The Aux/E (Auxiliary or Emergency) wire on a thermostat is a key component for heat pump systems. When paired with a Nest thermostat, properly identifying and wiring Aux/E ensures reliable heating performance, energy efficiency, and accurate temperature control. This guide explains what Aux/E means, how Nest uses it, wiring steps, common pitfalls, and troubleshooting tips to help homeowners.

What The Aux/E Wire Means In A Heat Pump System

Auxiliary (AUX) or Emergency (E) heat provides additional heating capacity when the heat pump alone cannot maintain comfort or when outdoor temperatures are very low. In many systems, the AUX/E function is activated automatically by the thermostat based on demand and outdoor conditions. The AUX/E circuit typically connects to a heat strip inside the air handler or furnace. In a Nest thermostat, AUX/E is commonly labeled as W2 or AUX depending on the model and wiring scheme. Understanding this distinction helps ensure correct wiring and control logic.

Nest Thermostat Wiring: Key Terminals To Know

Before connecting Aux/E, recognize how Nest terminals map to your equipment. Common terminals include:

Y – Compressor contactor (cooling) or heat pump compressor
G – Fan
R – 24V power
C – Common return for power (required for many smart thermostats)
O/B – Reversing valve (heat pump)
W2 / AUX / E – Auxiliary or Emergency heat

In heat pump setups, the W2 or AUX terminal is used to energize supplemental heat when necessary. The Nest will engage AUX/E based on outdoor temperature, heat demand, and system settings.

Wiring Aux/E On A Nest Thermostat: Step-By-Step

Wiring steps can vary with model (Nest Thermostat, Nest Thermostat E, or Nest Learning Thermostat). The following general process applies to most setups:

Turn Off Power: Switch off the HVAC circuit at the breaker to prevent shock or damage.
Remove The Cover: Access the control board or furnace/air handler to confirm existing wiring.
Label W2/Aux: If your current system labels AUX/E as W2, note this for identification during rewire.
Connect Aux/E To Nest: Attach the wire previously on W2/Aux to the Nest’s W2/Aux terminal (the exact label may vary by model).
Verify Other Connections: Ensure Y, G, R, C (if used), and O/B are correctly placed according to your equipment. Do not mix terminals.
Power On: Restore power and wake the Nest. It may prompt for system type or configuration choices.
Run A System Test: Use the Nest to run a test that engages cooling, heating, and AUX/E to verify proper operation.

If a C wire is not present, the Nest may still operate using power stealing, but a dedicated C wire improves reliability. Consider a C-wire adapter or adding a dedicated C wire if your HVAC control board supports it.

Common Scenarios And How Aux/E Behavior Changes

Some typical configurations and what to expect from Aux/E behavior:

Heat Pump With Auxiliary Heat: In cold weather, the heat pump runs along with AUX/E to meet demand. Nest may show AUX or W2 during peak heating periods.
Aux Heat Only (Non-Heat Pump Systems): For conventional furnaces, W or W2 typically controls the furnace burner; AUX/E labeling is less common.
Adaptive Settings: Nest learns usage patterns. If AUX/E engages frequently, review thermostat settings, outdoor temperature thresholds, and heat pump capacity.

Tips For Troubleshooting Aux/E Wiring

Issues with AUX/E can manifest as insufficient heat, thermostat not calling for AUX, or constant Auxiliary engagement. Consider these checks:

Check Wiring Continuity: Ensure the Aux wire is continuous from the control board to the Nest. Damaged insulation or loose terminals can cause unreliable operation.
Confirm System Type: Confirm the thermostat is configured for a heat pump with auxiliary heat in the Nest settings. Incorrect system type can misfire AUX signals.
Test In All Modes: Run heat pump-only cooling and heating tests, then test AUX/E to confirm it engages correctly during high demand.
Look For Fault Codes: Nest will display error codes if there are wiring or sensor issues. Note any codes and consult wiring diagrams.
Evaluate Thermostat Location: Extreme temperature or drafty placement can affect sensor readings and control accuracy; relocate if necessary.
Check Control Board Labels: Some boards label auxiliary heat as W2 or AUX. Verify with your unit’s schematic to avoid miswiring.

Safety And Compatibility Considerations

Safety is paramount when working with electrical components. Always power down the system before handling wires. If unsure, consult a licensed HVAC technician. Compatibility concerns include:

Thermostat Model: Not all Nest models support all wiring schemes. Verify that your model supports W2/Aux and O/B with your heat pump.
Control Board: Some boards require a dedicated C wire or have limited AUX/Auxillary heat control. Review the equipment manual for wiring diagrams.
Energy Efficiency: Proper Aux/E control can improve comfort and efficiency, but improper use may raise running costs. Settings like temperature differential and outdoor temp thresholds influence performance.

Alternatives If Aux/E Wires Are Not Available

If the Aux/E wire is unavailable or unreliable, consider these options:

Use Heat Pump With Auxiliary Safety Settings: Some systems can be configured to run emergency heat only under extreme conditions via programming adjustments.
Add A C Wire: Running a dedicated common wire improves Nest power stability and reduces false readings.
Separate Thermostat or Staging: In some setups, a separate control strategy or staging controller can manage aux heat more precisely.