In trying to figure out how to hook up this Ecobee 3 Lite to an in-floor, heating system that only uses a red and white wire for each zone to open and close the valve which carries hot water, the first thing everyone says is: you need a C wire. What I leaned is “C” is nothing more than a common ground wire. So, I added one and the thermostat came on but didn’t call for heat. Essentially, without a ground, there is no way to keep power to the thermostat continually.
This is not something the average homeowner should try because it isn’t common knowledge that 24v systems are powered by 110v transformers. Touch the wrong wire and you could be dead. If you’re a technician, read on.
Originally, there were only two wires, a red and a white. Thankfully there was a 4-wire thermostat cable in the wall. That allowed me to add the other 2 needed lines to make this work.
Here’s what was needed at the thermostat:
Rc: red wire carrying 24v constant. This came from the junction block where the hot lines come into the zone control box. You will need a voltage meter set to AC to test each and find the one that is constant 24v. I removed the red thermostat wire from the board and connected it right to the 24v so now there is a wire missing in controlling the call for the valve to open.
W1: white wire from the board for the zone you are working on.
W2: a new wire needs to take the place of the red you connected to 24v in this zone. In my case there was an unused blue wire available in the thermostat wire for this zone. I connected it in the controller in the slot next to the white wire in this zone.
C: wire that is common ground in your zone box.
Be sure to turn the power off or you risk frying the whole thing. Seriously, pay an electrician or for an hour’s work and smile when it works.
