How many wires do I need to install an ecobee thermostat?

This is dependent on the type of heating/cooling system you have, as well as factors such as the stages of heating/cooling it controls.

The most common need for additional wiring when installing an ecobee comes from the lack of a C wire. Depending on your system, the Power Extender Kit (PEK) included in the box with every ecobee may be able to provide a C wire connection. However, if your system is heat-only or your thermostat controls two separate systems (such as a boiler and air conditioning), the PEK will not be compatible.

The table below shows common types of systems, the minimum amount of wires needed, and if the PEK is compatible. These minimums are a baseline and assume only one stage of heating/cooling is present and that no accessories are installed.


Type of system

Minimum wires needed

PEK compatible?


(boiler or heat-only furnace)

3 (R, W, C)
2 with FAST-STAT Common Maker (R, W)






4 (R, C, G, Y)

3 if PEK is used


Forced-air furnace

(1 stage heating, 1 stage cooling)

4  (R, G, Y, W)




Heat pump with 1 stage of auxiliary heat

5 (R, G, Y, O, W)


Dual-transformer system

(boiler with air conditioning)

6 (RC, RH, G, Y, W1, C)

5 if the G wire is repurposed as C

(RC, RH, W, G, Y)



Some important things to note:


Heat-only systems

The PEK cannot be used with heat-only systems. In this case, the following options are recommended:

  • FAST-STAT Common Maker (sold separately)
  • External 24VAC transformer (sold separately), wired to RC and C on the ecobee. This is generally recommended for older boiler systems with no clear common connection point, or in boiler zone valve setups

More information is available here: 2-wire boiler/furnace installations


Dual-transformer systems

The PEK cannot be used with dual-transformer setups. In this case, the following options are recommended:

  • Run an additional wire to the air handler/exchanger side to use as C
  • Repurpose the G wire as a C. Connect this wire to C at the air handler/exchanger, and jumper G and Y together at the air handler/exchanger

NOTE: You will lose the ability to run the fan independently with the above method

More information is available here: Dual transformer wiring diagram


Check for extra wiring behind the wall

Sometimes there are unused wires wrapped around the thermostat’s wiring bundle. Assuming these wires run all the way to your heating/cooling system, you may be able to unwrap them and use one of them as C. This is useful if you are unable to install the PEK and do not wish to run new wires.


Accessory control

If you wish to use your ecobee4 or ecobee3 to control a humidifier, dehumidifier, or ventilator, additional wiring may be needed. These wires can usually be removed from your current accessory controller, such as your humidistat, and used in the ACC+ and/or ACC- terminals on your ecobee. Please see our article on installing accessories for more information.


To fully ensure compatibility, please use our compatibility tool or email




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