pressure relief valve blows off at 20lbs

Discussion in 'Boiler Forum' started by jpaul, May 5, 2013.

  1. jpaul

    jpaul New Member

    Messages:
    18
    Location:
    Connecticut
    Heat running, pressure builds to 20lbs, blows off some water until it builds back up to 20lbs and blows off some water again. Bell & Gosset expansion tank (2 years old), 11-12 year old boiler w/coil. Boiler temp was increasing from 175, 180, 185, 190, 195. Pressure never went beyond 20lbs and blew off some water every time once it hit 20lbs. I checked 15 minutes after I turned heat off and boiler not running. Boiler temp 205, 12lbs pressure. Settings at 140 & 180 degrees. Thanks for looking...Suggestions?
  2. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
    303
    Location:
    Minneapolis
    Had the boiler serviced by a professional lately?

    Fill valve on?

    Pressure relief valve tested or replaced in the last 5 years?

    Expansion tank full?
  3. jpaul

    jpaul New Member

    Messages:
    18
    Location:
    Connecticut
    Thank you for your reply. I have a cleaning annually. I believe the fill is an auto fill. When I tap the expansion tank top and bottom with a wrench sounds about the same. The top of the tank feels warm and the bottom feels cool.

    I just changed the relief valve and the boiler pressure goes up to 25 lbs and does not blow now, however I don't know if the boiler is operating properly.

    With two zones calling for heat the boiler temp was 170. I turned one zone off and the temp started rising. The temp went to 200 degrees and the burner turned off but the thermostat was set for 76 and was reading on 70 when it turned off.
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,317
    Location:
    New England
    Depending on how the boiler is set up, the thermostat may control the circulator, not the boiler. The boiler may be turned on by an end switch on the circulator or zone valve, but it may cycle up and down between the min/max aquastat settings on the boiler. IOW, it may be normal for your boiler to shut down while there is still a call for heat if it bounces off the max temp. If the pressure varies a lot between cool and hot, the expansion tank is suspect. the whole idea of the expansion tank is to moderate any changes. 25# is a little high for most systems, but could be okay for yours. More typical is in the 12-15# range. If the expansion tank rings when you hit it, the bladder is not likely shot, but the static pressure may not be set properly. To set the static pressure on an expansion tank, you have to depressurize the boiler otherwise, reading the pressure on the expansion tank will be exactly the same as the boiler pressure.
  5. jpaul

    jpaul New Member

    Messages:
    18
    Location:
    Connecticut
    Seem to be ok since I replaced the relief valve. Thanks for the help.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,317
    Location:
    New England
    Many residential systems use a 30# relief valve in their boilers. While there may be lower ones and there are higher ones, if it's opening at 20#, it is, as you found, the most likely suspect. But, with a properly sized expansion tank, precharged properly for your system, the pressure should not vary much during operation. Your expansion tank may be too small, or it may not have the proper pre-charge.
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