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Question about boiler static fill pressure

BigErl Member Posts: 38
Is the max pressure on a boiler rating plate determined by the pressure relief valve packaged with the boiler? I have to pump up 4 floors, about 50 feet. Using H x .433+5, I get a fill pressure of close to 27 psi. Can I set my fill valve to 27 psi and install a larger pressure relief valve (36 or 40lb)? Or do I risk damaging the block or components if the boiler pressure exceeds the number on the rating plate, 30 psi?


  • ScottSecor
    ScottSecor Member Posts: 864
    @BigErl there are some rules of thumb that might help you.

    First and foremost, the pump does not "pump up four floors," instead the static pressure fills the pipes with water and the circulator pump moves the water through the pipes. In other words, the water stays inside the pipes all summer when the pump is off.

    In simple terms, for every pound of pressure over zero the water will rise 2.3 feet. So for a typical four story building (with boiler in basement) the highest pipe would be about forty feet above the boiler. 40 feet divided by 2.3 equals about 17.4 pounds of pressure, let's add a few pounds for a slight cushion, so I'd go with 22 psi.

    If the boiler is rated at 30 psi, it could be dangerous to install anything more than a 30 psi relief. If the boiler is rated at 40 or 50 psi then it would be acceptable to replace the relief valve with either one.

    If the expansion tank is large enough and working properly, you should be able to sneak by with that 30 psi relief valve. If not you might be forced to use a larger expansion tank or add a second tank.

    Make sense?
  • BigErl
    BigErl Member Posts: 38
    Thanks Scott. The height to the top of the highest radiator is 48’. That gives me a fill pressure of 25.784. I called the manufacturer. They said I can install up to a 50lb pressure relief valve.
  • Jamie Hall
    Jamie Hall Member Posts: 23,652
    48 feet is right up there. True, that is about 26 psi -- but if the manufacturer of the boiler is content with a 50 psi relief valve, and the boiler is labelled to handle the pressure, I'd add about five pounds... and shoot for 30 psi at the expansion tank and the inlet to the circulating pump.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 22,479
    Also be sure the expansion tank is sized properly to that fill pressure.
    use the commercial tank size at the Amtrol site

    Tank pre-charge needs to be set also.

    Building height above tank times .5 is a easy number to calculate with.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    EBEBRATT-Ed Member Posts: 15,840

    When you replace the relief valve make sure that the BTU rating of the relief valve is large enough. It should meet or exceed the rating on the boiler name plate

    Also, any boiler with a relief valve over 30 psi is supposed to have a ASME rated expansion tank.
  • BigErl
    BigErl Member Posts: 38
    Thanks for all the replies. I have a follow up question about setting the expansion tank precharge. There are 2 b&g 60 tanks. Should I set both tanks to the system fill pressure or split the difference between the 2?
  • Zman
    Zman Member Posts: 7,607
    Both should be at the fill pressure.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    delta T
  • delta T
    delta T Member Posts: 884
    edited October 2019
    In my state (Colorado, according to my state boiler inspector), if we exceed 30 lb on the relief valve we have to install ASME rated expansion tanks, and they are VERY pricey, I would check with your AHJ to make sure.

    Last one I did, the ASME tank was 4 TIMES the cost of the non-ASME tank.