I have a Weil-McLain HE2 series 1 boiler and am attempting to attach a Honeywell RTH6500 wifi thermostat to the main floor. The house has 4 zones and therefor 4 separate thermostats. The main floor thermostat has an extra (green wire) in addition to a red and white at the thermostat. There is only heat, no AC, fueled by propane. I attached the thermostat on the main floor and with the Cwire attached to the neutral side of the external transformer, the thermostat powers up, but cycles off and on when calling for heat. There is an additional transformer inside the boiler unit with a pink and brown wire. Where should I attach the C wire at the boiler?enter image description here

  • What is the external transformer for, it doesn't appear on the schematic you included? From the schematic, it looks like one side of the transformer is grounded, so you may be able to attach C to ground. Though if there's a zone controller involved, all bets are off. You'll have to include the schematic for that as well.
    – Tester101
    Commented Sep 21, 2015 at 1:25
  • It is attached and powered by the boiler switch. Then the hot end of the transformer, which actually shows 29 volts, leads to each of the 4 valves and the control board where it shows Control 24VAC.
    – G. Merc
    Commented Sep 21, 2015 at 1:48
  • What model valves are you using? What is the volt-ampere (VA) rating of the external transformer?
    – Tester101
    Commented Sep 21, 2015 at 2:37
  • Taco zone valve 571-2. Screw #1 wire 26 volts connects to a wire where all 4 valve screw #1 are connected. Screw #2 wire is 26 volts and connects all valve screw#2 together and comes from the external transformer. Screw # 3 wire is neutral and connects all valve screw 3 together and comes from the external transformer. That transformer is a 40 amp.
    – G. Merc
    Commented Sep 21, 2015 at 12:11
  • I only want to add 1 wifi thermostat to one zone valve, doesn't it make sense to add that C wire to only that zone valve? Tried it, doesn't work. The thermostat blinks off and on.
    – G. Merc
    Commented Sep 21, 2015 at 12:15

2 Answers 2


Both this document (PDF) and this document (PDF), say that one 24V-40VA transformer can only support 3 zone valves. Since you have 4 valves, you're over the recommended number of valves. Once you added the demand of the WiFi thermostat, you likely overwhelmed the transformer. The answer here, may be to install an additional transformer.

There's a diagram in the second document, that shows how to wire in an additional transformer. This document also has a good explanation of how the valve actually works, which might be worth a read.

571-2 Taco-Zone valve additional zone wiring

Based on these documents, a WiFi thermostat should be wired in like this.

571-2 Taco-Zone valve additional zone wiring Wifi thermostat

Based on the your description of the wiring, it sounds like it's a bit different than that of the documentation. Incorrectly wired valves might also be the problem. Based on the documentation, terminals 1 and 2 are connected through a heater, while terminals 2 and 3 are connected only when the valve is open.

When the thermostat closes (calls for heat), current flows from terminal 1, through the heater, to terminal 2. Once the valve opens, a contact allows current to flow between terminals 2 and 3. So basically energizing 1 and 2 opens the valve, and energizing 2 and 3 signals the boiler to circulate.

  • Good catch on the 3 valves per transformer. Is that why they have that second (external-outside the boiler) transformer? And should I change that out to a 24-40? And where would I connect that C common wire for that one zone valve?
    – G. Merc
    Commented Sep 22, 2015 at 14:27
  • @G.Merc According to the document, you should have 2 external transformers for the valves. And that's in addition to the one in the boiler. So that's 3 total transformers. If you look at the last image in my answer, the blue line represents the C wire.
    – Tester101
    Commented Sep 22, 2015 at 19:47
  • OK, I think I got it. I'll give it a shot. I really appreciate all your time and responses. I'll let you know. Thank You.
    – G. Merc
    Commented Sep 23, 2015 at 0:40
  • Tester101- Do you think I can get away with swapping out that 24V-20VA with a 24-40VA? Do you think that would do it?
    – G. Merc
    Commented Sep 23, 2015 at 15:12
  • @G.Merc I'm just going off the documentation here, and it says you need two 24V-40VA transformers for your setup.
    – Tester101
    Commented Sep 23, 2015 at 15:20

Final Solution: added a 24VAC/20VA transformer. Wired it into the on/off switch on the side of the boiler. Needed 4 wires to the thermostat, but I only had a three, two of which ( red, white) were being used. So fished 2 more wires from the thermostat to the boiler. At the thermostat, removed the jumper between R and RC, and hooked one wire on RC. Hooked a second wire at C ( common). There were now 4 wires connected to the thermostat, R, RC, W, and C. At the boiler connected the two new wires to the new transformer, doesn't matter which sides. I now have 24 volts (actually like 28 but that's fine) to the R, C, and RC terminals at the thermostat. Works perfectly, Finished, End of Story.

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