Why Heat Pump Auxiliary Heat Is Always on and How to Fix It

Heat pump auxiliary heat always on can significantly raise energy bills and indicate system issues. This article explains how heat pump auxiliary and emergency heat work, common causes for the auxiliary heat staying on, step-by-step diagnostics, practical fixes, maintenance tips, and when to call a professional. Understanding and resolving constant auxiliary heat saves energy and prolongs system life.

Topic	Key Takeaway
Auxiliary vs Emergency Heat	Auxiliary (backup) heat supplements when the heat pump can’t meet demand; emergency heat bypasses the heat pump entirely.
Common Causes	Thermostat settings, defrost cycle issues, refrigerant shortage, faulty sensors, or compressor failure.
Diagnostics	Check thermostat mode, outdoor unit operation, air filters, refrigerant leaks, and error codes.
Fixes	Correct thermostat settings, replace filters, repair refrigerant leaks, or replace components; call HVAC pro for complex issues.

How Heat Pumps And Auxiliary Heat Work

Heat pumps transfer heat between indoor and outdoor units using refrigerant and a reversing valve to provide heating or cooling. When outdoor temperatures are moderate, the heat pump provides most heating efficiently.

Auxiliary heat (often electric resistance strips or gas furnace integration) engages when the heat pump cannot maintain the thermostat setpoint on its own or during defrost cycles. Emergency heat disables the heat pump and runs backup heat only, typically used when the heat pump is malfunctioning.

Why Auxiliary Heat Turns On Frequently

Auxiliary heat always on commonly results from control or mechanical problems rather than normal operation. Identifying the root cause avoids unnecessary energy use.

Thermostat Settings And Operation

A thermostat set to “Emergency Heat” or configured with high heat anticipator settings can force continuous backup heat. Some thermostats have a dual-fuel or backup setting that triggers auxiliary heat based on outdoor temperature sensors.

Insufficient Heat Pump Output

If the heat pump cannot supply enough heat because of low refrigerant, a failing compressor, or restricted airflow, auxiliary heat will run to make up the difference. This often becomes apparent on very cold days but can occur at moderate temperatures with system faults.

Defrost Cycle And Sensor Problems

During defrost cycles the outdoor coil heats to remove ice, and auxiliary heat may engage temporarily. However, a malfunctioning defrost control or sensor can cause prolonged auxiliary operation.

Electrical Or Control Failures

Faulty relays, contactors, control boards, or thermostat wiring shorts can send continuous power to auxiliary heat elements. These electrical issues should be diagnosed carefully to avoid safety hazards.

Diagnosing Why Auxiliary Heat Is Always On

Diagnostics combine simple homeowner checks and technical inspections by an HVAC technician. Start with basic checks that are safe and non-invasive.

Check Thermostat Mode And Settings

Confirm the thermostat is set to “Heat,” not “Emergency Heat.” Review any wiring changes, schedule programming, and outdoor temperature sensor settings. Reset the thermostat to defaults if uncertain.

Observe Outdoor Unit Behavior

With the system calling for heat, verify the outdoor unit fan and compressor are running. If the outdoor unit is off while the indoor is heating, the system may be on emergency or auxiliary heat only.

Inspect Airflow And Filters

Restricted airflow from dirty filters, blocked returns, or closed vents reduces heat pump performance. Replace or clean filters and ensure vents are open to rule out airflow issues.

Look For Error Codes And Diagnostic Lights

Many modern heat pumps and thermostats display error codes or flashing lights. Consult the unit manual for code interpretation. Codes related to defrost, low pressure, or compressor issues point to why auxiliary heat runs.

Check For Refrigerant Leaks

Signs of low refrigerant include reduced heating capacity, hissing sounds, or ice buildup on the evaporator coil. Refrigerant diagnosis and recharge require certified technicians due to EPA regulations.

Common Repairs And Solutions

Solutions range from user-level fixes to professional repairs. Prioritize safety when working around electrical components and refrigerant lines.

Thermostat Reconfiguration Or Replacement

If the thermostat is set incorrectly or failing to sense heat pump operation, reconfigure or replace it. Upgrading to a heat pump–compatible thermostat resolves miscommunication and can improve efficiency.

Fix Airflow Problems

Replace clogged air filters, clean return vents, and ensure the blower fan operates. Good airflow often restores the heat pump’s capacity and reduces auxiliary usage.

