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Weil Mclain boiler reset issue

I have a Weil Mclain Ultra 155 hot water boiler connected to a Weil Mclain Indirect Hot water tank. During the summer when there is no call for heat, the boiler errors out frequently with a code of E18. According to manual this means "Outlet water temperature was higher than 210 degrees. I reset basically every morning and then in works ok until night at which time it usually flips to the error code again. Does not ever happen during heating season. I have had a couple plumbers look at it but neither resolved the issue. They both just serviced the boiler. I am no boiler technician but this seems like it might be a thermostat issue or some thing of that nature? Anyone have any experience with this? I have called Weil Mclain when I have the code flashing but because I am not a liscensed plumber they basically don't want to hear from me, so that hasn't been helpful.


  • 109A_5
    109A_5 Member Posts: 1,385
    Hello @rlugg88,

    With the AC power to the boiler off, I would inspect all the wiring connections to supply temperature sensor (the upper item 29 in the drawing) and re-seat the connectors at the control board to where the supply temperature sensor wires connect.

    Also the supply and return sensors probably can be swapped and see it the symptoms change (both items 29). You may just have a flaky supply side sensor.

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System