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i have a 5 zone radiant floor, oil fired heating system. Each thermostat only has 2 wires running to it. i have a TACO SR506 switching relay that receives the wires from each thermostat and controls the circulators and burner. The TACO relay has a 24v transformer inside. Can i attach a single wire to the COM terminal and run a wire to each thermostat that i want to upgrade?

UPDATE: due to finished ceilings in the basement, i have no access to the thermostat wires and cannot snake a wire to the COM terminal on the transformer. But a ground is ground, right, so i am wondering if i could simply run a wire to a nearby ground wire inside an outlet?!?!? I tested this idea with a multimeter and i get 26V. it seems like it will work... what do you think???

  • Can anyone comment on the use of the house ground instead of the 'C' wire?? – Mark Feb 24 '16 at 18:56
  • Isn't there a Standard Accepted Answer for this by now? – keshlam Apr 20 '16 at 23:10
  • Is the transformer grounded on the secondary side (i.e. is the common terminal of the transformer grounded)? – Tester101 Dec 27 '16 at 17:34
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The "C" in C-Wire stands for "Common", so what you propose should work. The 24VAC transformer has two wires coming from it. One of the wires goes to the thermostat and that is what the thermostat actually switches on and off. The other wire is typically labeled as "common" and it goes directly to the relay(s) that the thermostat controls.

To say it another way... older thermostats didn't need power to run, so only having one wire from the transformer was fine. For a new powered thermostat, you need both wires from the transformer, and that's all the C-wire is - the "other" wire from the transformer.

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