Electrical – Where to connect thermostat C wire to Weil-McLain HE2 series 1 boiler

electrical

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

Best Answer

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.

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