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Boiler Pressure Question

Ludwig12 Member Posts: 2
Hi Everyone.

I have a 6 years old Vitodens 100 gas boiler with both hot water tank and radiant heating system connect to it. The boiler pressure has been steady at 12 PSI (0.8bar) over the last few years, however I recently noticed in the user manual that 12 PSI was deemed as the lowest pressure, so I tried to top it up. The interesting thing that I noted is that after I top it up to 16PSI or higher, it will gradually come back to 12 PSI over a few days then stay there. I tried it a few times and the result is the same.

I am truly puzzled over the following and google did not get me the answers.

1. Is what I am observing normal?
2. Is 12 PSI boiler pressure too low?

Thanks a bunch!


    JUGHNE Member Posts: 10,044
    You only need enough pressure to lift the water up to the highest point of radiation.
    12 PSI X 2.3'/psi = 27'. Usually OK for a 2 story house.
    How high is your highest radiation?

    Nothing is to be gained by higher PSI unless needed for height.
    Also if you raise the system pressure you need to raise the air pressure in the exp tank.....not always easy....to do so you must isolate/disconnect it from the system.

    Also 16 cold gets you closer to the 30 max that will open the relief valve.
  • Ludwig12
    Ludwig12 Member Posts: 2
    Thanks Jughne for the fast and educated response. The boiler is in the basement and yes, it is a two story house. I was reading online that says that if the pressure is too low, it would make the heating system less efficient. Glad to know that 12PSI is acceptable.

    Btw: it is 12/16PSI hot (I increased the pressure when the boiler is running with radiant heating on)
    JUGHNE Member Posts: 10,044
    As long as the system is completely full of water with all air removed.
    The circulator simply moves the water around like a Ferris wheel.
    The increase in pressure is from the expansion of the water as it is heated.
  • dab2
    dab2 Member Posts: 1
    How do I top off my steam boiler with garden hose. I turn off pipe flow and get water coming out of spicket. I have tried every pipe individually but can't figure it out. My contracter showed me how to do it once but I cant remember!
  • Jamie Hall
    Jamie Hall Member Posts: 19,954
    @dab2 , see my reply in your other thread. If this is a steam boiler, this thread is not even remotely relevant to what you say you are trying to do.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England