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    Thread: I guess I don't understand siphon as well as I thought. Help?

    1. #1
      combatwombat is offline Member
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      I guess I don't understand siphon as well as I thought. Help?

      So, I'm getting to the end of building a water garden, and I'm having some siphon issues. When the pump shuts off, I get backflow. I thought that I understood how to control that w/ anti-siphon piping/air admittance valves, but maybe I don't understand it as well as I thought, and hoping you all can help me not mess it up again.

      The pond is about 6,500 gallons and overflows via negative edge into a 2,500 gallon reservoir. I have (3) 2" flex pvc lines starting in the reservoir (to be connected to submersible pumps) that feed jets and other features inside the pond. The flex line is all inside the pond and travels above water level to get over the weir of the negative edge and into the pond.

      I noticed that after filling the pond, the unconnected ends of the hoses in the reservoir were spilling water back into the reservoir when left dangling down. They didn't stop until water level was equal between the pond and the reservoir. That must be the siphon effect since the apex of the piping is above water level.

      I will have check valves on these lines, but I know those eventually fail, so I was thinking I'd also add an air admittance valve to each line. But I'm not sure where to add it.

      I think you'd normally add them at the highest point in the piping? I've already covered that area with a giant boulder and other rock work. It's also a very visible location where I'd rather not have to try to hide 3 AAVs.

      Would it work to tee off of these 2" lines somewhere else—below pond water level—and then place a stand pipe with the vents attached above water level? Like this:

      Name:  IMG_4113.jpg
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    2. #2
      combatwombat is offline Member
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      Ok, I think typing this out has made me realize the answer is no, this won’t work. I think we can disregard this question. Thanks for letting me think out loud.

    3. #3
      icu2's Avatar
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      I'm not familiar with the valves you were speaking of so I didn't want to offer the wrong advise...
      The only thing I can relate it to is I use an anti siphon on my s/g filter to prevent it flowing back to the pump
      if the power failed so I just drilled a 3/8" or so hole in the pipe supplying water to the s/g filter and it
      dribbles out under pressure but it's enough to break the siphon if the pump is shut off. Here's an arrow where
      the hole would be in your example:

      Name:  Image1.jpg
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      Just fwiw...
      --Steve



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      combatwombat is offline Member
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      Thanks, Steve. I can probably get at these lines with a really long drill bit, so I’ll give that a shot. Any idea how to determine the size of hole to drill? How did you land on 3/8”?

    5. #5
      icu2's Avatar
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      I copied Kent Wallace.

      https://www.koiphen.com/forums/showt...823#post622823

      But I remembered wrong... it says it was 3/16". You might even be able to go smaller... you just need enough so it'll break the
      water tension and allow air to get sucked into the hole. I'd try a small one and see if it works and then if not, go just a
      hair bigger until it stops the siphon.
      --Steve



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      combatwombat is offline Member
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      Quote Originally Posted by icu2 View Post
      I copied Kent Wallace.

      https://www.koiphen.com/forums/showt...823#post622823

      But I remembered wrong... it says it was 3/16". You might even be able to go smaller... you just need enough so it'll break the
      water tension and allow air to get sucked into the hole. I'd try a small one and see if it works and then if not, go just a
      hair bigger until it stops the siphon.
      Great idea!

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      aquaholic is offline Supporting Member
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      You can use a check valve IN REVERSE to break your siphon. Place it where you indicate "Can siphon breaker go here". Use a hinge check valve. Only when the pump is running, the valve will stay closed.

      Check valves are not ideal though. Or place a hole where your red arrow is, right at the water level if you don't want to see a water spurt out. I would place another slightly larger hole (or slit) below the first hole for redundancy.

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      combatwombat is offline Member
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      Well, a 3/16" hole stopped 2 out of 3. I'm up to 5/8" on the last one and it's still going...

    9. #9
      combatwombat is offline Member
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      Quote Originally Posted by aquaholic View Post
      You can use a check valve IN REVERSE to break your siphon. Place it where you indicate "Can siphon breaker go here". Use a hinge check valve. Only when the pump is running, the valve will stay closed.
      I don't think that will work because air will not make it into the line to stop the siphon until water level in the pond lowers all the way to the tee that the standpipe comes off of.

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      combatwombat is offline Member
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      Well, the 5/8" hole finally got the stubborn one. Maybe I didn't wait long enough for the downhill pipe to clear before going up a size? The other pipes have longer tails and they stopped right away with the 3/16" holes, so I was expecting the same from the 3rd. Who knows. It seems fixed though!

