Honeywell Home T9 Wiring Diagram — Conventional and Heat Pump
The T9 uses standard 24V labels but the app’s configuration must match how you land O/B and Aux. This guide covers the usual terminal map and the mistakes that cause reverse-season operation.
Updated July 2026 · 6 min read · United States
Symptoms
- Heat runs when cooling is called (or vice versa)
- Aux heat never stages on a heat pump
- T9 boots then loses power without C
Likely causes
- O/B polarity configured opposite the reversing valve
- Missing C-wire
- Rc/Rh jumper left in place on dual-transformer systems
What to do
1. Conventional gas/electric mapping
Typical map: R (or Rc), C, W, Y, G. Keep the Rc-Rh jumper for single-transformer systems. Jumpers for dual fuel are not used on straight gas furnace + AC.
2. Heat pump mapping
Land O/B on the reversing valve wire, Y on compressor, G on fan, Aux/E on backup heat as labeled. In the T9 installer setup, set O/B to energize in cool or heat to match the outdoor unit.
3. Verify stages in the installer test
Run heat, cool, aux, and emergency heat tests. If the outdoor unit heats when you call for cool, flip the O/B setting — do not swap randomly at the board.
Stop and call a pro when
- Multi-stage communicating equipment
- Dual fuel with fossil furnace backup requiring outdoor lockouts
- Any uncertainty about which wire is O vs B
Related guides
- Nest C-Wire Install — Step-by-step Nest Learning Thermostat C-wire guidance: how to tell if you need a common wire, power connector options, and Rh/Rc/W/Y/G labeling.
- ecobee Without C-Wire — How to install an ecobee smart thermostat when you have no C-wire, using the included Power Extender Kit (PEK) with G/Y/R remapping.
FAQ
Does the T9 need a C-wire?
Honeywell strongly recommends a C-wire. Power-sharing modes exist but are less reliable than a true common.
Need a licensed tech instead?
If the guide says to stop — or the part swap did not clear the fault — get free quotes from licensed local HVAC contractors.
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