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expansion tank question

I found it odd today when checking pressure in a tank that there was essentially no water on the water side. The air side was good at 15 PSI, matching the system reading of 15 (boiler was cool and not circulating). The question came up "how can water enter the tank if air is in the line and there is not an air vent adjacent"? There is an air eliminator on the hydraulic separator, relatively close by, as well as one on the boiler heat exchanger. Due to the piping arrangement, it seems that it would be difficult for air to become entrained in the water loop. Then, the next question is "why does it matter if there is no water, so long as the pressure is good"?
I always assumed one side of the tank had air and the other side had water. thanks


  • Jamie Hall
    Jamie Hall Member Posts: 21,536
    There are no silly questions -- only silly answers. In your case, the way most expansion tanks are designed, it is quite likely that if the system pressure and the tank air pressure are the same, there will only be air in the tank (older hydro tanks don't work the same). When the system heats up and the water expands, the pressure increases and forces some water into the tank -- on the water side -- until the air is compressed enough to equal the additional water pressure.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • RayWohlfarth
    RayWohlfarth Member Posts: 1,318
    You want to be sure the system pressure is off the diaphragm before testing Air finds its way up because of the bouyancy. Wishing @Erin Holohan Haskell had a spell check on here LOL
    Ray Wohlfarth
    Boiler Lessons
    Erin Holohan Haskell
    JUGHNE Member Posts: 10,791
    What pressure shows when the system is at it's hottest?

    Ray, the spell checker warks fer me. Red under lining. ;)
    STEVEusaPAErin Holohan Haskell
  • Zman
    Zman Member Posts: 7,467
    The spell check may be a feature of your computer or browser. Mine works....
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • RayWohlfarth
    RayWohlfarth Member Posts: 1,318
    @JUGHNE Looks correct to me LOL
    @Zman Wow thats great Never knew that was on there. Thanks *Happy Guy*
    Ray Wohlfarth
    Boiler Lessons
  • Erin Holohan Haskell
    Erin Holohan Haskell Member, Moderator, Administrator Posts: 2,126
    Zman said:

    The spell check may be a feature of your computer or browser. Mine works....

    JUGHNE Member Posts: 10,791
    I don't know how I have it but it works.
    If it says add to dictionary, I am very good at repeating/typing a new version of the word until I get what I want.
    Often this is the reason I am slow to reply, trying for the correct spelling, grammar and punctuation.

    It is obvious many do not make use of this feature. ;)
    HVACNUT Member Posts: 5,255
    I cant stand when it chooses the words for me. I do just fine having words come out of my mouth without them going through my brain first. I dont need help from software.