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Boiler Temp Keeps Rising

13bravo13bravo Member Posts: 18
So I’ve had some other posts about replacing my boiler and adding a zone but I wanted to take a step back to what started this journey and pick the collective brain here again:

Back in early part of this year my pipes started thundering like crazy, went down and saw my cg7 WM boiler water temp was pushing 250. Long story short, the boiler heats the house up but when you turn it on the water temp keeps rising, it just won’t stop. I shut her down, it cools down, turn her back on, just keeps right on rising again and in a few hours back pushing 250 and need to cut power off again.

Is this the aquastat? The relay? Something else? Thanks in advance for your thoughts!


  • HVACNUTHVACNUT Member Posts: 3,508
    CG7? The more recent CGa and CGi use a control module. It seems like you needed a pro the first time you noticed it. What's the pressure on the boiler? How old is the relief valve?
  • Jamie HallJamie Hall Member Posts: 13,403
    This is hot water heat? Not steam? You have a serious control -- and safety -- problem.

    Assuming that this is hot water heat and not steam, the safety valve on your boiler should have released long ago, and put hot water all over your basement. If it didn't, it's defective or installed incorrectly.

    The control problem could be an aquastat. Could be control board. Could be a fired relay in the control.

    Whatever... you should turn the boiler off at the emergency switch, turn the breaker off (or take out the fuse), red tag everything, and not turn anything back on until you both find the control problem -- and get the safety valve replaced.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • ZmanZman Member Posts: 6,002
    If it wasn't for the thumping sound, I would think you have a bad temp gauge.

    If the temp is accurate, it's sounds like you have a deadly combination of a bad aquastat, high limit and relief valve. Shut it down so you boiler does not end up going through the roof!
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • 13bravo13bravo Member Posts: 18
    Thanks for all of the comments! It is a hot water system and it is a cg7 (coming on 30yrs old).

    I had shut it down by turning off the power and flipping the gas valve to pilot...I didn't see water anywhere so I am assuming the relief valve didn't work.

    The relief valve, I'm assuming, is at least 15 years old...may have been replaced when the water meter and line were updated. But if not, then it may be as old as the boiler.

    OK, so in addition to aquastat/relay/etc issue I also need a new relief valve is what I'm hearing...
  • Big Ed_4Big Ed_4 Member Posts: 1,433
    It could be the aquastat , if you have the cheap L4080 which has the range from 180* - 240* ( Why ???) When they start to go they tend to over ride . Where a L4006A the temperature falls short of it's set point when its starts to fail .When it ever does
    I have enough experience to know , that I dont know it all
  • hot_rodhot_rod Member Posts: 13,832
    When boilers get filled with mineral deposits they will often overshoot temperature also. Usually after the burner shuts down, you see the temperature continue to increase.
    You need to see if the burner is on or off when it overshoots.
    Deposits in a boiler can also cause that thumping or percolation sound.
    And it is usually old boilers that take on water continuously.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • 13bravo13bravo Member Posts: 18
    Burners seemed to keep going, it didn't appear to be after burners shut off
  • JUGHNEJUGHNE Member Posts: 7,183
    Could your gas valve when powered down not be closing completely?
    Could happen sometimes stuck open sometimes closed.
  • mattmia2mattmia2 Member Posts: 1,861
    If my math is right the boiling point at 250 f is about 15 psig so it should not have cased a 30 psi relief valve to open.
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