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Hot water system pressure continues to rise

jj_3 Member Posts: 10
Weird thing happening. Pressure in system continues to rise in an old 50 yr old boiler system. Has new PRV and #30 expansion tank, which replaced 15gal steel tank. No indirect fired heater either. Other than defective prv or tank not sure what else is left to check? Any thoughts out there? Thx. Jim


  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    IF, your pump is pumping towards the expansion tank, AND the pressure reducing valve is connected to the inlet side of the pump, every time the pump turns on, it creates a slight negative pressure that the fill valve sees, and sends in a little more water. Over time, the pressure rises and the relief valve will eventually pop off.

    That, or if you have an indirect DHW heater, the heat exchanger has failed and is cross connecting with the water in the boiler that normally operates at a lower pressure.

    If you turn off the make up valve, does it stop? If not, look elsewhere. If it does, look at the configuration/arrangement.


    There was an error rendering this rich post.

  • unclejohn
    unclejohn Member Posts: 1,833
    Old domestic water coil in boiler that may not be in use thats still plumbed in? By pass on the prv?
  • mikeg2015
    mikeg2015 Member Posts: 1,194
    Pressure regulator on boiler fill is set too high so expansion tank is waterlogged (full) when the system is cool.