is the honeywell t9 compatible with my heat pump? i'm getting weird heating and cooling at the same time

Diagnosis

The Honeywell Home T9 (RCHT9610WFSW2003) is fully compatible with heat pump systems, but the "simultaneous heating and cooling" phenomenon is almost always caused by a configuration mismatch between the physical wiring and the software settings in the Installer Setup (ISU). Specifically, this occurs when the thermostat is configured for a "Conventional" system instead of a "Heat Pump" system, or when a wire is incorrectly seated in the W terminal while the system is trying to engage the reversing valve via the O/B terminal.

When the T9 is incorrectly set to Conventional, it treats the Y terminal as cooling and the W terminal as heating. In a heat pump system, the Y terminal controls the compressor for both modes, and the O/B terminal tells the reversing valve whether to move heat into or out of the house. If you have a wire in the W terminal that is actually your auxiliary heat, but the thermostat thinks it is the primary heat source, it may trigger the auxiliary strips while simultaneously calling for the compressor to cool, leading to the "fighting" temperatures you are experiencing. You can confirm this by checking if the outdoor unit is running while the indoor vents are blowing hot air from the heat strips.

How to Fix It

To resolve this conflict, you must ensure the physical wiring matches the digital configuration. Follow these precise steps:

1. Shut off the power to your HVAC system at the circuit breaker to prevent shorting the 24V transformer.
2. Gently pull the T9 faceplate off the wall bracket.
3. Inspect the wiring terminals. For a standard heat pump, you should have wires in R (or Rc/Rh), C, Y, G, and O/B. If there is a wire in the W terminal, this is your Auxiliary/Emergency heat. Ensure no wires are bridging across terminals.
4. Snap the faceplate back on and power the system back up.
5. Enter the Installer Setup (ISU) menu. To do this, press and hold the Menu button until the screen changes to the advanced settings.
6. Navigate to "System Type" and change the setting from "Conventional" to "Heat Pump." This is the most critical step; without this, the T9 cannot manage the reversing valve.
7. Locate the "Reversing Valve" setting. Set this to "O" if your valve energizes in cooling mode (common for Rheem/Ruud) or "B" if it energizes in heating mode (common for Bosch/others). If you get cooling when you want heat, flip this setting.
8. Check the "Aux Heat" setting. Ensure it is set to "On" if you have a wire in the W terminal; otherwise, set it to "Off" to prevent the T9 from trying to call for heat that doesn't exist.
9. Save settings and exit the menu.

If That Didn't Work

• Check for a C-Wire Power Issue: If the T9 is power-cycling or rebooting, it may lose its configuration and default back to Conventional mode. Verify that you have a dedicated Common (C) wire connected. If you are using a power extender kit or a "C-wire adapter," ensure the wires are seated deeply in the push-terminals and not touching the metal plate.
• Verify Reversing Valve Logic: If the system blows cold air when you call for heat, but the Aux heat is working, your O/B setting is inverted. Go back into the ISU menu and switch the Reversing Valve from "O" to "B" (or vice versa). This does not require rewiring, only a software change.
• Inspect the Air Handler Control Board: If the T9 is configured correctly but simultaneous heating/cooling persists, check the control board inside your indoor unit. Look for a jumper wire that might be incorrectly placed on the "Heat Pump" vs "Conventional" pins on the board itself.
• Test for a Stuck Contactor: If the outdoor compressor runs constantly regardless of the thermostat setting, you likely have a welded contactor in the outdoor unit. This is a hardware failure where the compressor is "stuck on," making the T9's settings irrelevant.

Heads Up

• Avoid "Wire Shoving": The T9 push-terminals are sensitive. If a wire is slightly frayed, it can cause an intermittent short between the Y and W terminals, triggering both stages. Trim wires cleanly to 1/4 inch of exposed copper.
• C-Wire is Mandatory: Unlike older Honeywell models, the T9 requires a constant 24VAC power supply via the C-wire. Using "power stealing" methods or batteries will lead to erratic behavior and potential Wi-Fi disconnects.
• Aux Heat Calibration: To prevent the "blast of heat" feeling, adjust the "Aux Heat Differential" in the settings. Setting this to a higher degree prevents the auxiliary strips from kicking in too early during a heat pump cycle.
• Voltage Warning: Never touch the R wire to the C wire while the system is powered; this will instantly blow the 3-amp or 5-amp fuse on your HVAC control board.