I would like to replace my old thermostat with a new Ecobee3 thermostat. HVAC system is a Slantfin Galaxy GG75 boiler - single zone. (No air conditioner.)

Current set-up has only a red and white connected from thermostat to boiler. (see picture). There are unused wires from thermostat to boiler.

Connection at thermostat is to "T" and "TV" terminals (though not labelled).

Need help figuring out how the connect the common wire and where...

Thermostat manufacturer has suggested everything from not possible to get another transformer, to get a ran/isolation relay... No consistency on the best solution. Help Please.

picture of the existing wiring at boiler.  Brown wire containing 5 coloured wires comes from thermostat - only red and white in use.

enter image description here

  • NOTE: I've added the wiring diagram to my original comment above. Dec 11, 2015 at 2:02

1 Answer 1


From the pic, I'm guessing the transformer is the black device under the junction box. Looks yellow and purple coming off of that, you're connecting to the yellow, so the purple would be the common. To be sure, you'd want to connect a multimeter and verify the voltages to ground. It would also be helpful to find the wiring diagram for your boiler to be sure you're interpreting these wires correctly.

Assuming purple is the common, you'd run your wire to that, preferably through that wiring block where the other thermostat wires are connected on the left. There's no issue connecting the common to multiple places.

  • Looks good. Measure yellow and purple to make sure you have 24v A/C current.
    – JPhi1618
    Dec 10, 2015 at 19:33
  • Another clue that the purple wire is Common, is that it looks like the terminal is grounded in the junction box.
    – Tester101
    Dec 10, 2015 at 20:37
  • Thank you all for your help thus far.... So - can I connect a 'blue wire' from the thermostat directly to where the purple wire or green wire is connected to the "transformer" (I believe the "black box")? The first response above indicated connecting it to the "wiring block where the other thermostat wires are connected" however, there are no free terminals there...? Dec 10, 2015 at 20:47
  • Checked the voltage from purple to yellow/ground -- it was 27.1v Dec 11, 2015 at 2:20
  • 1
    @BMitch -- Correct - I meant to say "Voltage from yellow to purple/ground" is 27.1v. So - Im going to loosen the screw that has the Purple and Green/Ground at the transformer, and a connect a blue wire from the thermostat to that screw, and tighten back the screw terminal ---- then use blue at the thermostat as the common / c-wire . – Andrew MacPherson 13 mins ago Dec 11, 2015 at 20:14

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