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air vent sucking air

GB123
GB123 Member Posts: 1
Hey all.

I am filling a hot water system that is quite big. We are having trouble expelling any air from it. Some of the highest vent points are actually sucking air in when we open them....how can this be?

any help would be appreciated thanks!!!

Comments

  • icesailor
    icesailor Member Posts: 7,265
    Venting Radiator Systems.

    If you are filling a large multilevel building, you need to fast fill the system, get the pressure up, and vent from the top down. When  top floor radiator is filled, you know that there is enough pressure at the top of the system. Keep the pressure up, vent the top floor, then the second floor. Then the first.

    If you open a top floor vent, and the lower floors aren't vented, you can suck air.

    I work alone. I'm almost as old as dirt. My leg muscles are tired. If I turn on the fast fill on the PRV, I can go to the third floor, vent the radiators, and go to the second floor and vent a few,and then shut off the fast fill with enough pressure passing through the PRV. It's a matter of "feel". It works for me. Anything to keep me from having to make extra steps up stairs is a plus for me.

    The "correct" way is to do a floor at a time if you have help. But ant vent opened up on a floor above may suck air.
  • hot_rod
    hot_rod Member Posts: 22,024
    ideally

    You want about 5 psi at the highest point in the system for good air removal with float style vents, especially.



    So the system fill pressure needs to be adjusted to provide that pressure. Either a gauge at the high point or calculate the pressure and use a "accurate" pressure gauge at the boiler or expansion tank connection.



    So you need to know the height to the top of the system above the inlet to the expansion tank. A close rule of thumb is .433 X the height plus the 5 psi.



    So if the vent is 19 feet above the expansion tank connection .433 x 19 + 5 = 13.27psi required fill pressure. Some round the .433 to 5 to make the math easier, = 14.5psi



    The exact formula from Siggys Modern Hydronics take into account the density of the water. which changes with temperature Pressure= Height (density divided by 144)+5



    So fill water at 60F is 62.36 lbs per cubic foot density.



    19 x (62.36 divided by 144) +5 = 13.23 psi.



    You can increase the fill pressure to speed things up, just don't exceed the boiler relief valve setting. Some brands of Autofill valves have fast fill features built in and do not require a bypass :)



    Remember the pre-charge in the expansion tank needs to be adjusted to the fill pressure, before you pressurize the system.



    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • SWEI
    SWEI Member Posts: 7,356
    30 PSI boiler relief valves

    Can be a pain in a mid-rise building.  We finally resorted to shutting off the boiler loops and using street (or fill pump) pressure to fast-fill the distribution loops, then leave them at up to 100 PSI for at least 24 hours to test.  Final step is to drain the distribution side back to operating pressure and re-open the boiler loop isolation valves.
  • hot_rod
    hot_rod Member Posts: 22,024
    great idea

    to pipe the boiler, and even the expansion tank out of the loop. Higher pressure will blast air and dirt debris out quickly.



    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
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