K Terminal Wiring (Honeywell Wire Saver)

A wire on K means a Honeywell wire saver module is installed at the furnace, combining fan and cooling on one wire to free up a C wire. Only some thermostats understand K.

Detected system: 1-stage heat, wire saver on K (fan + cooling), C wire present · 4 wires

Smart thermostat compatibility

ThermostatVerdictWhat to know
Nest Learning (4th gen)Pro install recommendedNo K terminal. A pro must remove the wire saver and restore G and Y.
ecobee PremiumPro install recommendedNo K terminal. A pro must remove the wire saver and restore G and Y.
ecobee EnhancedPro install recommendedNo K terminal. A pro must remove the wire saver and restore G and Y.
Honeywell T9Works: direct swapDirect swap: every wire has a matching terminal.
Amazon Smart ThermostatWorks: direct swapDirect swap: every wire has a matching terminal.

Your wiring is pre-filled below. Change any terminal to match your wall exactly.

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

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?

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?

4Your wiring

Your system: 1-stage heat, wire saver on K (fan + cooling), C wire present · 4 wires

Which smart thermostats work with your wires?

Honeywell Home T9 Smart Thermostat

Works: direct swap
Wires from the wallHoneywell T9Rred · 24 V powerRCgreen · Common (C wire)CWwhite · Heat, stage 1WKyellow · Wire saver (fan + cool)K
WireColorJobConnect toNote
Rred24 V powerR
CgreenCommon (C wire)C
WwhiteHeat, stage 1W
KyellowWire saver (fan + cool)K

Install steps

  1. Turn off power to the furnace/air handler (switch or breaker) and the AC condenser. Confirm the old thermostat is dead.
  2. Photograph the old wiring, then label each wire with the letter it came off. Ignore short jumpers.
  3. Connect each wire to the Honeywell T9 terminal shown above. Cap unused wires one at a time.
  4. Restore power and finish setup in the app.
  5. Test heat, cool and fan. If a breaker trips or equipment runs nonstop, power off and call a pro.

What you need

  • Honeywell Home T9 Smart ThermostatThe thermostat itselfCheck price

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Confirm against the official Honeywell Home install guide. The app also checks your wiring during setup.

Frequently asked questions

Will the Nest Learning (4th gen) work with k wire (honeywell wire saver)?

Pro install recommended. See the Nest Learning (4th gen) wiring diagram.

Will the ecobee Premium work with k wire (honeywell wire saver)?

Pro install recommended. See the ecobee Premium wiring diagram.

Will the ecobee Enhanced work with k wire (honeywell wire saver)?

Pro install recommended. See the ecobee Enhanced wiring diagram.

Will the Honeywell T9 work with k wire (honeywell wire saver)?

Works: direct swap. Every wire has a matching terminal, so it’s a direct swap. See the Honeywell T9 wiring diagram.

Will the Amazon Smart Thermostat work with k wire (honeywell wire saver)?

Works: direct swap. Every wire has a matching terminal, so it’s a direct swap. See the Amazon Smart Thermostat wiring diagram.

Other wiring setups