Heat Pump Thermostat Wiring: O/B, AUX and E Made Simple

Updated 2026-09-12

A heat pump thermostat has a few jobs a furnace thermostat doesn’t:

• Y runs the compressor in both heating and cooling.
• O/B flips the reversing valve. O (energized in cooling) is used by most brands; B (energized in heating) by Rheem and Ruud. More on O vs B.
• AUX / W2 / W1 runs backup electric heat strips when the heat pump can’t keep up.
• E runs emergency heat only (compressor off). Often tied to AUX at the air handler. E wire details.

Dual fuel

If the backup heat is a gas or oil furnace instead of electric strips, it’s a dual-fuel system. Nest, ecobee and Honeywell T9 support dual fuel; Amazon says its Smart Thermostat does not.

After wiring

In the app, choose heat pump, set O/B correctly, and set aux heat type. Then test: cooling should blow cold within a few minutes; heating should blow warm without the aux indicator on (in mild weather).