Need to add a "C" Wire for an older Honeywell L8048G Controller with built-in 24V transformer in order to connect an Ecobee3 thermostat. Attached is a copy of my current wiring diagram (pre-Ecobee3 install). Would I connect my common wire up to the thermostat to the "blue" wire connected to terminal "T"?

Or, do I need to solder a lead to the back of the board; purchase an external 24V transformer; or, purchase a Fast-Stat Common Maker? The Ecobee seems to power up sometimes with the blue wire connected to it, and other times it appears "dead".

Please review current wiring and let me know if this is correct to the Taco valves and thermostat, and what my best solution is for getting a common wire to the Ecobee replacement thermostat. Thank you.

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    Your "attachment" didn't go through... Jan 1, 2017 at 19:02
  • Also, can you get us photos of this L8048G? I can't find it on Honeywell's site anywhere... Jan 1, 2017 at 19:06

1 Answer 1


It’s possible you don’t need a C (common neutral) wire when installing your Ecobee3. Check out the guide on using the Power Extension Kit that comes with your new thermostat: https://www.ecobee.com/faq/i-do-not-have-a-c-wire-or-common-wire-can-i-install-ecobee-residential-thermostats/

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