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Discuss System venting water in the Central Heating Forum area at

  1. fixitflav

    fixitflav Member

    I don’t know how I managed to give that impression, but if I did I apologise. I’m always ready to learn, that’s why I started this thread, and I’m grateful for the responses, which helped me fix the problem.

    On to the hydraulics!

    Take a situation where you’re pumping from a tank into a pipe with an inverted U, and the height from the top water level in the tank to the top of the U is say 3m, and it’s 1m from the top of the U to the end of the pipe.

    If the flow is reasonable, or there is a seal pot on the end of the pipe, (and any air has been expelled) there is 1m siphonic assistance, so the pump sees 2m head (ignoring any losses). In that case the pressure inside the pipe at the top of the U is below atmospheric, by 1m. So if you drill a hole in the top, air will enter, and break the siphon. The water coming over the U then free-falls down inside the pipe. That is sometimes desirable (siphon breakers aren’t unusual), but then the pump head rises to 3m.

    Of course, if the end of the pipe is below the tank water level, you can get flow with no pump, as you discovered all those years ago.

    BTW, I only put a lot of detail in to make sure we each understand the other’s viewpoint, not trying to be patronising!

    Happy to discuss further if you want.
  2. Ric2013

    Ric2013 Plumber Top Contributor!!

    I don't know. Was probably in a weird mood. No hard feelings this end. I also apologise - can't see what I was upset about either!

    Thanks for discussing this. I do find it interesting. I managed to follow your explanation. Took me a while to grasp the physics of what I think I understand intuitively, but got there in the end.

    So I take it your point, originally, was that, if the flow had been sufficient to fill the inverted U, then the head required to maintain that flow would have been significantly lower than that required to get the flow started. As such, your point was that the height of the inverted U above the F & E waterline is essentially going to have no effect on how much pressure would be required to maintain the flow, once full-bore flow is initiated and siphonic action is able to reduce the backpressure the pump would have to overcome?

    I suppose I was confused because, looking at it as a plumber, there wouldn't be any flow from the vent. Looked at it from an engineer's perspective, I suppose the pump does pull on the cold feed and push towards the vent. The only reason pumping over does not occur is that there is no significant resistance between the two pipes, hence the need to keep them as close together as possible.

    Still a little confused by what you mean by a seal pot - have tried Googling the term and can only assume it is some kind of equipment you would put there to prove a point under test conditions?
  3. fixitflav

    fixitflav Member

    OK, no problem!
    That's right, but I hadn't thought it through in that much detail. I didn't think there was any risk of a siphon forming, as there was only a trickle from the pipe. But it was enough to show something was wrong, and it couldn't be left like that.
    Yes, there is a rise in the overflow pipe, due to headloss between the 2 pipes, but if everything's OK it should be nowhere near the top of the U. Also a drop in level in the F/E tank, but much smaller due to greater area.
    In an earlier post I called it an overflow tank, but I changed it for more common usage. The end of the pipe is submerged about 25mm below the overflow weir on the seal pot, so it's still drowned at zero flow. This stops air getting in and breaking the siphon. My shot at a sketch attached.
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