Boiler pressure rises

Discussion in 'Boiler Forum' started by Leo, May 16, 2020.

  1. Leo

    Leo New Member

    Joined:
    May 15, 2020
    Location:
    British Columbia
    Allied Engineering Superhot MG-75 Nat gas boiler (14 years old) for hydronic home baseboard heating losing water from PRV. Found 2 gal exp tank had no air charge and full of water. Installed Maxtrol EX-15 exp tank with 14 PSI precharge, new 30 PSI PRV, and a new boiler 1/2" drain gate valve. I purged air from all 5 zones and from the boiler circuit. I have 14-16 PSI reading on tridicator so pressure reducing valve is working correctly, (I brought it down from 19 PSI as only have 14 feet of lift). I have twice lit the pilot, warmed the heat exchanger, then fired up the boiler. 2-3 minutes and I'm at 180 F and 29 PSI and climbing so had to shut down. WTF? I have a circulator at the boiler on the return side. Any ideas? What is the circulation pump supposed to sound like?
     
    Last edited: May 16, 2020
  2. Sylvan

    Sylvan Still learning

    Joined:
    Sep 23, 2004
    Occupation:
    plumbing - fire suppression - boiler inspector
    Location:
    New York
    I happen to like B@G series 100 circulator installed on the return side and it is quiet
     
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  4. Reach4

    Reach4 Well-Known Member

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    Sep 25, 2013
    Location:
    IL
    I never heard of Maxtrol. Sounds like you need a much larger expansion tank. Either that, or your fill valve leaks. I am not a pro.
     
    Last edited: May 17, 2020
  5. fitter30

    fitter30 Well-Known Member

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    Feb 2, 2020
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    Retired service tech
    Location:
    Peace valley missouri
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
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    Occupation:
    Retired Systems engineer for defense industry.
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    If the ET is working and the pressure rises that much, only three things that I can think of:
    - the autofill valve is leaking some
    - the heating coil of the potable water (either internal or if you have an indirect) may have a small leak in it, letting pressurized potable water into the heating circuit
    - the ET is undersized for your system
     
  7. Dana

    Dana In the trades

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    Jan 14, 2009
    Location:
    01609
    If the boiler is going from 14-15 psi to 29-30 psi and 140F to 180F in two minutes it means there is likely to be valves closed isolating the boiler from the system (probably not just isolated from the expansion tank, since it indicates zero flow.)

    If the flow is being blocked by a closed valve, the pump is probably cavitating audibly.
     
  8. fitter30

    fitter30 Well-Known Member

    Joined:
    Feb 2, 2020
    Occupation:
    Retired service tech
    Location:
    Peace valley missouri
    Pumps not pumping and expansion tank has a problem if it's piped correctly
     
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