Heat Pump Auxiliary Heat: When It Runs, Costs, Settings, and Fixes

Meta Description: Learn how heat pump auxiliary heat works, when it should turn on, how much it costs, and the best thermostat settings and fixes to reduce energy bills while staying comfortable.

When the weather turns cold, many homes see “Aux Heat” light up on the thermostat. This article explains heat pump auxiliary heat, when it should run, what it costs, and how to set controls to balance comfort and savings. It includes practical tips, tables, and troubleshooting steps based on common U.S. systems.

What Is Heat Pump Auxiliary Heat?

Heat pump auxiliary heat is a backup heat source that supplements the heat pump when extra capacity is needed. Most commonly, it is electric resistance “heat strips” inside the air handler. In some homes, it is a gas or propane furnace in a dual-fuel setup.

Key idea: Auxiliary heat provides fast, reliable heat when the outdoor unit cannot keep up, but it costs more per BTU than a heat pump.

Auxiliary Vs. Emergency Heat

Auxiliary heat is automatic. The thermostat or control board calls for it when the heat pump alone cannot meet the setpoint in a timely manner. “Aux” may appear on the thermostat display while the compressor still runs.

Emergency heat is manual. Switching the thermostat to EM Heat forces the system to run only the backup heater. Use EM Heat only for compressor failures, severe icing, or maintenance periods. Otherwise, it raises energy costs and can overheat ducts.

Heat Strips Vs. Dual-Fuel Backup

Electric heat strips convert electricity directly to heat with a coefficient of performance (COP) of 1. A 10 kW strip produces about 34,000 BTU/h. It is simple and reliable but expensive to run.

Dual-fuel systems pair a heat pump with a gas or propane furnace. Controls switch to the furnace below a set outdoor temperature. Dual-fuel can cut costs in very cold weather, especially where gas is affordable and electricity is expensive.

When Does Aux Heat Turn On?

Aux heat should engage only when truly needed. Frequent or constant Aux indicates control or sizing issues. Typical triggers include:

• Large Setpoint Changes: If the thermostat sees a big gap, many models add Aux to speed recovery. A difference of 2–3°F may be enough on some thermostats.
• Low Outdoor Temperature: Below the system’s “balance point,” the heat pump cannot meet the load alone, so Aux supplements. For older systems, this may be around 30–40°F. Cold-climate models may be comfortable down to 5–20°F without Aux.
• Defrost Cycle: The outdoor unit occasionally reverses to cooling to melt frost. The system may energize Aux to temper the indoor air during defrost.
• Time-Based Staging: Some thermostats bring on Aux if the setpoint is not reached within a time threshold (e.g., 10–20 minutes).
• Compressor Lockout: If configured, the thermostat can disable the compressor below a minimum outdoor temperature and rely on Aux or a furnace.

Rule of thumb: If Aux heat is on for long stretches when outdoor temperatures are moderate (above the mid-30s°F) and the setpoint is steady, something is off—check settings, filters, airflow, or equipment sizing.

Costs: How Much Does Aux Heat Cost To Run?

Aux heat is effective but expensive. Electric resistance heat delivers 3,412 BTU per kWh at a COP of 1. In contrast, a heat pump may deliver 6,800–13,000 BTU per kWh at COP 2–3.

Example: A 10 kW heat strip uses 10 kWh per hour. At $0.17/kWh (around the 2025 U.S. residential average), that’s $1.70 per hour. Running 4 hours per day for a month adds about $204 to a bill.

Costs vary widely by local energy prices, efficiency, and weather. The table below compares typical delivered heat costs.

Takeaway: Auxiliary electric heat often costs 2–3x more per BTU than a heat pump and may cost more than propane. Using controls to delay or avoid Aux can produce meaningful savings.

Best Thermostat Settings For Aux Heat

Modern thermostats provide several settings that determine how often heat pump Aux heat runs. Tuning them helps balance comfort and cost.

