Why is my heat pump auxiliary heat running so often?

Auxiliary heat runs frequently if your thermostat droop setting is too tight (e.g. 1°F instead of 3°F), if you use deep overnight temperature setbacks, or if your outdoor lockout temperature is disabled.

What temperature should I set my auxiliary heat lockout to?

For modern cold-climate heat pumps, set the auxiliary heat lockout between 5°F and 15°F (-15°C to -10°C). Above this temperature, only the high-efficiency compressor should run.

How much electricity do heat pump heat strips use?

Most residential heat pump heat strips draw between 10kW and 15kW of continuous power, costing roughly $1.80 to $2.70 per hour of runtime at standard electricity rates.

Heat Pump Auxiliary Heat: Stop High Electric Bills

System Specifications & Key Findings:Homeowners transitioning from natural gas or oil to cold-climate heat pumps are frequently horrified by their first sub-zero electricity bill ($600–$1,000+). The culprit is rarely the heat pump compressor itself; it is the improper engagement of backup electric resistance auxiliary heat strips. A 10kW to 15kW heat strip draws as much power as three electric ovens running simultaneously, operating at a dismal Coefficient of Performance (COP) of 1.0.

What Is Auxiliary Heat and Why Is It So Expensive?

Direct Answer: Auxiliary heat consists of electric resistance coils installed inside the indoor air handler that activate when the outdoor heat pump cannot satisfy the thermostat demand alone, or during defrost cycles. While a modern heat pump compressor operates at 200% to 350% efficiency (COP 2.0–3.5), resistance heat strips operate at strictly 100% efficiency (COP 1.0), consuming 3x to 4x more electricity.

When outdoor temperatures plunge below freezing, modern inverter-driven vapor-injection heat pumps continue to extract atmospheric heat efficiently down to -15°F (-25°C). However, standard smart thermostats (such as default Ecobee or Nest profiles) are often factory-configured with aggressive droop settings. If the indoor temperature falls just 2°F below setpoint, the thermostat prematurely fires the 10kW or 15kW resistance strips.

At an average electricity cost of $0.18/kWh, running a 15kW heat strip for 10 hours a day adds $27 per day—or over $810 per month—directly to your utility bill. This catastrophic waste occurs even when the outdoor compressor was fully capable of heating the home on its own with proper sizing, directly aligning with principles outlined in our forensic audit of Manual J heat pump sizing mistakes.

How Do You Lock Out Auxiliary Heat to Save Money?

Direct Answer: Access your smart thermostat’s installer settings and set an Auxiliary Heat Lockout Outdoor Temperature (typically between 15°F and 5°F / -10°C to -15°C depending on your home’s heat loss). Above this lockout threshold, the thermostat is physically prohibited from energizing the electric strips.

Locking out auxiliary heat forces the variable-speed inverter compressor to ramp up to its maximum operating frequency before calling for backup assistance. In addition, homeowners must adjust the compressor-to-auxiliary staging differential (temperature droop) from the default 1°F or 2°F up to 3°F or 4°F. This allows the heat pump time to recover temperature during morning warm-ups without triggering resistance heat.

Operating Mode	Heating Mechanism	Efficiency (COP)	Power Consumption (kW)	Est. Monthly Cost (300h)
Cold-Climate Compressor	Vapor-injection refrigerant cycle	2.2 – 3.2 COP	2.5 – 4.0 kW	$135 – $216 / mo
10kW Auxiliary Heat Strips	Electric nichrome wire coils	1.0 COP	10.0 kW	$540 / mo
15kW Auxiliary Heat Strips	Heavy electric resistance coils	1.0 COP	15.0 kW	$810 / mo
Dual-Fuel Natural Gas Backup	Condensing gas burner secondary	96% AFUE	0.3 kW (fan only)	Variable fuel tariff rate

What About Defrost Cycles and ‘Set It and Forget It’?

Direct Answer: Never use deep nighttime temperature setbacks (such as dropping the thermostat from 70°F to 62°F overnight) with a heat pump. Recovering 8°F in freezing weather triggers auxiliary resistance heat immediately. Maintain a steady setpoint 24/7 to ensure steady, high-efficiency compressor operation.

During winter operation, outdoor coils frost over and the heat pump reverses into cooling mode for 3 to 10 minutes to melt frost, during which resistance heat briefly tempers indoor discharge air. Proper sensor calibration reduces defrost duration, as detailed in our guide to sub-zero cold-climate heat pump COP curves.

Graham Ward’s Verdict:Auxiliary heat should be a rare emergency safety net during once-in-a-decade blizzards, not a daily heating source. Configure your thermostat lockout to 10°F (-12°C), eliminate deep overnight temperature setbacks, and let your variable-speed inverter do the heavy lifting it was engineered to do.

Related Cold-Climate HVAC Analysis:

To prevent auxiliary resistance strips from engaging during sub-zero heating cycles, review our complete forensic guide on Heat Pump Setback Myth: Why Night Drops Spike Bills.