I have a RayPak boiler that currently has a W and R wire only. In wall i have a 5 connector thermostat cable so I have three extra wires I can connect to.

I'm a little unclear if it matters if i splice into the yellow line or the red line that I drew on the picture. Its much easier to access the yellow but I'm having trouble figuring out from this diagram where the R and W wires that go to the thermostat are, also I'm assuming that to get the power flowing correctly the C needs to come off one of these terminals and not the other. Is that correct?

Any guidance is appreciated. I'm attempting to install an Ecobee3 which mentions here that I can could pull a new 24v wire to install it: https://ecobee.zendesk.com/hc/en-us/articles/227874667

Perhaps a better question is which of the 24v lines is actually the common one in this diagram?

enter image description here


The transformer has 2 terminals, commonly called R and C. For passive thermostats, the R terminal goes to the thermostat. The "C" wire is the other one.

What's happening is that the smart thermostat needs power, which it gets by attaching to both sides of the transformer. It already has access to one side (i.e. the R wire). It needs the other. So it's the one it's not already getting.

You could try a blind guess. Try attaching your C wire to either side of the transformer. If you get the wrong side, there'll simply be 0 volts between R and C, and the thermostat won't get power. If you get it right, there'll be 24 volts AC between R and C and happy thermostat. This will only go badly wrong if you attach to the wrong transformer, or wrong side of the transformer.

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  • It took me 3 tries - but turns out the yellow wire - which i stupidly marked with a red line is the one to use! – Jeef Dec 1 '16 at 12:51

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