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Boiler temperature overshoot?

MrScott Member Posts: 15
Our house has hydronic heat.  The boiler stopped running.  I saw low system pressure; bumping the fill valve didn't fix it (pressure dropped again); replaced the stuck-open pressure relief valve at the boiler with a boiler-rated 30PSI valve. 

During the first test cycle the new relief valve operated briefly:  for a 120° setpoint it climbs to 165° over about 10 minutes before shutoff (code Oy).  Is this usual?  Temperature swing seems excessive.  Could this be a case of too large of a boiler and this was set to increase the minimum run time and efficiency?  I suspect a pump or two need replacing (one is noisy), and am having a HVAC contractor come clean and maintain the boiler this week

Variable ticky grindy noise from the boiler circulation pump (replacing this week).  

 - Would you put in a filter?  Water is black.  Which type, magnetic, bypass filter?
 - Would you add corrosion inhibitors?  I don't know what is in the water now.  Maybe I can find out who worked on it last.

Some of the equipment:
Buderus GB 142-24 propane-fired boiler.
Buderus ST300 storage tank.
Grundfos circulation pumps (UPS15-58 FC/LC at boiler circulation loop, UP26-64 F circulates tank to boiler).

Thank you!


  • geno907
    geno907 Member Posts: 12
    two things.
    1. replace the pump if you hear grinding.
    2. sounds like a leak. get someone who has a leak detector. and fix the line.

    if the low water cutoff keeps tripping, there is a reason.
    always remember, boilers are bombs.

    let us know what you decide to do.
  • Derheatmeister
    Derheatmeister Member Posts: 1,473
    Have the system fluid analyzed for magnetite or other contaminants.
    Read the Boiler manufacturers Instructions carefully and understand the requirements on the aluminium HX !!!
    Do NOT just add ANY chemicals or Antifreeze that are not approved by the manufacturer (Bosch) as it could cause premature failure to the HX which will not under warranty !
    If your circulating pump is making grinding noises you may want to replace it.
    Check for leaks or None Oxygen diffusion barrier type Tubing such as "Aquapex" Or PB pipe.
  • Jamie Hall
    Jamie Hall Member Posts: 21,542
    Hot water system? If the new relief valve operated at all the pressure is much too high and that leads me to wonder if the expansion tank is not working. Waterlogged, bladder, gone, or simply not set to the correct pressure.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England