Outdoor Lockout Temperatures

• Aux Heat Lockout (Max Outdoor Temp for Aux): Prevent Aux from running above a chosen outdoor temperature. Many homes set 30–40°F. Cold-climate heat pumps may push this into the 20s°F.
• Compressor Lockout (Min Outdoor Temp for Heat Pump): For dual-fuel, below this temperature the system uses the furnace only. Typical 10–25°F, depending on rates, comfort, and capacity.

Choose lockouts based on comfort, equipment capability, and energy prices. Test incrementally: move the Aux lockout a few degrees at a time and watch comfort and run times.

Setbacks And Scheduling

• Use small setbacks (0–3°F) in heating season. Large setbacks often trigger Aux during recovery, erasing savings.
• Schedule earlier start times so the heat pump can recover slowly without Aux. Many smart thermostats have “adaptive recovery.”
• Keep fan on Auto unless advised otherwise by a pro; “On” can blow cool air during defrost and increase drafts.

Staging And Time-To-Temp Controls

• Heat Pump To Aux Runtime: Sets how long the system tries the compressor before adding Aux (e.g., 20–45 minutes). Longer delays reduce Aux usage.
• Max Temp Differential: Controls how many degrees away from setpoint triggers Aux. Use a smaller differential to delay Aux.
• Compressor Protection: Minimum off-time and low ambient limits help reliability and may affect how often Aux is needed.

Brand-Specific Notes

Menu names vary; consult the user manual or installer settings.

• Ecobee: Look under Thresholds for “Compressor Min Outdoor Temp,” “Aux Heat Max Outdoor Temp,” and “Heat Pump To Aux Runtime.” Extend runtime and lower Aux reliance where comfort allows.
• Nest: “Heat Pump Balance” (Max Comfort/Off/Max Savings) adjusts Aux aggressiveness. Advanced settings include “Alt Heat Lockout” and compressor lockout temperatures.
• Honeywell/Resideo: Installer setup includes system type and heat stages. Some models allow Aux lockout and heat stages timing. Verify correct wiring for W2/Aux and E.

Pro tip: If Aux is frequent, verify the outdoor temperature sensor reads correctly. A faulty sensor can force unnecessary Aux or lockouts.

Improving Comfort Without Aux Heat

Reducing the home’s heat loss allows the heat pump to carry more of the load before Aux is needed.

• Seal Air Leaks: Weatherstrip doors, caulk gaps at trim and penetrations, and seal attic hatches. Air sealing often delivers big comfort gains.
• Add Insulation: Attic insulation to R-38–R-60 in colder regions is typically cost-effective. Insulating rim joists and knee walls helps too.
• Improve Airflow: Replace clogged filters, keep returns clear, open supply registers, and ensure adequate duct sealing.
• Outdoor Unit Care: Clear snow, leaves, and debris. Maintain 18–24 inches of clearance for airflow, which reduces frosting and improves capacity.
• Windows And Shades: Close shades at night to reduce losses; open on sunny days to capture solar heat.

These steps can reduce Aux run time, improve comfort, and help the system defrost more efficiently.

Troubleshooting Common Aux Heat Problems

Aux Heat Runs Constantly

• Clogged Filter Or Airflow Issues: Replace the filter, check registers, and inspect ducts for kinks or leaks.
• Low Refrigerant Or Compressor Problem: Heat pump may underperform; call a licensed HVAC technician for diagnostics.
• Thermostat Settings: Aux lockout too high, aggressive staging, or large scheduled setbacks can cause frequent Aux.
• Outdoor Sensor Error: A bad sensor may signal colder temperatures than actual, forcing Aux.
• Undersized Equipment: In extreme cold, Aux may be necessary; consider a cold-climate heat pump upgrade.

Aux Heat Never Comes On

• Wiring: Verify the W2/Aux terminal is connected; some thermostats combine W2/E and require correct configuration.
• Tripped Breaker: Electric heat strips have separate breakers or fuses; check panel and air handler.
• Lockout Settings: Aux lockout may be set too low; adjust carefully if needed.
• Defective Heat Strip Or Relay: Requires electrician or HVAC service to test elements, sequencers, or boards.

