my old thermostat had a jumper between rc and rh, what do i do with it for the honeywell t9?

Diagnosis

The confusion stems from a fundamental design change between traditional mercury or basic digital thermostats and the Honeywell Home T9 (RCHT9610WFSW2003). In older systems, a physical metal jumper wire was required to bridge the Rc (Cooling power) and Rh (Heating power) terminals when the HVAC system utilized a single 24V transformer to power both functions. Without this jumper, the thermostat would only power one half of your system, leading to a situation where your heat works but your AC doesn't, or vice versa. The Honeywell T9 handles this bridge electronically through an integrated R-switch located on the wall plate. If you attempt to install a physical jumper wire into the T9 terminals, you risk creating a short circuit or damaging the internal switching logic of the device. This is because the T9 is designed to intelligently route power based on the switch position, and adding a manual jumper overrides these safety and routing protocols, potentially blowing the 3-amp or 5-amp fuse on your furnace's control board.

How to Fix It

Follow these precise steps to configure your power connection without the old jumper:

1. Remove the physical jumper wire from your old thermostat's backplate and set it aside; it is not compatible with the T9.
2. Locate the R-switch on the T9 wall plate. This is a small sliding toggle located near the R terminals.
3. Identify your wiring: If you have only one red wire coming from your wall, this is a "single-transformer" system.
4. Slide the R-switch to the **UP** position. This internally bridges the Rc and Rh circuits, replicating the function of your old jumper.
5. Insert your single red wire into the terminal labeled **R**. Ensure the wire is stripped to exactly 3/16 of an inch and pushed firmly into the spring-loaded connector.
6. If you have two separate red wires (one for heating and one for cooling), slide the R-switch to the **DOWN** position. This separates the circuits, allowing the T9 to draw independent power from two different transformers.
7. Insert the heating power wire into **Rh** and the cooling power wire into **Rc**.
8. Snap the T9 thermostat faceplate onto the wall plate and power up the system to verify both heating and cooling stages engage.

If That Didn't Work

• Check for "No Power" Error: If the screen remains blank after installation, verify that the C-wire (Common wire) is securely connected to the C terminal. The T9 requires a constant 24VAC power source; unlike some older models, it cannot rely solely on batteries for full functionality.
• Inspect the Furnace Fuse: If you accidentally installed the jumper and the unit won't power on, go to your HVAC air handler. Open the panel and check the purple 3A or orange 5A blade fuse on the control board. If the filament is broken, replace it with an identical amperage fuse.
• Verify Voltage with a Multimeter: Set your multimeter to AC voltage. Probe the R and C wires. You should read between 24V and 28V. If you read 0V, the issue is with your transformer or a tripped breaker, not the T9 switch position.
• Reset the R-Switch: If the system is heating but not cooling (or vice versa), toggle the R-switch to the opposite position, reboot the thermostat by removing it from the wall for 30 seconds, and re-seat it.

Heads Up

• Never use a "jumper wire" of any kind on the T9; the internal switch is the only approved method for bridging Rc and Rh.
• Ensure all wires are stripped cleanly. Frayed copper strands touching adjacent terminals can trigger a short that kills the control board.
• If you are using a C-wire adapter/power extender kit, the R-switch must still be set based on whether you have one or two red power wires entering the wall.
• Always turn off the breaker to your HVAC system before touching the R (power) wires to prevent sparking the control board.