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Maximum Working Pressure

A boiler has a listed 75 psi MWP. Should it be running at this pressure or slightly below (70-74psi) with a 50 psig rated T&P valve which is dripping? Two building with 2 hot water coils installed in 2 fan coils. One is located next to the boiler in the basement and the second is located on the second floor above a drop ceiling. Approx. 20' above the boiler. Another company replaced the tubes recently and I was at the location for a separate issue and noticed this. This is a 688K input boiler. Should this not be running at much lower pressure, say 12-15 psig? Am I wrong and if so, would someone please educate me?

Thanks, Mike C. in St. Louis


  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    Static fill pressure is dictated by....

    The height of the system above the heat source (1/2 PSI per vertical foot plus 5 PSI) and/or operating pressures above 200 degrees F (to avoid flashing to steam).

    If every thing is set up ideally (pumping away from the boiler and the expansion tank) and operating temperatures below 180 degrees F, you would only need a fill pressure of 1/2 PSI per vertical foot of system elevation above the heat source, with a minimum fill pressure of 12 PSI (if the boiler is located on the top of the building).

    So, if the system is in a 5 story building with 10' per floor, and the fan coils are in the ceilings, and the boiler is in the basement, then the system height above the boiler is roughly 55 feet, then you would need a cold fill pressure of 28 PSI (55 times .5 = 27.5 rounded up to 28) PLUS 5 PSI for a total of 33 PSI static cold fill.

    The extra 5 PSI is there to avoid flashing to steam at the top of the system and to aid in burping bubbles out of the system with manual purge points near the top of the system.

    The actual number, is .434 PSI per vertical foot. I use .5 just to be sure. If you are on the marginal edge, you may want to calculate precisely.

    The maximum recommended OPERATING pressure (maximum operating temperature and pressure) is typically 80% of the pressure relief valve setting. So, in your case, 75 PSI times .8 = 60 PSI.

    Check your expansion tank. 9 times out of 10, a seeping relief valve indicates a failed or water logged expansion tank.


    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
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