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WiFi thermostat stopped working

Jim Connell
last month

I have a Honeywell (Resideo) WiFi thermostat (RTH9585WF) controlling a Weil-McLean steam boiler. It's been working fine for a few years. The other day I got up and the screen of the thermostat was dark. The usual stuff (unplugging and plugging it in, cycling the power) did nothing. I measured the voltage at the thermostat baseplate and it was 27V. I called Resideo tech support and they said that I needed to check the transformer.

I bought a new thermostat of the same model, and plugged it in to the existing baseplate. It stayed dark. So I'm guessing it's not the thermostat. I checked the contacts on the baseplate and they look fine with no bent pins.

I measured the voltage at the transformer and it was also 27V. The transformer is a special Weil-McLean part and costs about $125.

Nothing has been disturbed at the boiler or the thermostat and I can't imagine that the wiring has just gone bad, plus the voltages at the thermostat are what they should be.

My question is whether the 27V coming out of the transformer is high enough to shut down the thermostat. I don't think so. What else should I try?

Thanks very much!


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