Heating & Cooling

How the ecobee Power Extender Kit (PEK) Works: Explained

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If you have an old analog thermostat and want to upgrade to a new smart one with ecobee, you might be thinking it’s going to be a big job. While it may take some modifications to your current HVAC system, it’s not as difficult as you may think. 

For older HVAC systems without a constant power supply (C-Wire), you can use an ecobee Power Extender Kit (PEK). This kit allows you to use an ecobee smart thermostat on older systems that do not have much digital technology. The PEK is easy to install and doesn’t require you to run any new wiring.

The ecobee PEK makes converting your system super easy, so there is no reason to be intimidated or think you can’t enjoy the benefits of a smart thermostat. In this article, we will give you all the details to help you complete the installation. 

What Does PEK Wire On ecobee Do?

It’s likely that when you think of a thermostat, you imagine a brass hemisphere on a hallway wall that never really did anything. It was just something your Dad told you never to touch or adjust. 

Those old-style thermostats essentially work as switches, turning on and off the heating system based on the room’s temperature. This was facilitated by a liquid metal, such as mercury, in a glass tube on a metal coil.

These systems often require only two wires, one with electricity flowing to the thermostat and one back to the furnace. When the thermostat is warmer than the set temperature, the switch is open, and the electrical circuit is open or not completed.

The metal coil constricts when the temperature drops, causing the mercury to contact the electrode and complete the circuit. When the circuit is closed, electricity flows and tells the thermostat to turn until the circuit is interrupted again.

This was a simple and ingenious system that worked reliably well for setting just one temperature. 

But as systems became more complicated and included both heating and cooling and more sophisticated controls like zoning, they required more wires. 

Depending on your home’s and HVAC system’s age, there may be 2 and 5 wires running to the thermostat. One wire that is important for the ecobee is the Common wire, also called the C-Wire. 

The C-Wire gives a constant power supply of 24 volts to the thermostat and is usually present on some digital and all smart thermostats. It is required to run any thermostat that uses WiFi like your ecobee. What a PEK does is provide a constant source of power even without a C-wire.

Read on to know more about how that works.

How Do You Know if You Need the ecobee Power Extender Kit?

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To find out if you need an ecobee Power Extender Kit, you will need to check the wires on your existing thermostat. You can do this in 4 easy steps:

  1. Turn off the power to the thermostat either at the breaker box or with the switch usually located next to the furnace.
  1. Check that the power is off by adjusting the temperature and seeing if the furnace or air-conditioner starts up.
  1. Remove the thermostat from the wall by pulling it away from the wall mounting plate. Most snap on and off but check to see if any screws are holding it in place if that doesn’t work. 
  1. Look at the wires in the terminal on the back of the thermostat and check how they are labeled.

All systems are slightly different, but most likely, you will see 2 to 5 wires labeled Y, G, C, U, or A. If the C-wire is present, then you can use ecobee as is, without the PEK. 

If the slot labeled ‘C’ is empty, you will also need to install the PEK to have the full function of your ecobee.

How Does the ecobee PEK Work?

The ecobee PEK provides the constant electrical power needed to run the display and connect to WiFi in systems without a C-Wire. 

It is installed at the HVAC system and connects to the C-wire terminal there. But, since the wires running to your thermostat don’t have a C-wire, it instead uses the R-Wire. 

In this case, ‘R’ stands for red wire and is the wire we discussed earlier that carries electricity to the thermostat in a normal situation. When using the ecobee PEK, constant power will be sent through the R wire to supply the thermostat. 

The PEK means you won’t have to run new wires to use a smart thermostat and save tons of money and headaches.

A note here, if you see wires labeled ‘Rc’ or ‘Rh’ instead of just ‘R,’ that is normal. These stand for Red-cooling and Red-heating and will go to either your air conditioner or furnace. 

How to Install the ecobee Power Extender Kit

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ecobee designed the PEK to be easy and convenient to install. Since the existing wires will most likely already be stripped and secured to the terminals, the only tool you will need is a small flat head screwdriver. 

The installation will vary slightly depending on whether or not your system has a control board, and we will cover both below. 

ecobee PEK Adapter Installation with a Control Board

If your air conditioner or furnace has a control board, the installation process will be more intuitive as everything will already have a label. You can use the following steps to install your PEK:

  1. Turn off power to the HVAC system before doing work with any wiring. Although it is low voltage and most likely will not harm you, it can cause shorts that may cause other issues to your furnace or air conditioner. 
  1. Locate where the thermostat wires are connected to the control board. There will often be a terminal bus bar that locks in the wires using a screw or snap system. It is good to take a picture of this before doing any work, just for reference later on if things get mixed up. 
  1. Locate the wires labeled R, G, W, and Y, and use the enclosed stickers to label them so they will be easy to work with later. 
  1. Remove the four wires from the terminal.
  1. Attach the four wires to the respective receptacles in the PEK.
  1. Attach the five wires (R, G, W, Y, and C) from the PEK to the control board terminal.
  1. Go back to the location of the old thermostat, and install the four wires into the ecobee wall mount as follows: R wire -> Rc or Rh terminal, G wire -> C terminal, Y wire -> PEK terminal, and W wire -> W1 terminal. Snap the ecobee in place.
  1. Restore power and check that the ecobee is working correctly.