Repair Defrost Controls And Sensors

A technician can test defrost thermistors, control boards, and timers. Replacing faulty defrost controls stops unnecessary auxiliary activation during or after defrost cycles.

Address Refrigerant Or Compressor Issues

Low refrigerant or compressor faults are common causes of insufficient heat. Certified HVAC technicians should locate leaks, repair piping, and recharge refrigerant. Compressor replacement may be necessary for severely damaged units.

Replace Electrical Components

Faulty relays, contactors, or heating element controls cause continuous auxiliary power. Qualified technicians should inspect and replace damaged electrical parts and verify safe operation.

Energy And Cost Implications

Auxiliary heat typically runs on electric resistance or fossil fuel and is much less efficient than a heat pump. Prolonged auxiliary operation increases energy consumption and utility bills considerably.

Estimating cost impact: if a 5 kW electric strip runs 24 hours, it can consume 120 kWh daily. At residential electricity rates of $0.15/kWh, that adds about $18 per day. Minimizing auxiliary runtime provides substantial savings.

Preventive Maintenance To Avoid Constant Auxiliary Heat

Regular maintenance reduces the risk of auxiliary heat running unnecessarily. Annual or biannual inspections catch issues early and ensure the heat pump operates efficiently.

• Schedule professional maintenance annually, including refrigerant checks and electrical inspections.
• Replace or clean air filters every 1–3 months during heavy use seasons.
• Keep outdoor unit clear of debris, snow, and ice for proper airflow and heat exchange.
• Install or calibrate outdoor temperature sensors if the system uses them for staging auxiliary heat.

When To Call An HVAC Professional

Some signs require an HVAC technician: persistent auxiliary heat, lack of outdoor unit operation, refrigerant leaks, unusual noises, or error codes. Professional service is essential for refrigerant work and complex electrical diagnostics.

Choose a licensed, insured technician experienced with heat pumps. Ask for itemized estimates and warranties on parts and labor. For systems under manufacturer warranty, contact the dealer first.

Smart Thermostats And Control Strategies

Smart thermostats can optimize heat pump staging and reduce unnecessary auxiliary heat. Features like adaptive recovery, outdoor sensor integration, and dual-fuel settings improve efficiency.

Strategies include using moderate setpoint changes, avoiding “Emergency Heat” unless instructed, and configuring temperature differentials to allow the heat pump to work before backup engages.

When Emergency Heat Is Appropriate

Emergency heat is appropriate when the heat pump is failed or the compressor is inoperative. It provides temporary heating while awaiting repair but is not a long-term solution due to cost and lower efficiency.

If emergency heat is used frequently, it indicates the heat pump needs service or replacement. Analyze repair vs. replacement costs for older systems to determine long-term value.

Frequently Asked Questions

Does Auxiliary Heat Running Mean The Heat Pump Is Broken?

Not always. Auxiliary heat may run briefly during startup, extreme cold, or defrost cycles. Continuous operation often indicates a problem with the heat pump, controls, or thermostat and should be investigated.

Is Emergency Heat The Same As Auxiliary Heat?

No. Emergency heat forces the backup heat only and bypasses the heat pump, while auxiliary heat supplements the heat pump when needed but allows the heat pump to operate.

Can A Homeowner Fix Auxiliary Heat Problems?

Homeowners can check and change thermostat settings, replace filters, and clear outdoor unit obstructions. Repairs involving refrigerant, compressors, or internal controls require certified HVAC technicians.

How Much Will Repairs Cost?

Costs vary widely: thermostat replacement $100–300, defrost control repair $200–600, refrigerant leak repair and recharge $200–1,000+, compressor replacement $1,000–3,500+. Obtain multiple quotes and verify qualifications.

Key Takeaways

• Auxiliary heat always on is usually a symptom, not the root cause.
• Begin with thermostat checks and airflow fixes before escalating to refrigerant or electrical diagnostics.
• Regular maintenance prevents many causes and reduces energy costs.
• Contact a licensed HVAC technician for refrigerant handling, compressor issues, or complex control faults.

For households experiencing continuous auxiliary heat, prompt diagnosis and targeted repairs restore efficiency, improve comfort, and reduce utility bills. Professional evaluation is recommended for persistent or unclear issues to ensure safe, effective resolution.