I want to add a C wire to support a smart thermostat. The existing wire is 2-conductor (R+W). I can run a new 18/3 easily, but I don't see an obvious place to attach the new C wire on the burner/transformer side.
The control is a Hydrostat 3150. The thermostat is connected to terminals marked T + T (which correspond to R + W on the thermostat end), but I don't see any other identifiable low-voltage wires. The transformer is soldered to the PCB board but it's clearly visible, and if I were sure which wire to use, I could probably just put an alligator clip on it. I am pretty sure (from the circuit diagram) that all the rest of the screw connectors (L1 L2 C1 C2 B1 B2 ZC ZR) are 120VAC.
Is there a place I can tap into the hot 24V output of the transformer for a C wire? Or is adding another transformer the best bet?
Best Answer
You'll need to use another transformer
Your control does not provide any access to the C terminal of the transformer, as per the ladder diagram in the manual. As a result, you'll need to run something more than a 3-wire cable, with the new transformer connecting to Rc/C on the thermostat and the existing T-T terminal wiring connecting to Rh/W on the thermostat.