Why Heat Pumps Stop Heating in Cold Weather and What to Do

Heat pumps can struggle in cold weather, leaving homeowners with reduced comfort and higher energy bills. This article explains the common causes, diagnostics, and practical solutions to restore reliable heating. Understanding how cold affects heat pump operation helps identify whether a simple fix, a component replacement, or a system upgrade is needed.

Symptom	Common Causes	Quick Action
Little Or No Heat	Refrigerant Leak, Frosted Coil, Compressor Fault	Check Airflow, Call Technician
Heat Pump Frequently Cycling	Defrost Cycle Issues, Thermostat Fault, Oversized Unit	Reset Thermostat, Professional Check
Outdoor Unit Icing	Defrost Malfunction, Low Refrigerant	Manual Defrost, Schedule Service

How Heat Pumps Work In Cold Weather

Heat pumps transfer heat from outdoors to indoors using a refrigerant cycle and a compressor. In heating mode, the outdoor coil acts as an evaporator, absorbing ambient heat, while the indoor coil releases heat into the home. Most modern heat pumps include a defrost cycle to remove frost buildup on the outdoor coil during cold, humid conditions.

Performance drops as outdoor temperature falls because less thermal energy is available to extract, and airflow or refrigerant problems compound the issue.

Common Reasons A Heat Pump Is Not Heating

1. Low Refrigerant Charge

Low refrigerant reduces the system’s ability to absorb and transfer heat. Unlike a gas, refrigerant circulates in a closed loop, so a loss typically indicates a leak that must be located and repaired. Symptoms include reduced heating capacity, long run times, and ice forming on the outdoor coil.

2. Defrost System Failure

The defrost cycle periodically melts frost on the outdoor coil. If the defrost control, sensor, or valve fails, the coil can remain iced, blocking airflow and stopping heat transfer. Frequent or prolonged defrost cycles are common signs of a malfunctioning defrost system.

3. Dirty Or Blocked Airflow

Restricted airflow from dirty filters, obstructed supply/return vents, or a blocked outdoor unit significantly lowers performance. Restricted airflow reduces heat exchange at the indoor coil and may cause the system to run continuously without producing adequate heat.

4. Faulty Compressor Or Electrical Issues

The compressor is the heart of the refrigerant cycle. Electrical faults, worn components, or internal compressor failure can prevent proper pressurization, eliminating heating. Tripped breakers, burnt wiring, or failed capacitors also cause intermittent or no operation.

5. Thermostat Or Controls Misconfigured

Incorrect thermostat settings, wrong mode selection, or communication errors between thermostat and heat pump can make the system appear to be not heating. Software or sensor faults in modern thermostats may also prevent commands from reaching the heat pump.

6. Inadequate Sizing Or Installation Issues

An undersized heat pump may struggle on cold days, while an oversized unit may short-cycle and feel unreliable. Poor installation practices—such as improper refrigerant charge, bad refrigerant lines, or incorrect airflow setup—can produce poor performance when temperatures drop.

How To Diagnose The Problem

Diagnosing a heat pump that won’t heat requires a systematic approach combining visual checks, basic tests, and professional diagnostics. Follow these steps to narrow causes before calling a technician.

Visual And Quick Checks

• Verify thermostat is set to heat mode and set temperature is above room temperature.
• Check air filters and replace if dirty; inspect supply and return vents for obstructions.
• Look at the outdoor unit: is there heavy ice buildup or visible damage?
• Check circuit breakers and the outdoor disconnect switch to ensure power is available.

Temperature And Airflow Tests

Measure temperature difference between supply and return vents. A healthy heat pump in heating mode typically produces a supply that is 10–20°F warmer than return air under normal conditions. Significantly lower delta indicates a problem with heat transfer.

Listen And Observe Operational Behavior

Note unusual sounds such as buzzing, clanking, or high-pitched whining from the outdoor unit. Observe whether the system cycles frequently, runs continuously with poor output, or never starts. These clues help identify electrical, compressor, or control issues.

When To Call A Professional

If basic checks don’t restore heating or if there’s ice on the outdoor coil, refrigerant concerns, or electrical problems, schedule a licensed HVAC technician. Refrigerant work and compressor/service tests require tools and certifications for safe, legal repair.

Immediate Steps Homeowners Can Take

Improve Airflow

Replace or clean filters, remove furniture or debris blocking vents, and ensure the outdoor unit is clear of leaves, snow, and ice. Good airflow often restores most of the lost capacity.