      Will have to wait and see how much flow I lose through that gaping hole, though. Guess I can always choke it down with a barbed fitting or something later if I need to.

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      aquaholic is offline Supporting Member
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      The reverse check valve will work. Tried and proven.

      Hole is simpler and safer though.
      Why are you trying to get the smallest sized hole? I would go bigger and add a second hole lower down (slightly under water). A slit has less chances of blockage but two holes or slits best. Biofilm or detritus blockage would be disastrous if stopping the siphon is important.

    12. #12
      combatwombat is offline Member
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      Quote Originally Posted by aquaholic View Post
      The reverse check valve will work. Tried and proven.

      Hole is simpler and safer though.
      Why are you trying to get the smallest sized hole? I would go bigger and add a second hole lower down (slightly under water). A slit has less chances of blockage but two holes or slits best. Biofilm or detritus blockage would be disastrous if stopping the siphon is important.
      I’m clearly no plumbing genius, but the reason I’m thinking it won’t work is because water always seems it’s own level. So if the tee is 12” below water level—with a standpipe above water level—then the standpipe will have 12” of water in it. No air will be introduced to the main line to break siphon until water level falls 12”.

      If I’m correct about that, then it seems the AAV I originally proposed would work just like a reverse check valve would. That’s basically what an AAV is, after all.

      But maybe I’m wrong.

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      aquaholic is offline Supporting Member
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      Understand your concern. You could move the reverse check valve to the higher water level point to provide reassurance comfort. Replace either of the elbows with a TEE where pipe goes over your wall.

      But using drilled holes is best as long as they can't clog.

    14. #14
      Enrgizerbunny is offline Senior Member
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      Air admittance valve can go anywhere in the line above the water line and it should break the siphon. That's how plumbing vents work - and what you're putting in is exactly that : a valve used to create a "vent" where a vent stack didn't exist. That's why they're one way - to prevent sewer gas from getting into the building. A siphon creates a vacuum in the entire line, and it doesn't stop until gravity difference goes away (equal level) or air is introduced into the system. If you are installing a tee and putting the valve in I think the only issue is that once it is below the water line, it isn't under vacuum anymore.

    15. #15
      combatwombat is offline Member
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      Quote Originally Posted by Enrgizerbunny View Post
      Air admittance valve can go anywhere in the line above the water line and it should break the siphon. That's how plumbing vents work - and what you're putting in is exactly that : a valve used to create a "vent" where a vent stack didn't exist. That's why they're one way - to prevent sewer gas from getting into the building. A siphon creates a vacuum in the entire line, and it doesn't stop until gravity difference goes away (equal level) or air is introduced into the system. If you are installing a tee and putting the valve in I think the only issue is that once it is below the water line, it isn't under vacuum anymore.
      I really want to understand this, and I think I'm still not getting it. If I install a tee, standpipe, and air admittance valve where shown in my drawing above, it will stop siphon, but not until the water level in the pond reaches the same level that the top of the tee is at. If the tee is 2' below pond water level, then the line will continue to siphon until water level drops 2' and air can be introduced to the main line.

      Is that right? Or am I still not getting it?

      Bonus question: Anyone have a clue why a 5/8" hole would be insufficient to immediately break siphon on a 2" line? Seems general wisdom is that a 3/8" hole is as big you'd ever need and 3/16" is more common.

      The outlet of the pipe inside the pond is 4.5' deep and I drilled the hole at the high point of the line, so it is completely exposed to air. It does work in that the siphon is broken eventually, but it takes about 15 minutes of draining at what looks like about 1-200 gph before it stops completely. Not the end of the world, but I thought it should work a lot faster than that.

      I don't think it's just draining the water beyond the high point because there's only about 10' of pipe from the siphon breaker hole and the pump vault, so there should only be about 2 gallons of water to drain back after siphon breaks.

    16. #16
      nota is offline Senior Member
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      I tryed to build a suction syphon to move muck but no luck very weak short lift
      so it got placed to suck air in to well water on the input side to get decent O2 in the ground water
      it does that very well

      I found very little on syfons or siphons but fire dept's call them e-ductors or eductor pump or jet eductors or jet pumps
      https://www.grainger.com/search/pump...E&gclsrc=aw.ds

    17. #17
      icu2's Avatar
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      Quote Originally Posted by combatwombat View Post
      I really want to understand this, and I think I'm still not getting it. If I install a tee, standpipe, and air admittance valve where shown in my drawing above, it will stop siphon, but not until the water level in the pond reaches the same level that the top of the tee is at. If the tee is 2' below pond water level, then the line will continue to siphon until water level drops 2' and air can be introduced to the main line.

