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Just a thought about the "B" dimension.

Hello everybody. I just finished tuning my freshly piped two pipe return system. I had issues with water spitting out of my return vents so I went to the book and read about the "B" dimension. The book was right and I got it figured out. I have 32" in height so the max cut out I can use comes in at about 1.2 psig. As I was thinking about this i was wondering if you were to increase the diameter of the vertical pipe that leads to the wet return if you could actually decrease the "B" dimension. The way i see it, its a battle of pressures. Head pressure vs. boiler pressure. If you were to increase pipe diameters from 1.250 to 2.5 you would double your head pressure going to the boiler. I would imagine this would make some sort of difference. Any thoughts ?


  • Jamie Hall
    Jamie Hall Member Posts: 19,149
    Sorry... pressure doesn't work that way. Pressure in a liquid is purely related to the depth of the liquid or the height of the water column. It has nothing to do with the area or pipe size.

    And I agree -- a lot of people have a lot of trouble with this concept, but I assure you that that's the way it works.

    Your solution will be to invest in a vapourstat, rather than a pressuretrol, and set it -- for starters anyway -- at about 12 ounces cutout and 6 ounces cutin.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Chapstick
    Chapstick Member Posts: 64
    Thanks Jamie. That's the solution I came up with too. I figured there was an explanation for why it doesn't work that way. There's no way I thought of something that the dude that wrote that book didn't !
  • KC_Jones
    KC_Jones Member Posts: 5,094
    Think of it this way, if you swim in the ocean do your feet get crushed when you are treading water? For what you are saying to be true the weight of the top 5' of the entire ocean would be pressing on your feet. You could also think of a round swimming pool as a giant pipe...similar concept as the ocean. Remember PSI is pounds per SQUARE INCH so if you increase pipe size you spread the "weight" out over a larger area so the amount of pressure on an area (PSI) does not increase. Jamie is right it's a tough one to understand sometimes.
    2014 Weil Mclain EG-40
    EcoSteam ES-20 Advanced Boiler Control
    Boiler pictures updated 2/21/15
  • Chapstick
    Chapstick Member Posts: 64
    Great way of explaining KC. I was actually going in that direction of thought but hadn't quite gotten there yet.
  • Chapstick
    Chapstick Member Posts: 64
    So if you were standing underneath a 50' foot diameter water tank and had a 1" ball valve to drain it it wouldn't matter if it had 10' of water or 20' of water, it would still fill a water jug in the same amount of time.
  • Paul48
    Paul48 Member Posts: 4,469
    No..........There would be a difference in the time between 10 and 20, but if the tank was 25' diameter, the 10' time and 20' time would be the same as the 50' diameter.
  • Paul48
    Paul48 Member Posts: 4,469
    There's an old way of finding level at some distance. You use clear tubing, and fill it with water. The water will always find level. It wouldn't matter if one end of the tube was 2" and the other was 1/2".
    JUGHNE Member Posts: 9,822
    Think of gravity, that diameter of the tank doesn't matter, it is the height of the actual water above you that determines the pressure. You may notice that water towers are placed on a hill if possible. The height of the tower plus the hill elevation produce the best pressure possible without operating a pump. That is a huge tank up there but its still only has a standpipe connecting to it that might be 10 to 16" in diameter.
  • dennis53
    dennis53 Member Posts: 53
    Pouring wet concrete into a wall form is similar. The thickness of the wall does not matter- only the height. There is the same amount of pressure at the bottom of an 8" thick wall as a 36" thick wall, given equal heights.
  • Chapstick
    Chapstick Member Posts: 64
    Gotcha. Thanks for the clarification guys.