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One zone on boiler overheats and blows pressure relief valve

I have a Dunkirk boiler with two zones heating a two story house.  The second floor zone has been functioning all along. I had a bad Grundfos pump on the 1st floor. It moved freely with a screwdriver but would not move when calling for heat.  I replaced the pump. The new pump comes on and off just fine.  While trying to refill and bleed the system I noticed the really old pressure reducing-Auto fill valve was so plugged with debris that it was not working so I replaced that along with installing a new ball valve before it. If I fire up the boiler with only the second story zone running it works fine. Pressure on the boiler is 15 when cold. The cold pressure on the one year old expansion tank reads 15psi cold as well. When I run the second floor zone it runs perfectly. It shuts of at 180 and runs at about 20psi on the boiler. This zone has been functioning fine all week.  When I turn on the first floor zone the pump turns on but I get no circulation in the zone unless I manually open the flow control valve. Once open I can hear the water flowing through.  Within minutes of doing this the temp runs away and blows the pressure relief valve. I have tried this with the water fill supply both off and on. I am stumped. Any ideas out there?


  • STEVEusaPASTEVEusaPA Member Posts: 1,445

    Are you sure the circ is pointing in the right direction?  If so, if there's no blockage (you mentioned the crud) I think it's air.  Air blockage, no flow thru the boiler, runs up on temp and pressure.

    BTW, how are you reading pressure on the expansion tank? 

    The pressure shouldn't change all that much with a properly working system.  My boiler (2 high temp zones, 4 radiant, plus indirect) hardly changes pressure of a pound or 2 all winter.
  • Steve WhitbeckSteve Whitbeck Member Posts: 669

    either the pump is not producing what it is rated at or it is installed backwards.

    It can't be air in the system since when he raises the flow control he gets water flow.

    My bet is the pump is installed backwards.
  • Tstar1234Tstar1234 Member Posts: 3

    Yes I am checking the pressure at both the boiler and the expansion tank. The pump appears to be installed correctly. It is the same as the other one. Could it be wired backwards?
  • Tstar1234Tstar1234 Member Posts: 3

    What a fool I am! I was looking at the front cover when I installed it and not the arrow on the side! Yes the pump was installed backwards :) Thanks for your help!!
  • We have all been there

    Luckily this website is here to help us all double-check what we have done!--NBC
  • WeezboWeezbo Member Posts: 6,232
    safe bet....

    the circ was installed in the wrong direction.

    thats three votes none opposing . go with it . :)
  • WeezboWeezbo Member Posts: 6,232
    when i read the post ...

    two folks had replied.

    there's something different happening with that.


    *~//: )

    when it refreshed after my post i see that you have made the connection .. sometimes we flip the cartridge so someone looking at things sees it looking like its going the right way. normally because it is. which reminds me i need to flip the wiring around on a circ before someone tells me its not right. what that is requires an allen key . what is happening for you is you need a couple Mexican Metric crescent wrenches..
  • steveraysteveray Member Posts: 8
    checking tank pressure

    You need to check the tank pressure when there is no pressure in the system otherwise it will always read the same as the system pressure . If you have system pressure at zero and the tanks at 15 then the tanks right. If the pressure still goes up when it gets hot you need a bigger tank.
This discussion has been closed.


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