      Is that right? Or am I still not getting it?

      Bonus question: Anyone have a clue why a 5/8" hole would be insufficient to immediately break siphon on a 2" line? Seems general wisdom is that a 3/8" hole is as big you'd ever need and 3/16" is more common.

      The outlet of the pipe inside the pond is 4.5' deep and I drilled the hole at the high point of the line, so it is completely exposed to air. It does work in that the siphon is broken eventually, but it takes about 15 minutes of draining at what looks like about 1-200 gph before it stops completely. Not the end of the world, but I thought it should work a lot faster than that.

      I don't think it's just draining the water beyond the high point because there's only about 10' of pipe from the siphon breaker hole and the pump vault, so there should only be about 2 gallons of water to drain back after siphon breaks.
      That does seem like a long time for the siphon to stop. Just thinking out loud, I wonder if putting the hole in the underside of the pipe, but still exposed
      to the air, would stop it any faster? The thought being if the air is entering the pipe when the pump stops at the top, and the "hump" over the pond edge
      is pretty small, maybe the water is able to continue to flow. If at the bottom the air would rise as it entered the pipe and might break it quicker?

      As someone pointed out, the 3/16" hole is pretty small for your scenario. In mine where it is in route to a s/g filter, the debris has been prefiltered and
      so I've never experienced any sort of clogging in the years it's been running. But where yours is coming from a sub pump from the bottom of the other
      pond, I think it might be worth making it a tad larger. It also isn't going to a filter, so my smaller hole was also an attempt to keep as much water going
      into the actual filter media as possible. Fwiw, here's what mine looks like:

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      --Steve



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      aquaholic is offline Supporting Member
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      I don't understand why you want to break the siphon. Is your sump volume too small or too low for the pond water level?

      Depending on the water velocity, a siphon would / should easily suck in air even if the water level was 12 inches lower but the safest place is somewhere above the water level.

      To answer your last question, a smaller sized hole pulls in air but much of the air is expelled with the siphoning water. It is taking a few minutes for enough air to slowly accumulate sufficiently to break the surface tension - water attracting (polarity) water. A larger sized hole would fix this. However even a larger sized hole can easily clog without redundancy. Why can't you drill a bigger hole? All the water goes back to the pond regardless.

      The reverse check valve is a method to producing a dynamic sized hole. No hole when water pump is flowing, very large hole as soon as you need one. A 50mm sized check valve was what I had in mind.

      Is your plumbing too difficult to experiment with? That's what I do when I'm unsure what will work. You could have a rotation collar around your drilled hole. Fully opened is a perfect circle through both the pipe and collar. Rotating the collar reduces the hole size as you need.
      Last edited by aquaholic; 05-24-2022 at 08:20 AM.

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      combatwombat is offline Member
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      Thank you, Steve. Good idea about the hole in the bottom instead of the top.

      I don't understand why you want to break the siphon. Is your sump volume too small or too low for the pond water level?
      Primarily to save space in the reservoir. The reservoir is 2500 gallons. My desire is to reduce "water in motion" as much as possible so that I have as much space as possible available for rain water storage. I am piping my gutters into the reservoir to try to make the pond as self-sustaining as possible.

      Water level in the reservoir, I expect, will fluctuate a lot vs. pond water level. In the winter, it will be totally full and, in the summer, I expect I'll have to add water occasionally to keep it from running dry. There is an 6" drop from pond level to reservoir overflow, so there will always be a minimum 6" difference between the two.

      Why can't you drill a bigger hole?
      I can! I was just concerned that since a 5/8" hole wasn't doing the trick fast enough for me, that something else might be off and didn't want to keep making it bigger and bigger without consulting the fine folks here first.

      Is your plumbing too difficult to experiment with?
      Nope! I will do just that. Unfortunately, I am now frantically dealing with a bunch of pinhole leaks in my (relatively new) RPE liner. So this is now taking a back seat to that fiasco, but I'll return to this once I have that sorted.

      You could have a rotation collar around your drilled hole. Fully opened is a perfect circle through both the pipe and collar. Rotating the collar reduces the hole size as you need.
      That's clever, too!

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      Enrgizerbunny is offline Senior Member
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      Quote Originally Posted by combatwombat View Post
      I really want to understand this, and I think I'm still not getting it. If I install a tee, standpipe, and air admittance valve where shown in my drawing above, it will stop siphon, but not until the water level in the pond reaches the same level that the top of the tee is at. If the tee is 2' below pond water level, then the line will continue to siphon until water level drops 2' and air can be introduced to the main line.
      .
      That's correct from what I understand about siphons.

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