installing a honeywell t9 and i have two R wires, rc and rh, where do they go?

Diagnosis

The presence of two separate "R" wires (typically labeled Rc and Rh) indicates that your HVAC system uses two independent power sources: one for cooling (Rc) and one for heating (Rh). This is most common in older homes with separate furnace and air conditioning transformers, or in systems where the heating and cooling systems are located in different areas of the house and powered by different circuits. In a standard single-transformer system, a single R wire provides 24VAC power to both the heat and cool functions. However, when you have two separate wires, the thermostat must be physically configured to accept both inputs without causing a short circuit or blowing a fuse in your control board. If you simply plug both into a single terminal or ignore the jumper setting, the thermostat will either fail to power on or will not be able to trigger one of the two systems, leaving you without heat or air conditioning.

How to Fix It

To correctly integrate dual power sources into the Honeywell Home T9 (RCHT9610WFSW2003), follow these precise steps to configure the hardware and wiring:

1. Locate the R-Switch: Look at the wiring backplate. Between the Rc and Rh terminals, there is a small, sliding plastic switch. This switch determines whether the thermostat expects a single power source (jumpered) or two separate sources.
2. Set the Switch Position: Slide the R-switch to the OFF or DOWN position. This breaks the internal connection between the Rc and Rh terminals, allowing them to operate independently.
3. Insert the Cooling Power Wire: Take the wire coming from your cooling transformer (typically red) and insert it firmly into the Rc terminal.
4. Insert the Heating Power Wire: Take the wire coming from your heating transformer (typically red) and insert it firmly into the Rh terminal.
5. Verify Connection: Gently tug on each wire to ensure the spring-loaded terminal tabs have gripped the copper core. If the wire slips, strip an additional 1/4 inch of insulation to ensure a deep seat.
6. Mount and Power Up: Snap the T9 faceplate onto the base. Once powered, navigate to the Settings > Equipment menu on the screen to ensure the thermostat recognizes both the heating and cooling stages.

If That Didn't Work

• Check for a Common Wire (C-wire): The T9 is a Wi-Fi enabled device and requires constant power. If you have dual R wires but no C-wire, the thermostat may power cycle or reboot constantly. Ensure you have a wire in the C-terminal. If you don't, you will need to install the C-Wire Adapter (Honeywell part #C-Wire-Adapter) or run a new 18/5 thermostat wire.
• Test for Voltage Mismatch: Use a multimeter to measure the voltage between Rc and C, and Rh and C. Both should read approximately 24VAC to 28VAC. If one reads 0V, you have a blown fuse on your furnace or AC control board. Check the 3-amp or 5-amp automotive-style fuses on the equipment boards.
• Verify Wire Colors: Do not rely solely on wire colors. In some older installs, a blue or yellow wire may have been used as a power source. Confirm with a voltmeter that the wires you are plugging into Rc and Rh are actually the 24V hot leads.
• Reset the Equipment Configuration: If the system doesn't trigger, go into the T9 Installer Setup menu. Ensure the "System Type" is set to "Conventional" and not "Heat Pump" unless you specifically have a heat pump system, as this changes how the Rc/Rh terminals are utilized.

Heads Up

• Avoid Shorts: Never touch the exposed copper of the Rc and Rh wires together while the system is powered; this will instantly blow the transformer fuse in your HVAC equipment.
• Wire Gauge: Ensure you are using 18-gauge thermostat wire. Using thicker house wiring (14-gauge) can damage the small terminal tabs on the T9 backplate.
• Firm Seating: The T9 terminals are "push-to-connect." If the wire is not pushed in at least 1/3 of the way, the thermostat may intermittently lose power, causing the Wi-Fi to disconnect.
• Power Cycle: After changing the R-switch position, it is recommended to flip the breaker for your HVAC system off and back on to "soft reset" the control board logic.