Honeywell Home T9 Smart Thermostat wiring guide

Room-by-room sensors and the widest label support: accepts K (wire saver), L/A, Rh/Rc and E terminals. Needs a C wire or wire saver.

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What it supports

Conventional systems	Up to 2 heat / 2 cool stages
Heat pumps	2 compressor stages, 1 aux stage, separate E terminal
Dual fuel	Supported
Dual transformer (Rh/Rc)	Supported
Power / C wire	Needs a C wire, or a Honeywell wire saver with 4 wires (Y moves to K).
Line voltage / millivolt	Not compatible

Where your old wires go

Old terminal	Honeywell T9 terminal
R (single transformer)	R
Rh + Rc (dual transformer)	Rh
C (common)	C
W / W1 (heat)	W
W2 (2nd-stage heat)	W2 / Aux
AUX / W2 on a heat pump	W2 / Aux
E (emergency heat)	E
Y / Y1 (cool)	Y
Y2 (2nd-stage cool)	Y2
G (fan)	G
O, B or O/B (reversing valve)	O/B
K (wire saver)	K
L / L/A (service light)	L/A

B, X and K depend on your system. Use the tool to check. Confirm with the official guide.

Honeywell T9 with common wiring setups

Setup	Verdict
2-Wire Thermostat (Heat Only)	Works with an adapter
3-Wire Thermostat (Heat + Fan, No C Wire)	Works with an adapter
4-Wire Thermostat Wiring (No C Wire)	Works with an adapter
5-Wire Thermostat Wiring (With C Wire)	Works: direct swap
Cooling-Only Thermostat Wiring (AC + Fan)	Works: direct swap
2-Stage Heat & 2-Stage Cool Thermostat Wiring	Works: direct swap
Heat Pump Thermostat Wiring (5 Wires, No C Wire)	Works with an adapter
Heat Pump Thermostat Wiring (6 Wires With C Wire)	Works: direct swap
Heat Pump Wiring With AUX and E (Emergency Heat)	Works: direct swap
Heat Pump With a B Wire (Rheem / Ruud Style)	Works: direct swap
B Wire as Common (Trane / American Standard Style)	Works: direct swap
Rh and Rc Wires (Dual Transformer) Thermostat Wiring	Pro install recommended
K Terminal Wiring (Honeywell Wire Saver)	Works: direct swap

Check your own wiring

1Snap a photo of your old thermostat’s wires (optional)

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AI reads the terminal letters and wire colors, then you double-check them. Photos are not saved.

2Tick every terminal that has a wire

Pull the faceplate off your old thermostat. Use the letter printed next to each wire, not its color. Skip short jumper wires between two terminals.

More labels (W3, X, X2, K, PEK, L, S, T, U, HUM, DEHUM)

3A few quick questions

Do you have a heat pump?

YesNoNot sure

A heat pump is an outdoor unit that both heats and cools. Heat pump thermostats usually have an “EM HEAT” or “AUX” setting.

Do any of these apply? Thick wires joined with wire nuts, 120/240 V labels, or electric baseboard heaters (line voltage) Gas wall/floor heater or fireplace that runs without household power (millivolt) Numbered terminals (1, 2, 3, 4 or A, B, C, D), e.g. Carrier Infinity or Lennox iComfort

4Your wiring

Tick at least one terminal above and your wiring shows up here instantly.