What Aux Heat Lockout Means

“Aux heat” is your backup heating stage (electric resistance or gas). A lockout temperature tells the thermostat not to use auxiliary heat until the outdoor temperature falls below a set point. Set correctly, your primary heat pump does the heavy lifting where it’s efficient, and expensive backup stages only run when truly needed.

From an efficiency standpoint, this revolves around your system’s balance point — the outdoor temperature where heat pump output matches your home’s heat loss. Above that point, the pump is typically cheaper than resistance heat. For background on how heat pump performance changes with temperature, see the U.S. Department of Energy.

Heat Pumps: Balance Point & Best Practices

Most modern air-source heat pumps remain cost-effective into the mid-30s °F. As a practical starting point, we set aux lockout at 35 °F and tune ±5 °F after a few days of real-world use.

• Field test: On a mild evening, raise your setpoint by 2 °F and confirm the thermostat does not call aux above your lockout. Longer but cheaper compressor runs near the balance point are normal.
• Our observation: In our wake-up recovery tests (pre-heat disabled), aggressive morning ramps triggered aux on multiple systems even with a 35 °F lockout. A 20–30 minute pre-heat window before wake-up prevented aux calls while maintaining comfort.

Furnace & Dual-Fuel: Economic Balance Point

Dual-fuel setups pair a heat pump with a gas furnace. Here, lockout governs fuel switching: run the heat pump until gas becomes cheaper or delivers better comfort. Typical switchover points land between 30–45 °F, but the best value depends on your gas/electric rates and envelope.

Pro tip: Use a reliable outdoor sensor or weather feed. Bad outdoor readings cause nuisance switchovers. We saw fewer false aux calls when the outdoor sensor was shaded, ventilated, and mounted away from exhausts.

Brand-Specific Settings (Ecobee • Nest • Honeywell)

Ecobee Smart Thermostat Premium

Path: Settings → Installation → Thresholds → Aux Heat. Choose Automatic (uses weather + runtime) or Manual (e.g., 35 °F). Ecobee works well with occupancy-aware SmartSensors. Official docs: Ecobee Support.

Nest Learning Thermostat (3rd-Gen)

Nest uses “Balance” strategies rather than a simple device-side manual lockout. In the app/web, set a heat-pump cutoff (often 30–45 °F) or pick Max Savings to delay aux longer. Schedules and Nest Temp Sensors influence room priority. Official docs: Google Nest Help.

Smart Thermostat

Nest Learning Thermostat (3rd-Gen)

“Max Savings” mode pushed aux later than default in our trials. Morning recovery benefited from a gentle pre-heat; time-block sensors lack live motion, so align schedules carefully to prevent aux during unoccupied periods.

Pros

• Simple app-driven cutoff via Balance
• Strong learning/scheduling features

Cons

• No true manual aux lockout on device
• Sensors don’t use live occupancy data

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Honeywell Home T10 Pro

Installer Menu: Code 0350 → Aux Heat Lockout. You can set both compressor and aux lockouts (separate values) with an outdoor sensor. T10 sensors include humidity + occupancy, which improved shoulder-season staging in our tests. Reference manual: Honeywell Home.

Smart Thermostat

Honeywell Home T10 Pro

The dual lockouts (compressor + aux) gave us the tightest control. A 35 °F aux and 15 °F compressor lockout avoided short aux bursts while letting the furnace take over in deep cold.

Pros

• Separate compressor/aux lockouts
• Occupancy + humidity room sensors

Cons

• Requires installer menu access for full control

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Comparison Table (Lockout Controls)

Values are starting points; fine-tune for your climate, envelope, and utility rates.

Step-by-Step: Set Aux Heat Lockout

1. Confirm outdoor temperature source (sensor or weather feed). If wired, mount in shade with airflow.
2. Enter brand settings: Ecobee Thresholds; Nest Balance/Heat Pump; Honeywell installer code 0350.
3. Choose starting lockout: 35 °F for aux on most air-source pumps; dual-fuel may differ.
4. Test staging: Raise the setpoint by 2 °F above ambient — confirm aux stays off above your threshold.
5. Tune after 48–72 hours based on comfort/runtime; adjust ±3–5 °F as needed.

Optimization: Sensors, Occupancy & Schedules

• Occupancy matters: With Ecobee or Honeywell room sensors, occupied rooms weigh more, reducing aux calls. Nest sensors follow time blocks; align schedules with real usage.
• Thermostat placement: Avoid cold drafts or sun-exposed walls; see our thermostat placement guide.
• Away/Vacation modes: Large setbacks often trigger aux on recovery. Use gradual ramps or pre-heat windows to avoid hitting aux as you return.
• Utility programs: Pair with rebates and demand response to monetize reduced aux runtime during peak events.
• Further reading: geofencing vs occupancy and best thermostats for heat pumps.

FAQ

Will a strict lockout make my home feel colder?

Near the balance point, expect longer heat-pump cycles and gentler temperature rises. If recovery feels sluggish, nudge lockout up by 3–5 °F or add a short pre-heat before wake-up.

Do I need a physical outdoor sensor?

Ecobee and Nest can rely on weather services; Honeywell T10 benefits from a dedicated sensor for precise dual lockouts. We recorded fewer nuisance aux calls with a properly mounted physical sensor.

What’s the difference between compressor and aux lockout?

Compressor lockout stops the heat pump when it’s too cold to run efficiently; aux lockout prevents backup heat from engaging too early. Dual-fuel systems often use both to control switchover.

What starting values should I try?

As a baseline: aux at 35 °F, compressor at 15 °F (where supported). Then adjust based on comfort, utility rates, and actual runtime data.