ecobee3 installation & troubleshooting

The ecobee3 is designed for 24VAC HVAC systems. Millivolt or Line-voltage will not be directly compatible. The sub-base terminals will not accept wires that are larger than 18-gauge and should be a solid strand type wire otherwise difficulty will occur when inserting the wires into the terminal blocks. It helps to understand the basics of  your HVAC equipment that you have. For example Conventional HVAC system vs. Heat-Pump, how many stages of heat/cool the system, etc. The ecobee3 attempts to automate the configuration based on wires inserted however should there be an issue, having HVAC equipment details will aid in your configuration & trouble-shooting.


Auto-Wire Detection Issues

ecobee3 is designed to automatically detect wires inserted into the terminal blocks. This relies on a minimum AC voltage threshold (approx. 11VAC) for detection to work. Here are the scenarios that could possible effect wire detection either during or after the installation.

  • Heating boilers that require only an open/close dry-contact.
  • Zone control boards that may/ may not provide a voltage signal to be detected.
  • Ancillary controls that will create a voltage drop.
  • Any type of equipment that uses a form of isolation relay.
  • HVAC equipment using DC voltages. (Prevalent in commercial applications, don't know of any in residential).
  • Millivolt systems- typically fireplaces.
  • High/Low limit controls within HVAC equipment. Some brands will disrupt power to R therefore the thermostat will be powered off during a high limit scenario, other brands will disrupt power from W only to stop the burner control.
  • A condensation pump with safety switch. The Y output from t-stat will be wired in line with an overflow safety switch and when activated (opened) will break 24V to AC control circuit.  This will cause ecobee3 to think a wiring change has occurred.


Power Issues

During your trouble-shooting process, the key is identifying and verifying your current wiring configuration. Check the thermostat sub-base and HVAC control module or any other component included into the configuration. Important things to determine are:

  • Is the current HVAC setup using one or two transformers? You may not know this however a simple way is to check “did the existing thermostat have a jumper between RH/RC”? In certain cases it had two wires (one each) for RH & RC. This does not always mean you have two transformers, it could be two wires wired directly together at the R(24V) terminal at the furnace control board. You will need to confirm this by accessing the HVAC control board.



  • Are you using the PEK? If so confirm exactly how this is wired? If wires are not getting detected on first-run setup go through the normal steps of double-checking wiring at thermostat, PEK and furnace control board. Verify you have/have not any other ancillary devices/controls inline that are causing insufficient voltage to be not detected by E3 auto-wire detection. 11VAC is the minimum threshold for this to work. When using PEK the R-wire from PEK to ecobee3 sub-base must connect to the RC terminal (as per manual).


Important !  The C wire at ecobee3 should not be left connected in the C terminal block



Wiring/Installation  - Conventional HVAC & Boiler


  • Note 1: with a 2 transformer configuration RC & C must be connected to the same transformer.
  • Note 2: with a 2 transformer configuration PEK module cannot be used.



Wiring/Installation - Heat Pump with Air Handler/Furnace



  • All heat-pumps have a default mode. Is the default mode heating or cooling? This important to know because it will be required during the configuration of the O/B terminal. If incorrectly configured the heat-pump will provide heat output when it should be cooling and vise-versa when in cooling mode.
  • If a wire is detected on the O/B terminal the ecobee3 will automatically assume it is going to be configured for heat-pump control.


A link for various ecobee3 wiring diagrams:



        Wiring/Installation – Using PEK module

PEK wiring.PNG


Note: R(24V) will always connect to the RC terminal on ecobee3 when using PEK.

  • One word of caution, if PEK is wired at the furnace, the C wire at ecobee3 should not be left in the C terminal block!  The false 24VAC on C may confuse the ecobee3 detection algorithm into thinking PEK is not installed.  If PEK is installed and ecobee3 thinks there is no PEK, then the first time the fan is turned on, it will blow the furnace fuse and cause failure.   

PEK Installation video link:


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