Installing Ecobee3 but lack a C wire

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Old 06-25-17, 10:30 AM
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Installing Ecobee3 but lack a C wire

Hello All,

In short I am attempting to install an Ecobee3 but my current thermostats lack a C wire so I need to find where I can pull the 24V from.

I recently purchased my first home so I am rather new to the world of thermostats and HVAC. My house has 2 zones (upstairs and downstairs) and is heated through forced hot water. Here is a link to a series of pictures I will be referencing in this post. Imgur: The most awesome images on the Internet . The photos are numbered for convenience.

The current thermostats are a Honeywell model, as seen in pictures 1, 2, and 3. As seen in picture 3 it appears that the red and white wires are installed backwards.

A honeywell L8148J is installed in the furnace but I can not find an easy or obvious place to pull 24V from. See pictures 4, 5, and 6.

I believe an additional 24V transformer is installed, as seen in picture 9, but I am not 100% certain that what I photographed is in fact a transformer. It does however look like there is a C terminal marked on that "transformer". If this is in fact a 24V transformer then I imagine my best bet would be to connect to that C terminal and run a wire to where I plan to attached the Ecobee3 Power Extender Kit.

Please let me know if my logic is sound or if you have any advice. Every comment is appreciated.

Thank you.
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Old 07-08-17, 05:56 AM
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You must use the PEK (power extender kit).
Old 07-09-17, 08:26 AM
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The Honeywell L8148J transformer 24 VAC leads are connected to terminals TV and Z-W. Try connecting the C wire to Z-W.

The black wires from the transfomer to T1 & T2 are the secondary 24 volt leads. With a meter You can verify the voltages between C and Z-W by using the black wire points. One black should be zero volts to C and 24 to Z-W. The other black should be zero to Z-W and 24 to C.

Last edited by doughess; 07-09-17 at 10:23 AM.
Old 07-09-17, 01:11 PM
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Welcome to the forums.

TV = red or R
T = white or W
Z = green or Common.

Use the third wire in the thermostat cable for your C wire.
The PEK module is not needed here.
Old 01-12-18, 11:09 AM
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This is a very commonly asked question. Each installation has it's own peculiarities which is best handled in a dedicated thread so feel free to start a thread with your question.
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