How-to guide

ecobee PEK Wiring: Installing the Power Extender Kit

By Ismail Farooq · Updated 2026-10-01

The Power Extender Kit (PEK) is a small module that ecobee includes with the Smart Thermostat Premium and Enhanced. It lets the thermostat get the constant power it needs from a four-wire system that has no C wire, without running new cable.

Do you need the PEK?

• You have a C wire, or a spare wire you can connect to C at both ends: you don’t need the PEK. A real C wire is simpler and more reliable.
• You have exactly four wires on R, W, Y and G (a furnace with AC, no C wire): this is what the PEK is for. See the 4-wire setup.
• Two or three wires: the PEK needs four, so it can’t help. See the 2-wire and 3-wire setups for the options.
• Heat pumps, dual transformers and other setups without a C wire: check ecobee’s own compatibility guidance for your system before relying on the PEK, or run new 18/5 cable. Adapters like this assume one transformer.

How it works

Normally five wires are needed: R and C for power, plus W, Y and G for heat, cooling and fan. The PEK sits at the furnace end of your four-wire cable. It connects to all five terminals on the furnace board, and uses the four wires in the wall to carry both power and the three control signals. The thermostat and the PEK work together to tell the signals apart.

Wiring overview

Always follow the diagram in ecobee’s instructions for your kit. In outline:

Step by step

1. Turn off power at the furnace or air handler.
2. Photograph the furnace board and the thermostat wiring before disconnecting anything.
3. At the furnace, disconnect the four thermostat-cable wires from the board’s R, W, Y and G terminals. Leave the outdoor unit’s wires on the board as they are.
4. Connect the PEK’s leads to the board’s R, C, W, Y and G terminals.
5. Connect the cable’s four wires to the matching terminals on the PEK.
6. Secure the PEK inside the cabinet as the instructions show, away from burners, the flue and moving parts, and put the furnace door back on firmly. Most furnaces cut power while the door is off.
7. At the wall, connect the ecobee: R to Rc, W to W1, Y to Y1, and the former G wire to PEK.
8. Restore power. The thermostat should start up and show the PEK in its wiring list. Finish setup, then test heat, cool and fan.

If something doesn’t work

• Thermostat doesn’t turn on. Check the furnace door switch, service switch and the 3 or 5 A fuse on the board. Then confirm the PEK’s R and C leads are on the board’s R and C, and every wire is tight.
• Heat, cooling or fan does the wrong thing. Two wires are usually swapped at the PEK, so a signal arrives on the wrong terminal. Check each wire goes to the same letter on the PEK that it was on at the board, and that the wire on the PEK terminal at the thermostat is the one from G.
• A wire isn’t detected. Reseat it with fresh, straight copper. See thermostat wire not detected.
• The fuse blows. A bare wire is touching another or the metal cabinet. Turn power off and find it before replacing the fuse. See the troubleshooting checklist.

Switching from ecobee to another brand

Only ecobee thermostats understand the PEK’s signals. If you replace an ecobee that used a PEK with another brand, remove the PEK at the furnace, reconnect the four cable wires to the board’s R, W, Y and G, and then use the new brand’s own power option. See no C wire options for each brand.

PEK or new wire?

If the furnace is easy to reach, for example directly below in an unfinished basement, running new 18/5 or 18/7 cable gives you a real C wire plus spares and works with any thermostat. The PEK is the better choice when pulling new cable through finished walls isn’t practical. See thermostat wire and running new cable, or tick your wires in the wiring tool to see every option for your system.