my honeywell t9 says it's heating but cold air is blowing, or the aux heat kicks on when it's set to cool

Diagnosis

This behavior is a classic symptom of a "Reversing Valve" configuration error or a wiring mismatch between the T9 thermostat and your heat pump's control board. In a heat pump system, the O/B wire controls the reversing valve, which determines whether the system is in cooling mode or heating mode. If the T9 is configured to energize the O/B terminal in the wrong state (or not at all), the system will physically operate in cooling mode even while the thermostat display indicates "Heating." Essentially, the compressor is running, but the refrigerant is flowing in the wrong direction. When you experience auxiliary heat (electric heat strips) kicking on during a call for cooling, it typically means the thermostat is sending a 24V signal to the Aux/W2 terminal while simultaneously attempting to engage the compressor. This happens if the T9 is set to a "Conventional" heating system instead of a "Heat Pump" system in the installer settings, or if the O/B wire is incorrectly landed in the W terminal. You can confirm this by checking if the outdoor unit is running while the indoor vents are blowing cold air during a heat call; if the outdoor fan is spinning but the air is freezing, your reversing valve is not being triggered correctly.

How to Fix It

To resolve this, you must verify the physical wiring and then correct the software configuration in the T9 Installer Setup menu.

1. Shut off power to your HVAC system at the main circuit breaker panel to prevent shorting the 24V transformer.
2. Pull the T9 thermostat off the wall plate. Inspect the wiring on the UWP (Universal Wall Plate). For a standard heat pump, you should see a wire in the O/B terminal and a wire in the Aux/W2 terminal. Ensure no wires are bridged or touching.
3. Snap the thermostat back on and power the system back up.
4. Enter the Installer Setup menu: Press and hold the center of the screen to access the main menu, navigate to Settings, then select Equipment.
5. Locate the System Type setting. Change this from "Conventional" to "Heat Pump".
6. Navigate to the Reversing Valve setting. This is the most critical step. If your system is a Rheem or Ruud model, the valve usually needs to be energized in "Heat" (B). For almost all other brands (Carrier, Trane, Lennox), it must be set to "Cool" (O).
7. Save the settings and exit the menu.
8. Set the thermostat to "Heat" and increase the set point by 5 degrees. Wait 3-5 minutes for the compressor to engage and verify that warm air is now flowing from the vents.

If That Didn't Work

• Swap the O/B Logic: If you are unsure of the brand of your heat pump, toggle the Reversing Valve setting from "O" to "B" (or vice versa) in the Installer Setup. If the system was blowing cold during heat, flipping this logic switch is the most common fix for the T9.
• Check for a Blown Fuse: Inspect the 3-amp or 5-amp blade fuse on the indoor air handler's control board. If the fuse is blown, the thermostat may show it is "Heating," but the signal never reaches the reversing valve or compressor, leaving only the auxiliary heat strips to function.
• Inspect the O/B Wire Integrity: Check for a break or a loose connection at the air handler side. If the O/B wire is disconnected at the control board, the reversing valve will remain in its default position (usually cooling), regardless of what the T9 commands.
• Factory Reset: If settings are glitching, perform a full factory reset via the Reset option in the Settings menu to clear any conflicting configuration profiles and start the equipment setup from scratch.

Heads Up

• C-Wire Requirement: The T9 requires a constant 24V C-wire for stable operation. If you are using a "power extender" or "add-a-wire" kit, ensure the common wire is seated deeply in the terminal to prevent intermittent power drops that can reset equipment settings.
• Aux Heat Delay: Be aware that the T9 has a built-in "Auxiliary Heat Lockout." If the system just switched from cool to heat, it may take several minutes before the compressor engages to protect the equipment from "short-cycling."
• Voltage Warning: Never touch the wires together while the power is on. Touching the R (power) wire to the C or W wires can instantly blow the transformer or fry the T9's internal circuitry.
• Filter Maintenance: If you have corrected the wiring but the air is only "lukewarm," check your air filter. A clogged filter reduces airflow, which can cause the heat pump to trip its internal high-pressure switch and shut down the compressor.