why does my sensi thermostat say 'reversing valve' when i try to set up heat pump?

Diagnosis

The "Reversing Valve" prompt on the Emerson Sensi ST55 is not an error message, but a critical configuration step. In a heat pump system, the reversing valve is the component that physically changes the flow of refrigerant to either move heat into the house (heating mode) or pull heat out of the house (cooling mode). Because different manufacturers use different electrical logic to trigger this valve, the thermostat must be told whether the valve should be energized during the cooling cycle or the heating cycle. If this setting is incorrect, your system will behave opposite to your request: when you turn on the AC, the unit will blow hot air, and when you request heat, it will blow cold air. This happens because the ST55 is sending power to the "O/B" terminal based on the wrong logic. You can confirm this by setting the thermostat to "Cool" and feeling the air at the vents; if it is hot, your reversing valve setting is inverted.

How to Fix It

To correctly configure the reversing valve on the ST55, you must access the Equipment configuration menu. Follow these precise steps:

1. Press the Menu button on the thermostat faceplate.
2. Navigate to Settings and then select Equipment.
3. Select Heat Pump from the list of equipment types.
4. You will now see the Reversing Valve prompt. You must choose between O and B.
5. If you have a Rheem, Ruud, or Trane system, select O (this energizes the valve in Cooling mode).
6. If you have a Carrier, Bryant, or Lennox system, select B (this energizes the valve in Heating mode).
7. Press Done or Save to commit the changes.
8. Test the system by setting it to "Cool" and ensuring the air is cold; then switch to "Heat" and ensure the air is warm.

If That Didn't Work

• Verify the Physical Wiring: Pull the thermostat faceplate off the wall and inspect the wiring. Ensure the wire connected to the "O/B" terminal is securely clamped and not touching any other wires. If the wire is loose, the signal will never reach the valve regardless of the software setting.
• Check for Auxiliary Heat Conflict: If the system is blowing cold air in Heat mode but the reversing valve setting is correct, your system may be jumping straight to "Aux Heat" (electric strips) because the heat pump compressor isn't engaging. Check the "Aux Heat" settings in the menu to ensure the lockout temperature is not set too high.
• Reset the Equipment Profile: Sometimes the ST55 retains a cached equipment profile from a previous setup. Go to Menu > Settings > Reset and perform a factory reset. This clears all logic boards and allows you to start the equipment selection from scratch, ensuring no conflicting "Conventional" settings are overriding the "Heat Pump" logic.
• Inspect the Contactor: If the thermostat is configured correctly but the valve isn't switching, the 24V signal may be reaching the outdoor unit, but the reversing valve solenoid itself may have failed. Use a multimeter to check for 24VAC at the O/B terminal on the outdoor unit's control board during a call for heat.

Heads Up

• Avoid Frequent Switching: Do not rapidly toggle between Heat and Cool modes while testing the reversing valve. Doing so can cause "short cycling," which puts immense stress on the compressor and can lead to premature failure.
• C-Wire Importance: The ST55 WiFi features require a stable power source. If you are using a "power stealing" setup without a C-wire, you may experience intermittent reversing valve triggers or random restarts. Always install a dedicated 24V C-wire for WiFi thermostats.
• Seasonal Check: If you notice the system is blowing lukewarm air during the first heat cycle of winter, check the outdoor unit for ice buildup. If the reversing valve is working but the coils are frozen, you may need to adjust your defrost settings in the advanced menu.