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No power at thermostat - transformer issue??

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4lane
4lane Member Posts: 22
in Thermostats and Controls
I have an old Weil McLain gas boiler and the smart thermostat suddenly had no power, so no heat. At the boiler I found it had power, pilot was lit - I didn’t find anything wrong. After some time flipping the breaker and trying to figure out why there was no power back to the thermostat, it suddenly came back on. Is this a sign of a potential transformer issue or where should I start to track down the issue?

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  • HVACNUT
    HVACNUT Member Posts: 5,841
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    Is your smart thermostat wired for 24v?
    What model boiler? Its standing pilot but is there a motorized flue damper?
    Is that the only thermostat or are there other zones, or maybe an indirect water heater?
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,546
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    @4lane

    How long has the smart stat been installed? o you have a common wire run to it?
  • Jamie Hall
    Jamie Hall Member Posts: 23,330
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    Not a transformer issue -- a thermostat issue, almost certainly.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • 4lane
    4lane Member Posts: 22
    edited December 2021
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    It’s an ecobee thermostat which has been in place with C wire for about 3 months with a née three wire put in. It’s a Weil McClain CGM-6. There is no motorized flue and is the only thermostat connected.
  • Jamie Hall
    Jamie Hall Member Posts: 23,330
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    Do you get power between C and R? At the thermostat? At the other end of the cable?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • 4lane
    4lane Member Posts: 22
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    My voltage tester doesn’t test down to 24v unfortunately. If the wires had 24v at either end if the golden question.
  • Jamie Hall
    Jamie Hall Member Posts: 23,330
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    Get a multimeter -- they're cheap at the big box -- and find out.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    WMno57
  • HVACNUT
    HVACNUT Member Posts: 5,841
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    4lane said:
    It’s an ecobee thermostat which has been in place with C wire for about 3 months with a née three wire put in. 
    Boy you weren't kidding when you said old Weil McLain. 

    Is that supposed to read, "with a new 3 wire put in"?

    The thermostat is wired R, W, and C, and at the boiler, on the control relay you should have 2 leads labeled thermostat. One lead from R on the transformer to R on the thermostat. And one lead from G on the transformer to W on the thermostat. C on the transformer to C on the thermostat. 
    Is it possible R and W are reversed?
  • 4lane
    4lane Member Posts: 22
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    No it’s wired correctly. Again, it has been in operation without issues for 3 months. And it’s working normally now so I can’t test for power at both ends of the thermostat wire. So my question is still: What could have lead to the situation where there was no power at the thermostat but the boiler had power. Yes the thermostat could have lost its connection somehow but that’s a remote possibility. Could the transformer have had an issue where it stopped working and wasn’t feeding power back to the thermostat?
  • Jamie Hall
    Jamie Hall Member Posts: 23,330
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    Your description sounds like a bad connection. Hard to say where.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • HomerJSmith
    HomerJSmith Member Posts: 2,441
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    Not enough information about your sys. This is the way it is suppose to work. A 24 V transformer has two 110 V leads and two 24 V leads. One 24 V lead is 24 Vac and the other 24 V lead is the secondary common. The thermostat is just a temperature activated switch with added functionality. A switch just turns something on. It could turn on a zone valve, a zone valve controller, switching relay, aquastat, etc. Take a zone valve, the thermostat closes the switch and turns on a motor in the zone valve that has a lever that depresses a micro switch which closes and turns on the aquastat and energizes the gas valve and the circulator. The power comes from the aquastat transformer 24 Vac to the zone valve tie point then (uninterupted) to the thermostat R connection thru the switch to the W connection to the zone valve motor to the aquastat relay running the pump and gas valve to the aquastat transformer common. That's a circuit.

    A smart thermostat has WiFi functionality which runs off of a charging battery which is why you need a wire from the C terminal on the thermostat to the common on the aquastat transformer. 24Vac from the transformer goes to the R connection (uninterupted) on the thermostat thru the battery charging circuitry to the C connection back to the common on the aquastat transformer. That's a circuit.

    So how do you know that you don't have any power at the thermostat with out measuring it? You should see 24Vac between R and C and 24 Vac between R and W.

    You need to supply more information or pics of your sys.

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