Switch To Auxiliary Heat Temporarily

Use the heat pump’s auxiliary or emergency heat (usually electric strip heat) only when necessary, because it is more expensive to run. This provides immediate warmth while diagnosing the underlying issue.

Defrost The Outdoor Unit Manually

If the outdoor coil is heavily iced, temporarily turn the system off and gently remove ice once safe. Avoid chipping at the coil, and follow the manufacturer’s guidance. Restarting may allow the built-in defrost cycle to resume normal operation.

Reset The System

Turning the heat pump off at the thermostat and at the outdoor disconnect for a few minutes can sometimes clear control faults. This is a low-risk step but not a fix for persistent mechanical or refrigerant problems.

Repairs And Maintenance Professionals Perform

Leak Detection And Refrigerant Service

A technician will use gauges and leak-detection tools to find and repair refrigerant leaks, then recharge the system to factory-designed pressures. Proper refrigerant charge is essential for heat transfer and compressor protection.

Defrost System Repair

Repairs may include replacing defrost control boards, sensors, or reversing valves. Technicians verify defrost timing and sensor calibration to prevent excessive icing or unnecessary defrost cycles.

Electrical Diagnosis And Compressor Work

Electrical troubleshooting identifies failed contactors, capacitors, relays, and wiring faults. Compressor testing and potential replacement require experience; technicians also check oil levels and system pressures to assess compressor health.

System Tune-Up And Preventive Maintenance

Annual maintenance should include cleaning coils, checking refrigerant pressures, inspecting electrical components, lubricating moving parts, and verifying airflow. Routine service reduces cold-weather failures and prolongs equipment life.

Upgrades And Alternatives For Colder Climates

Cold-Climate Heat Pumps

Modern cold-climate heat pumps (including variable-speed and inverter-driven models) maintain better capacity at subfreezing temperatures than older units. Upgrading to a cold-climate model can eliminate many cold-weather performance problems.

Dual-Fuel Systems

Dual-fuel setups combine a heat pump with a gas furnace. The furnace takes over at low temperatures where heat pump efficiency drops, optimizing comfort and cost. A properly configured control automatically switches between systems.

Supplemental Heating Options

Sized correctly, supplemental electric resistance heaters, mini-split systems, or ductless heat pumps can provide targeted heat for hard-to-warm areas. These reduce strain on the main system during extreme cold.

Energy And Cost Considerations

Running a heat pump that’s underperforming increases energy usage and utility bills. Identifying and fixing the underlying cause often yields significant savings. Consider these cost factors when deciding on repair versus replacement:

• Age Of Unit: Older units typically have lower efficiency and higher likelihood of component failure.
• Repair Cost Vs. Replacement: Major repairs such as compressor replacement can approach the cost of a new high-efficiency unit.
• Energy Savings: New cold-climate heat pumps can reduce heating costs versus older models or electric resistance heat.

Signs It’s Time To Replace The Heat Pump

Replacement should be considered when the unit is beyond expected life (typically 10–15 years) and experiences frequent failures, reduced efficiency, or when repair costs exceed 50% of replacement cost. Upgrading can improve comfort, reliability, and long-term operating costs.

Preventive Tips To Avoid Cold-Weather Failures

• Schedule professional maintenance annually before the cold season.
• Keep outdoor units free of snow, ice, and debris year-round.
• Replace filters every 1–3 months depending on usage and indoor air quality.
• Ensure proper landscaping to keep a clear airflow radius around the outdoor unit.
• Install a smart thermostat or controls that optimize operation and detect abnormal behavior early.

Useful Resources And Standards

Homeowners should consult authoritative sources for guidance: the U.S. Department of Energy (DOE) for efficiency and heat pump basics, Environmental Protection Agency (EPA) rules on refrigerants, and local utility programs that offer rebates for cold-climate heat pump upgrades. Energy.gov and EPA.gov provide helpful references.

Final Practical Checklist Before Calling A Technician

1. Confirm thermostat settings and power to the system.
2. Replace air filters and clear vents.
3. Inspect and clear the outdoor unit of snow and debris.
4. Note any unusual sounds or patterns and measure supply/return temperature difference.
5. Document how long the system has been running and any prior repairs.

Providing this information to a technician speeds diagnosis and helps determine whether a service visit will be a simple repair or a replacement conversation.

Key Takeaway: Many cold-weather heat pump problems are caused by airflow restrictions, defrost failures, low refrigerant, or aging equipment. Timely maintenance, professional diagnosis, and considering modern cold-climate options restore reliable heating and improve efficiency.