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No power to the thermostat

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kiemside
kiemside Member Posts: 10
in Gas Heating
I have a steam and gas boiler and there is no power to the thermostat. The thermostat is wired, mercury, round Honeywell.

I opened up the thermostat and used a multimeter to check for voltage on the wires on the wall and got no voltage. I disconnected the vent damper from the wires and also used the multimeter to check the wires and got 24V. What does this mean? If the transformer is bad, I shouldnt have any 24V anywhere right?

I also used the multimeter at the transformer and only got 24V between the C and R terminals. Nothing anywhere else.

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  • Jamie Hall
    Jamie Hall Member Posts: 23,286
    edited November 2018
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    Bad connection or broken wire somewhere. At the thermostat, if you take it off the wall and everything else is OK -- all your safeties closed (normal) and there is power at the transformer, you should read 24 volts, more or less, between the two wires. If you don't, somewhere there is a break or bad connection.

    Start with the other end of the thermostat wires. Disconnect them, then check if there is 24 volts on one of the two wire connections (or between them) there? If there is, reconnect and go back to the thermostat. 24 volts there? No? Double check: with the thermostat connected, but the wires at the other end disconnected, turn the thermostat all the way to call for heat. You should then read zero resistance between the ends of the disconnected wires. if not... wiring problem.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • kiemside
    kiemside Member Posts: 10
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    Do I suppose to always have 24V in the wires going to the vent damper? I thought only when you turn on the thermostat, then the thermostat sends power to the vent damper. If that is the case, then if there is no power at the thermostat, there should be no power at the vent damper too right?

    There are 24V in the wires going to the vent damper but it won't open.
  • Jamie Hall
    Jamie Hall Member Posts: 23,286
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    You need to trace out your wiring rather carefully. The vent damper is one of the safeties in your system; if it won't open, the system won't run -- for obvious reasons. You really need, at this point, to start with the transformer and trace out the circuits through all the safeties and the thermostat and then to the boiler and back to the transformer.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England

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