Malfunctioning defrost controls can leave the outdoor coil iced and block cooling.

• Visible heavy frost or ice on the outdoor coil during mild conditions is abnormal and reduces cooling capacity.
• Short or missing defrost cycles are often caused by faulty sensors, control boards, or the defrost thermostat.
• While the unit is off, allow ice to melt naturally. Persistent icing after thaw indicates a professional inspection is needed.

Compressor And Refrigerant Circuit Failures

The compressor is the heart of the refrigeration cycle. If it fails or the refrigerant circuit is blocked, the heat pump will not cool.

• Signs of compressor failure include the outdoor unit running but no cooling effect, loud clunks on startup, or frequent tripping of overload protectors.
• A locked rotor or internal mechanical failure usually requires compressor replacement, which can be expensive and might prompt consideration of system replacement if the unit is older.
• Technicians can test for proper pressures, motor operation, and internal electrical continuity to diagnose compressor health.

Short Cycling And Thermostat Location Problems

Short cycling—where the system turns on and off frequently—reduces cooling and increases wear. It can be caused by oversized equipment, refrigerant issues, or thermostat placement.

• Thermostats near heat sources, direct sunlight, or in drafty hallways may get incorrect temperature readings and cause cycling. Repositioning or using remote sensors helps.
• Oversized units reach setpoint quickly but dehumidify poorly and cycle more, leading occupants to perceive inadequate cooling.
• Monitor run times and cycle frequency to share with an HVAC technician for proper diagnosis.

Routine Maintenance To Prevent Cooling Failures

Regular maintenance reduces the risk a heat pump is not cooling properly. Preventive tasks preserve efficiency and extend equipment life.

1. Replace air filters every 1–3 months depending on use and filter type.
2. Schedule professional tune-ups annually, ideally in spring before cooling season, to check refrigerant, electrical components, and airflow.
3. Keep outdoor unit clear of debris and pressure wash the fins gently if dirty (power washing can damage fins).
4. Clean indoor coils and condensate drain lines to prevent clogs and microbial growth.

Troubleshooting Checklist Homeowners Can Use

This checklist helps determine whether a DIY fix is possible or a professional is required. Follow safety precautions when accessing electrical panels or outdoor units.

Step	Action	Possible Outcome
Thermostat	Confirm mode, setpoint, and batteries	Often resolves simple misconfigurations
Filter	Inspect and replace if dirty	Improves airflow and temperature
Outdoor Unit	Clear debris and check fan operation	May restore full cooling
Breakers	Check and reset tripped breakers	Restores power if safe to reset
Ice/Frost	Switch to fan mode to defrost	Thawing may temporarily restore cooling
Refrigerant Signs	Look for hissing or frost; call tech	Requires certified repair

When To Call A Professional

Homeowners should contact a licensed HVAC technician when the issue involves refrigerant, complex electrical components, compressor failure, or persistent performance problems despite basic troubleshooting.

Situations calling for professional service include: visible refrigerant leaks, repeated tripped breakers, burnt electrical components, non-operational compressor, and replacement of major components. A technician can perform diagnostics, EPA-compliant refrigerant service, and safe repairs.

Costs And Replacement Considerations

Repair costs vary widely. Simple fixes like filters or thermostat replacement are inexpensive. Refrigerant leak repair, capacitor or fan motor replacement, and control board work are moderate. Compressor replacement or a full system swap is expensive and may justify replacing older units with a new, higher-efficiency heat pump.

When deciding on replacement, consider system age, frequency of repairs, SEER/HSPF ratings, refrigerant type (older R-22 systems are phased out), and long-term energy savings. Many utilities offer rebates for high-efficiency heat pump upgrades.

Key Takeaways And Best Practices

Regular maintenance and prompt attention to airflow and thermostat settings resolve many instances where a heat pump is not cooling. Homeowners should perform basic checks—filters, vents, thermostat, outdoor clearance—before calling a technician.

Professional intervention is necessary for refrigerant leaks, electrical failures, compressor issues, and major mechanical repairs. Keep records of service, model, and maintenance to assist technicians and inform replacement decisions.

Addressing cooling issues quickly preserves comfort, prevents secondary damage like frozen coils or electrical failures, and helps maintain efficiency and indoor air quality.