Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Problem from an incorrectly installed thermostat?

BillF Member Posts: 1
My heating contractor recently installed a digital Wi-Fi thermostat for me. Within about two weeks, both my fan motor and the relay for the igniter, limit switch, and fan were burned out. The service technician also discovered that I had charred and melted connections on the relay. He discovered that the circuit was carrying an excessively high amperage. In troubleshooting the problem the tech learned that with the new thermostat the connector to the fan side of the limit switch was not needed. When he disconnected this connector the high amperage went away. The tech told me that having the fan limit connected would not generate the high amperage problem. He said that it was most probably an issue of older wires in the same conduit bleeding through, and that by disconnecting the fan limit the bleed through went away. My question is whether the tech’s explanation is plausible or is he feeding me a line to cover his mistake. Thanks for your help.


  • Jamie Hall
    Jamie Hall Member Posts: 21,105
    Not really enough information. One would need a wiring diagram of the installation -- and of all the various oddments connected -- and then test all the wires. But the short answer -- yes, a short in the wiring somewhere could easily cause the problem. Among other possibilities...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • pecmsg
    pecmsg Member Posts: 3,878
    What Digital WiFi Stat?
    HVACNUT Member Posts: 5,025
    Do you mean the primary control (with the reset button)?

    A limit should NEVER EVER be bypassed or disconnected.

    Is there air conditioning being controlled from the same thermostat?

    Worst case he needed a common for the thermostat.

    The control side of the furnace should be low volt while the operating side is line volt. So how did a thermostat burn up line voltage parts? Could happen, but that's one reason for using relays in the first place.

    It's quite possible this is a coincidence. Was the WiFi stat working?

    For the limit example alone, I would call someone else.
    EBEBRATT-Ed Member Posts: 13,437
    Sounds fishy to me
  • Jolly Bodger
    Jolly Bodger Member Posts: 209
    Something does not sound right. as @HVACNUT said, the thermostat is low volt and the parts that burned out are high volt. Could be a case where the thermostat and the fan/limit were trying to run multiple speeds of the fan motor, that would burn it out.
  • Jamie Hall
    Jamie Hall Member Posts: 21,105
    I hope I'm wrong, but rereading I have to ask... the low voltage thermostat wires are in a separate conduit from any high (line) voltage wires? They can't possibly come in contact?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England