Installing the ecobee PEK to an HVAC System Without a Board

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Installing the PEK to an HVAC system without a board is not difficult but will require some additional steps and extra care during the process. The main issues with not having a board are:

  • No labeled terminals indicating which wires are the R, G, W, and Y
  • Instead of simple terminal receptacles, you will need to use wires nuts to connect two wires

The process for this is as follows:

  1. Turn off power to the HVAC system and thermostat before opening it or doing any work.
  1. Locate where the thermostat wires connect to the furnace or air conditioner.
  1. Locate the schematics diagram, usually on the furnace door or the back of the air conditioning unit. 
  1. Use the stickers that came with the ecobee to label each wire as either R, G, W, or Y. Refer to the schematic as you go, to help you identify the correct wires. Because your existing system did not use the C-wire, you will need to locate using the schematic diagram. 
  1. Once you have located and labeled all the wires, you can remove the wire nuts. Remove the old thermostat wires and attach the wires coming from the PEK into their respective locations. It may be easiest to do this one by one to avoid getting mixed up. A note here; if you connect stranded wires to solid wires, do not just push them into the wire nut, as generally, that will only grip the solid wire. Instead, twist the two wires securely together, then insert into the wire nut and secure them. 
  1. Once all the R, G, W, Y, and C wires from the PEK have been connected to the wires in the HVAC system, you are ready to join the old thermostat wires into the respective terminals of the PEK unit
  1. Now, you are ready to install the ecobee mounting plate. Connect the four wires into the ecobee wall mount as follows: R wire -> Rc or Rh terminal, G wire -> C terminal, Y wire -> PEK terminal, and W wire -> W1 terminal. Snap the ecobee unit into place.
  1. Restore power and confirm that the ecobee is working properly.

Can You Use ecobee if Your Thermostat Only Has 2 Wires?

You can still use an ecobee when replacing a thermostat with only two wires. However, you may require additional materials. 

Sometimes, the thermostat cable will contain 4 or 5 wires, but only two are in use. In this case, you can still use an ecobee. 

However, in cases where there are only two wires, such as in a heat-only system, you will not be able to use the ecobee with just the PEK unit. Instead, you will need to run new wires to the thermostat or install an external 24-volt transformer. 

The first step will be to remove your old thermostat and check if there are any unused wires in the cable behind the wall. It will only use two wires in some cases, but the thermostat cable will contain other wires. 

The number of wires present will determine how you configure your wiring later on. 

Next, check your furnace to see the wiring configuration. Follow the thermostat cable to see where the wires are connected and note the labels. Most likely, it will have a label indicating one of the following configurations:

  • R, W, and C
  • Th, TR, and Th-Tr
  • T/T

How to Setup to R, W, C Configuration

If the thermostat cable used more than just the two wires, simply hook up one of the unused wires to the C connector at both the furnace and the ecobee wall plate. 

If there are no extra or unused wires in the thermostat cable, you may need to run a new wire or purchase a Fast-Stat Common Maker. 

How to Setup to Th, Tr Configuration

This configuration usually means that the thermostat wires connect to a gas valve. A gas valve means that you will need to locate the 24-volt transformer and hook up a write to it. 

As with the R, W, C configuration above, you will either utilize one of the unused wires in the thermostat bundle or use the Fast-Stat Common Maker. 

How to Set-up to T/T Configuration

If you have a T/T configuration, you have a boiler using an Aquastat system. An Aquastat is like a thermostat, but while a thermostat monitors air temperature, the Aquastat monitors water temperature. 

In this case, you will most likely need to purchase an additional 24-volt power supply to run the ecobee. 

Does ecobee Include a PEK?

Currently, the ecobee 3 lite and the ecobee 4 come with the PEK in the box. It is only the ecobee 3 that does not come standard with the PEK. 

If you are upgrading your system from one ecobee to another, you should be aware that the PEK for the original ecobee 3 is not the same as the ones that come with the ecobee 3 lite or the ecobee 4. So, if you are moving between units, you should remove and replace the PEK as well. 

Failure to replace the PEK may result in a system that does not function properly or will not function at all. 

If you are unsure which ecobee model you have, you can check in the ‘About’ section of the main menu. 

Does ecobee Work With Millivolt Systems?

Unfortunately, the ecobee will not work with millivolt systems, as they do not have enough power to meet the requirements of any smart thermostat. 

If you have a system that uses millivolts instead of a 24-volt transformer (such as a wood stove), you will need additional hardware. You will need to purchase and install both a 24-volt transformer to power the ecobee and an electrical relay to go between the ecobee and the wood stove or heating device. 

Since this requires modification of your heating system, you may want to call ecobee technical support or hire a professional to complete such a setup.


We hope that this article has helped you understand how to use ecobee with your HVAC system. Depending on your system, there are various ways to modify it to use an ecobee smart thermostat. 

In most cases, the only thing you will need is the PEK, which ecobee includes with most models. In other cases, you may need to purchase a 24-volt transformer or additional wires. 

Some systems that cannot use the ecobee thermostats are the DC HVAC systems in recreational vehicles and the high voltage systems in some baseboard heaters. 

Other than those cases, there should be a way to take advantage of the ecobee smart thermostat to increase your comfort while saving energy in your home. 



Hubert Miles | Licensed Home Inspector, CMI, CPI

Hubert Miles is a licensed home inspector (RBI# 2556) with more than two decades of experience in inspection and construction. Since 2008, he has been serving South Carolina through his company, Patriot Home Inspections LLC. As a Certified Master Inspector, Hubert is dedicated to providing his expertise in home inspections, repairs, maintenance, and DIY projects.