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correct pressure hot water boiler

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Glenn Sossin_2
Glenn Sossin_2 Member Posts: 592
Residential systems usually have the feed valve set at 12psi. This would be the pressure when the system sits idle and has cooled down.

As soon as you start applying heat energy, the water will expand. Thats why you have an expansion tank. This tank needs to be sized properly to accomodate the water volume and btu's of the system. When the boiler is operating you will typically see an increase of approximately 4 -10 lbs - this is a function of the water temperature change and water volume of the heating system. If your operating pressure is much higher than this - you may have a problem, probably an undersized expansion tank.

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  • larryp3
    larryp3 Member Posts: 4
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    correct pressure hot water boiler

    What is the correct ststem pressure in a close loop baseboard heating system?
  • larryp3
    larryp3 Member Posts: 4
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    correct pressure hot water boiler

    My system pressure is 5 psi at temp of 170F with no load. When sytem fires as temp goes to 190F cut off, press rises to 14 psi. As temp drops to 170F press drops to 5 psi. Is this normal?
  • Brad White_9
    Brad White_9 Member Posts: 2,440
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    As Glenn said

    for a single-story to a 2-story house, 12 PSIG is standard. 5 PSIG may well work if it is a one-story ranch or cape in that the lift may only be five to eight feet. 5 PSIG will lift to 11.55 feet and give you some top pressure cushion.

    That said, the fact that you shoot from 5 to 14 PSIG for only a 20 degree rise indicates insufficient expansion tank volume or a defective tank. Normally the water expanding from room temperature to 200 degrees is about 4% of volume. You seem to be doing that and more in a much narrower band (without knowing your system volume, this is just a projection).
  • Glenn Sossin_2
    Glenn Sossin_2 Member Posts: 592
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    Maybe

    Agree with Brad and you also may have a defective feed valve- maybe theres a valve that provides water to the feeder (press reduc valve) is closed and you have a very low grade leak. Maybe your press guage is calibrated incorrectly. Before you go too far, I would confirm the pressure. If it really is that low, you need to check this out.

    Operating at such a low pressure could potentially cause flashing on the suction side of the circulator - then you might become air bound.

  • Brad White_9
    Brad White_9 Member Posts: 2,440
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    Feed Valve

    I would agree with the feed valve premise if the pressure remained high or kept climbing to relief valve lift-off even after she cooled down. The fact (stipulated) that the pressure returned to 5 PSIG after it cooled leads me to the expansion tank fault. Just my $0.02.

    Brad
  • Glenn Sossin_2
    Glenn Sossin_2 Member Posts: 592
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    Agree pressure change is indicitive of a expansion tank issue. Maybe it's a painted steel tank and getting water logged, or the bladder in the pnuematic tank is pierced?

    I still think there is a potential issue with the feeder too. If the feed valve was working properly and was supplied with water, the system would be idle at 12 psi not 5 psi.
  • LarryC
    LarryC Member Posts: 331
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    5 psi idle pressure

    Home owner here.

    Strictly speculation. What if the expansion tank is undersized or waterlogged, and the feedwater valve WAS set to 12 psi originally. The system, when heated up, could lift the relief every heating cycle. Then somebody either reduced the feedwater valve pressure regulator setting or isolated the feedwater valve until the system didn't spray water every cycle. If water continued to leak into the expansion tank, the system pressure would continue to drop until we arrive at the present day condition.
  • Glenn Sossin_2
    Glenn Sossin_2 Member Posts: 592
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    Should always be pressurizing the boiler

    The feed valve should always be pressurizing your boiler. Thats what it's purpose is. Even if there was a leak somewhere, that valve would continue to maintain the minimum pressure it is set to - usually 12psi on residential systems. If you really do have only 5psi, I would look for the valve that supplies the feeder. It could be closed.

    While its possible, its unlikely that someone would intentionally try to adjust a feed valve down that low. You need a competetent plumber to diagnose whats wrong.
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