Heat Feels Cool Or Drafty

• Normal During Defrost: Brief cool air while the outdoor unit melts frost. Some systems use Aux to temper air.
• Fan Speed Too High: Excess airflow can reduce supply temperature; an installer can adjust blower settings.
• Duct Leaks: Leaks in attics or crawlspaces can dilute supply air. Seal with mastic or have ducts professionally sealed.

High Bills After Cold Snap

• Aux Runtime: Check thermostat energy reports for Aux usage spikes.
• Setbacks: Large recoveries may have triggered Aux. Use smaller setbacks in winter.
• Utility Rates: Some utilities apply winter peak rates; consider time-of-use strategies where available.

Burning Smell

• Dust On Heat Strips: A slight odor is common the first few Aux cycles each season. If it persists, shut off power and have the system inspected.
• Electrical Odor Or Smoke: Turn off the system at the breaker and call a professional immediately.

Wiring, Staging, And Controls Basics

Knowing common terminals helps with setup and troubleshooting. Always cut power before opening equipment, and consider hiring a pro.

• O/B: Reversing valve control for heat/cool mode (varies by brand).
• Y1/Y2: First and second stage compressor.
• W2/Aux: Auxiliary heat call. On some thermostats, W2 and E are combined.
• E: Emergency heat call, typically manual from thermostat.
• G: Indoor fan; R/C: 24V power and common.

Staging logic can be temperature- or time-based. Single-stage heat pumps rely more on Aux during cold weather. Two-stage or variable-speed systems modulate capacity and often need Aux less frequently.

Installer configuration matters: Misidentified system type or heat stages can cause short cycling, poor comfort, or excessive Aux usage. Verify dip switches and thermostat setup match the equipment.

Heat Pump Aux Heat During Defrost

In heating mode, outdoor coils can frost. The system periodically reverses to cooling to defrost. Indoor air can cool briefly during this cycle.

• Aux During Defrost: Many air handlers energize Aux during defrost to maintain supply temperature.
• Frequent Defrost: Can signal poor airflow, low charge, or obstructed outdoor coil. Clear debris and schedule service if defrosts are very frequent.
• Cold-Climate Units: Advanced controls and variable-speed compressors reduce defrost penalties and keep higher capacity in low temperatures, delaying Aux.

Cold-Climate Performance And Reducing Aux Dependence

New cold-climate heat pumps maintain strong capacity at low temperatures thanks to variable-speed compressors, vapor injection, and smart controls. Many achieve 70–100% of rated capacity at 5°F.

Benefits: Less Aux usage, smoother comfort, quieter operation, and lower bills. If Aux runs often below freezing, a cold-climate upgrade can pay back through reduced winter electricity use.

• Sizing: Slightly larger low-ambient capacity (not oversized ducts) helps withstand cold snaps without Aux.
• Controls: Optimized outdoor lockouts and longer compressor runtimes limit Aux engagement.
• Ducted Vs. Ductless: Ductless heads near cold rooms can reduce whole-house Aux calls by directly serving high-load spaces.

Energy Tips And Incentives In The U.S.

Financial incentives can make upgrades that reduce Aux usage more affordable.

• Federal Tax Credit (25C): Up to 30% of costs, capped amounts, including up to $2,000 for qualifying heat pumps each year. Also credits for insulation, air sealing, and energy audits.
• Utility Rebates: Many utilities offer rebates for cold-climate heat pumps and smart thermostats with Aux controls.
• State Programs: Emerging rebates (HEEHRA) and state incentives vary. Check the Database of State Incentives for Renewables & Efficiency (DSIRE) at dsireusa.org.

Tip: An energy audit can identify air sealing and insulation upgrades that reduce Aux runtime at low cost.

Aux Vs. Emergency Vs. Defrost: What The Signals Mean

How To Read And Use Energy Data

Tracking Aux runtime helps target changes. Many smart thermostats show hourly usage and label “Aux” or “Alt Heat.”

• Identify Patterns: Note outdoor temperatures when Aux starts. Raise Aux lockout a few degrees at a time if comfort allows.
• Compare Bills: See how changes in setbacks or schedules affect cost during similar weather.
• Use Weather-Normalized Tools: Some utilities offer tools that normalize for temperature, clarifying the impact of Aux control changes.

