my ecobee enhanced keeps switching to 'aux heat' even when it's not that cold out, running up my bill.

Diagnosis

The issue stems from a misconfiguration of the "Aux Heat Lockout" or "Heat Pump Balance" settings within the Ecobee Smart Thermostat Enhanced. In a heat pump system, the unit extracts heat from the outside air. As the outdoor temperature drops, the heat pump's efficiency plummets. To prevent the house from freezing, the system engages Auxiliary Heat (electric heat strips or a gas furnace). When the thermostat is configured with a lockout temperature that is too high—for example, 40°F (4°C) instead of 30°F (-1°C)—the system triggers the expensive backup heat even during mild autumn or spring days. This behavior is often exacerbated by a poorly placed outdoor temperature sensor or a "Comfort Setting" that is set too aggressively. If the thermostat perceives a rapid drop in temperature or if the "Heat Pump Balance" is set to "Prefer Aux Heat," it will bypass the energy-efficient compressor and jump straight to the resistive strips. You can confirm this by checking the main screen: if you see "Aux Heat" active while the outdoor temperature reads above 40°F, your lockout thresholds are set too high.

How to Fix It

To stop the premature engagement of auxiliary heat, you must manually adjust the lockout thresholds and the balance settings. Follow these exact steps on your Ecobee Enhanced interface:

1. Tap the Main Menu (three horizontal lines) on the home screen.
2. Navigate to Settings and then select Installation Settings.
3. Select Equipment and then tap on Heat Pump.
4. Locate the Aux Heat Lockout setting. This is the temperature below which the aux heat is allowed to turn on. If it is currently set to 40°F, lower it to 30°F or 35°F depending on your local climate and the SEER rating of your outdoor unit.
5. Navigate back to the Equipment menu and find Heat Pump Balance. Ensure this is set to "Prefer Heat Pump" rather than "Prefer Aux Heat." This forces the system to attempt to reach the set point using the compressor before calling for backup heat.
6. Check the Compressor Lockout setting. Ensure this is set to a temperature that reflects the absolute minimum operating limit of your specific heat pump model (typically between 15°F and 25°F) to prevent the compressor from freezing while still avoiding unnecessary aux heat.
7. Save changes by tapping the checkmark or exiting the menu.

If That Didn't Work

• Check the Outdoor Sensor Calibration: If you have a wired outdoor sensor, it may be reporting a temperature lower than the actual ambient air. Go to Settings > Installation Settings > Sensors and check the outdoor reading. If it is off by more than 3 degrees, apply a temperature offset to calibrate the reading.
• Adjust the 'Heat Differential' Setting: If your system is "short-cycling" into aux heat, increase the Heat Differential Temperature (found in Advanced Settings). Increasing this from 0.5°F to 1.5°F prevents the system from triggering a heat call for minor fluctuations, which often prevents the aux heat from kicking in during brief cold snaps.
• Inspect the O/B Reversing Valve Wire: If the system is blowing cold air and then suddenly jumping to Aux Heat, the O/B wire may be loose or the "Reversing Valve" setting is incorrect. Go to Installation Settings > Equipment > Heat Pump and toggle the Reversing Valve setting between "On Cool" and "On Heat" to ensure the heat pump is actually heating before the aux heat takes over.
• Disable 'Smart Recovery': Sometimes the "Smart Recovery" feature tries to pre-heat the home to reach a target temperature by a specific time. Because it needs to heat the home rapidly, it ignores the lockout settings and engages Aux Heat. Turn this off in Settings > Preferences.

Heads Up

• Electric Bill Warning: Auxiliary heat strips use significantly more kilowatt-hours than the heat pump compressor. Even a few hours of unnecessary aux heat per day can increase your monthly utility bill by 30% to 50%.
• Filter Maintenance: A clogged air filter reduces airflow across the coils, causing the system to struggle to reach the set point. This "struggle" often triggers the thermostat to call for Aux Heat to make up the difference. Change your 20x20 or 20x25 filter every 30-90 days.
• Avoid 'Auto' Mode: Using "Auto" (Heat/Cool) can sometimes cause the system to swing wildly between modes. For winter months, keep the thermostat locked in "Heat" mode to ensure the lockout logic remains stable.