installed the honeywell t9 but now my air conditioner runs at the same time as the heat. what's going on?

Diagnosis

This issue is almost always caused by a "Dual-Call" configuration error, where the thermostat is sending a signal to both the compressor (cooling) and the heating elements (auxiliary or backup heat) simultaneously. In the Honeywell T9 (RCHT9610WFSW2003), this typically happens when the system is misidentified during the initial setup—specifically, treating a heat pump system as a conventional furnace/AC system, or vice versa. When the T9 is configured for a conventional system, the W terminal is used for primary heat and the Y terminal for cooling. However, in a heat pump system, the Y terminal handles both heating and cooling (by reversing the refrigerant flow), while the W terminal is reserved exclusively for "Auxiliary Heat" (electric heat strips). If you have a heat pump but the thermostat thinks it's a conventional system, it will trigger the W terminal for heat while the Y terminal is still active, causing your AC compressor and your heat strips to fight each other. You can confirm this by checking the "Equipment Setup" menu on the T9 screen; if it lists "Conventional" but you have a heat pump, the logic is flawed.

How to Fix It

To resolve this conflict, you must first verify the physical wiring and then reconfigure the internal software logic of the T9.

1. Pop the T9 faceplate off the wall bracket. Locate the wiring terminals.
2. If you have a heat pump, ensure there is only one wire going to the O/B terminal (the reversing valve) and one wire to the Y terminal.
3. Check the W terminal. If you have a wire in W, it must be designated as "Aux Heat." If you do not have auxiliary heat strips in your air handler, this wire should be disconnected and capped with a wire nut to prevent phantom signals.
4. Snap the faceplate back on and enter the setup menu: Press Menu > Equipment > System Type.
5. Change the selection from "Conventional" to "Heat Pump."
6. Navigate to Reversing Valve and select whether your valve energizes in "Cooling" (O) or "Heating" (B). Most brands like Carrier or Trane use 'O', while Rheem or Ruud typically use 'B'. Selecting the wrong one here will cause the system to blow cold air when you want heat.
7. Go to Aux Heat and select "On" only if you actually have electric heat strips. If you have a dual-fuel system (heat pump + gas furnace), select "Dual Fuel."
8. Save the settings and exit the menu. Test the system by calling for cooling first, then heating, ensuring only one set of vents/units is active at a time.

If That Didn't Work

• Perform a Factory Reset to clear any corrupted installation profiles. Go to Menu > Advanced Setup > Reset. This forces the T9 to go through the "Guided Setup" again, where you can accurately input your equipment type.
• Inspect the C-wire (Common) connection. If the T9 is not receiving consistent 24VAC power from the C-terminal, it can occasionally "ghost trigger" relays, causing the W and Y terminals to bridge momentarily. Ensure the wire is stripped to exactly 1/4 inch and seated firmly in the terminal block.
• Check for a "Short Cycle" or "Crossover" in the wall cavity. If the wires are pinched or the insulation has rubbed off, the 24V signal from the Y wire can leak into the W wire. Pull the wall plate out 2 inches and inspect for any copper-on-copper contact.
• Verify the Reversing Valve setting again. If the AC runs when you call for heat, but the heat strips also kick in, you have the correct "Equipment Type" but the wrong "Reversing Valve" polarity (O vs B). Flip this setting in the Equipment menu.

Heads Up

• Never jump the R and W terminals while the system is powered on to "test" the heat; doing so on a heat pump system can cause a short that damages the reversing valve solenoid.
• If you are using a C-wire adapter (Power Extender Kit), ensure the adapter is installed at the furnace/air handler end, not the thermostat end, or the T9 will experience erratic switching.
• Avoid setting your "Heat" and "Cool" setpoints too close together (e.g., 68 and 72). Leave at least a 4-degree "deadband" to prevent the system from rapidly switching between heating and cooling modes.
• Always turn off the breaker to your HVAC system before removing the faceplate or adjusting wires to avoid blowing the 3-amp or 5-amp fuse on your control board.