Two zones/thermostats each with 3 wire cable running from boiler room. It appears only 2 of the wires are hooked up (Red and White). There is a 3rd unused and not-hooked-up wire that is dark blue, for each zone.

I'm looking to provide power to the 3rd wire for each zone so it can function as Common.

I have one 24v transformer at the boiler side. I have a Hydrostat 3250 that does not have a Common wire 24v hookup.

How can I hook up the 2 unused wires so they can provide power to a smart thermostat (Ecobee)?

I have two FastStat Common Makers that I'm prepared to use, but I'm wondering a) if they're even needed considering I already have a 24v transformer nearby, and b) if needed, how would I hook them up?

Hydrostat 3250

setup with 2 zones

24v transformer

unused wires for each zone

zone valves

1 Answer 1


The two spare wires can be used as C. You definitely have C, it's one of the terminals on the transformer on the wall. We can't tell which one from the information you've given us.

Your Hydrostat controller doesn't need it because it uses 120 volts. Your Ecobees do need it and you don't need the Faststat. Just trace the wires, one of the terminals on the transformer will go to thermostat R. Connect the blue wires to the other side of the transformer. It looks like the wire from the transformer goes to where the wires from the thermostats are, so you shouldn't need to run any wires, just hook them up.

  • I traced the wires coming from the transformer. Red wire coming from transformer C goes to thermostats. White wire coming from transformer R goes to the zone valves (actuators). So to confirm - the blue wires should connect to the White wires which go to the zone valves?
    – jflbball
    Feb 25 at 5:22
  • Almost certainly. You can confirm ... The other two wires on the valve should go to the thermostat and the boiler. The valve's third terminal, that isn't those two, is C. That should be those white wires.
    – jay613
    Feb 25 at 9:40
  • i tried connecting to white. No luck. Also tried connecting to red. No luck. The ecobee powers up using the 24v wall adapter to the electrical outlet, but won't power up when hooking the blue line to C. Any other ideas?
    – jflbball
    Mar 5 at 3:17

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