Quick Calculations To Inform Settings

Use simple math to understand the stakes and set expectations.

• Per-Hour Cost: Heat strip kW × electricity price = hourly cost. Example: 10 kW × $0.17 = $1.70/hr.
• Monthly Impact: Hourly cost × hours per day × days. Example: $1.70 × 4 × 30 = $204.
• Delivered BTU Advantage: COP 3 heat pump delivers ~10,236 BTU per kWh; Aux delivers 3,412 BTU per kWh. The heat pump can be ~3x as efficient.

Safety And Maintenance Notes

Aux heat elements and gas furnaces get hot and draw high current. Safety and maintenance matter.

• Electrical Checks: Ensure dedicated breakers for heat strips are correctly sized and labeled. Loose connections can overheat.
• Combustion Safety: For dual-fuel, maintain venting, gas pressure, and CO detection. Schedule annual service.
• Clean Coils And Filters: Dirty coils increase frosting and force Aux. Replace filters every 1–3 months.
• Thermostat Firmware: Update smart thermostats; manufacturers improve Aux algorithms over time.

Common Questions About Heat Pump Aux Heat

Why Is My Heat Pump Aux Heat On?

It usually indicates the system needs extra capacity due to a large setpoint change, low outdoor temperature, a timed stage-up, or a defrost cycle. If Aux is frequent in mild weather, check filters and thermostat settings.

Is Auxiliary Heat The Same As Emergency Heat?

No. Aux is automatic support, typically alongside the compressor. Emergency heat is a manual mode that disables the compressor and uses only the backup heat source.

What Temperature Should Aux Heat Come On?

There is no universal temperature. Older systems may need Aux in the mid-30s°F; cold-climate models can avoid Aux well below 20°F. Set Aux lockout as low as comfort allows, then test.

Should I Use Setback Temperatures With A Heat Pump?

Use small setbacks. Large setbacks often trigger Aux during recovery, negating savings. Adaptive recovery in smart thermostats can help minimize Aux.

How Do I Reduce Aux Heat Usage?

Lower Aux lockout gradually, extend time-to-Aux delays, reduce setbacks, improve insulation and air sealing, keep filters clean, and clear the outdoor unit for airflow.

Can Aux Heat Damage My System?

Aux is designed for the system. The downside is cost, not damage. However, incorrect wiring, oversized heat strips, or stuck relays can overheat ducts. Have an installer verify sizing and controls.

Is Dual-Fuel Cheaper Than Electric Aux?

It depends on local prices. With low gas and high electricity prices, dual-fuel can be cheaper below a switchover temperature. With average electricity and high propane, a cold-climate heat pump may be more economical.

Setting Priorities: Comfort, Cost, And Equipment Health

Aux heat is a tool for comfort and reliability. The goal is not eliminating Aux entirely but using it strategically when needed. Small setting changes and home upgrades can cut Aux runtime significantly while preserving comfort.

• Comfort First: If parts of the home run cold, consider airflow balancing or adding a ductless head rather than relying on Aux.
• Cost Awareness: Monitor Aux hours and test settings during cold spells to find a practical balance.
• Protect Equipment: Do not disable safeties or necessary lockouts. Schedule regular maintenance.

Checklist To Optimize Heat Pump Aux Heat

• Verify Thermostat Type: Correctly configured for heat pump with Aux.
• Set Aux Lockout: Start around 35°F, then adjust based on comfort and performance.
• Set Compressor Lockout: Dual-fuel only; set 10–25°F depending on rates and equipment.
• Increase Time-To-Aux: Allow longer compressor runtime before Aux.
• Limit Setbacks: 0–3°F in heating season with early start enabled.
• Maintain Airflow: Clean filters, open registers, and clear the outdoor unit.
• Audit The Envelope: Seal leaks, upgrade insulation, and consider storm windows or efficient glazing.
• Track Results: Use thermostat reports and bills to confirm reduced Aux runtime.

With the right controls, maintenance, and building improvements, a home can stay warm while keeping heat pump Aux heat in its proper role—there when needed, but not running the show.