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How often should the boiler limit switch be reached?

finiuch Member Posts: 6
We have a boiler/radiator setup and the boiler is likely from the mid eighties. The only electrical pieces are the aquastat and the burner valve.

On long set-back recoveries (every morning), the boiler will reach its 180F limit in about an hour of straight running and cuts off the 110V to the transformer. This in turn powers off the burner and keeps the circulator running. It also powers off the thermostat which is annoying. The radiators are free of air and the circulator runs fine.

Is there a solution to this?


  • SWEI
    SWEI Member Posts: 7,356
    How long has this been happening?

    Deep setbacks are generally not a good idea with radiant heat.  Did you recently acquire the property?
  • finiuch
    finiuch Member Posts: 6
    Suspect something is wrong with the boiler/setup

    No, but the problem have only been made noticeable recently.

    When we moved in 4-years ago, I drained the system (needed to remove and by-pass a rad), filled and bled the rads. I replaced the old mercury t-stat with a digital battery powered one and scheduled for deep set backs.

    Recently replaced the tstat with another one with no batteries and noticed that the aquastat was cutting it off after a while. As far as I can guess, the old battery powered tstat could have been cut off by the aquastat all the time and not cared. The water does reach the 180F set on the aquastat, the pressure remains at around 12-15psi, and the circulator does come-on and stay on.

    I've thought about moving the transformer out of the aquastat circuit so when it trips, it cuts off only the burner valve not the 24V common line to the tstat. But I don't want to muck with a system I barely understand.

    p.s. thanks for responding.
  • kcopp
    kcopp Member Posts: 4,256
    I agree w/ the ...

    thoughts on deep set backs. 5 degrees at most.

    Sounds like the new t-stat is a power robbing/ stealing t-stat. I would use a t-stat that is battery powered... 5 bucks the problem will be gone.
  • Terry O
    Terry O Member Posts: 67
    Power stealong stats are often....

    a problem. See the Honeywell link below:

    Terry O
  • finiuch
    finiuch Member Posts: 6
    not a power stealing thermostat

    The tstat uses the common(C) wire from the transformer. The battery powered tstat didn't have this problem because when the aquastat would cut the power to the transformer (and therefore the common wire), the tstat wouldn't lose power.
  • Paul48
    Paul48 Member Posts: 4,469
    edited April 2013

    It sounds like it's not radiant. How many wires do you have to that thermostat? When was the last time the boiler was cleaned and serviced?
  • Paul48
    Paul48 Member Posts: 4,469

           The thermostat is just a switch that operates the relay in the aquastat,just like a light switch in your home.In your home the "hot" comes from the breaker panel to a light switch, then to a light bulb, then returns to the panel as a neutral,forming a circuit. In an aquastat, the 24 volt "hot" comes off one side of the transformer, then goes to one side of the relay coil. Then the wire goes from the other side of the coil to one "T" connection. From that "T" connection the wire goes out to the thermostat, through a set of contacts, and returns to the other "T" connection, and finally to the common connection of the transformer.

           If my description is hard to follow, draw it on a piece of paper.
  • finiuch
    finiuch Member Posts: 6
    edited April 2013
    3 wires to the thermostat

    C,W,R and go to the tstat. The C and R are connected to the two poles of the transformer and provide power to the tstat. The tstat relay connect W to R and energizes the burner gas valve to call for heat.

    I'm sure it has been serviced in a while. However, I know there is no air the system and the expansion tank is not waterlogged.

    curious, what kind of service would fix the boiler limit issue?
  • Paul48
    Paul48 Member Posts: 4,469
    edited April 2013

    Is the boiler oil-fired or gas? Soot reduces efficiency. Annual service is a must. If you can't remember exactly when it was serviced last, you may be only getting 50% out of the boiler. A gas boiler can easily get 1/16" of soot a year, and that's a 4.5% reduction alone. Oil would be worse still.Your fuel dollars are going up the chimney.
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