Help me understand how my overflow / sump works to not flood!

Discussion in 'Saltwater Beginners' started by johnny872005, Jul 28, 2015.

  1. johnny872005New MemberMember

    Hi All,

    I'm new to the hobby and have a used 120 gallon I picked up off of someone.

    What I'm struggling to understand is how the overflow and sump work. Specifically, I'm trying to understand how, in the event of a power failure (and thus the return pump isnt pushing water out), the tank does not keep flooding into the sump until the water level reaches the height of my intake overflow.

    Below is a quick diagram I put together showing my basic setup.


    From the image, you can see the intake is below the water line, and gravity naturally pushes water down to the sump. Also, the tube from the intake overflow sits above the sump and just dumps the water down into the sump.

    What I'd like to understand is: In the event of a power failure, how do I stop the tank from dumping all the water that is above the intake hole? From what I understand, my sump should only really have to hold the water that's in my plumbing, but from my understanding of the diagram above, I think water would keep flowing down.

    Is there something I'm missing or overlooking? Any help is appreciated!!
  2. LiterallyHydroWell Known MemberMember

    The most simple way to prevent flooding, is to keep the water level in the sump low enough so that if the power went out, when the tank drains into the sump it won't go all the way to the top of the sump.

    The way I fill sumps is to first set up the overflow in the aquarium and let it drain water into the sump until it stops (When the water level in the display tank goes below the overflow) and then fill the sump to the top. Now, when the power goes out, the overflow will drain water from the display tank down into the sump until the sump is full and stop.

    The best measures to take against a sump flooding is to set it up and fill it to the right level from the beginning, instead of having the power go out and having an unfortunate situation like that.

    Edit: That was too much rambling and a little bit circular. Let me try to explain this better.

    The sump won't take water from the whole tank, it will only fill with water that can flow into the overflow. When the water level falls a bit, the overflow should not be able to remove water anymore. So you fill the sump only enough to be able to fill with that amount of water.

  3. johnny872005New MemberMember

    Ok, I understand what you're saying. But I guess thats a bit surprising to me, since that means my water line is pretty fixed.

    So to make sure I understand what you're saying, I should:
    1. Empty out the sump
    2. Fill the tank until water starts to overflow into the sump
    3. Then fill the remainder of the sump
    4. Turn on my return pump

    Thus, if the power goes out, then I've already ensured there is not too much water. Correct?

    If so, then my question is - doesnt that mean I'll have a slow amount of overflow into the tank, since it's just barely skimming into the overflow? Wouldnt this make a water change hard too, since the water height is just slightly enough to go into the overflow to begin with?

    My tank is a 120 gallon, and I think my sump is ~20-25 gallon. Are those ratios ok? I would of expected I could of had the water level higher, but that doesnt seem to be the case.


  4. SlugWell Known MemberMember

    Raise your intake so that once the water level drops it breaks the suction. Also make sure your return is above the water level, or if it's not drill a hole in the section above the water level to stop a reverse siphon.
  5. SlugWell Known MemberMember

    Side note, it would also help to know is this tank already drilled? HOB overflow box? Reef Ready box? What are we dealing with as far as current overflow might help with a specific method.
  6. johnny872005New MemberMember

    Hey Slug, thanks for the info - thats the kind of questions I had. For starters, it's a 120 gallon tank with two pre-drilled holes. Hope that helps.

    As for your comments:
    1. What do you mean raise it so it breaks the suction once the water level drops? Do you just mean make sure the intake is just barely below water level, so once it fills the sump beneath no more water will flow in?

    2. My return line has a one-way valve near the pump which should stop water coming back down - but why would there be a return siphon there to begin with?
  7. DoubleDutchFishlore LegendMember

    It means the intake (to the sump) should be an overflow only overflowing when water is pumped into your tank. When pumping into the tank stops the overflow will stop almost immediately and no more water will flow into the sump..

    The one way valve prevents water returning into the sump when the pump fails / syphoning water out of the tank that way.
  8. SlugWell Known MemberMember

    Exactly. Just physically have the tube raised so that the 90 degree elbow is in the water just enough to have an established siphon without it sucking air and such. That way if the power goes out and it continues sucking, eventually the siphon will break due to it sucking more air than water and no more will flow to the sump. The other side to this is you must make sure your return is not submerged either as this will create a siphon on the other side hence why I said either keep it above water and spray it into the tank or have it submerged but drill a hole in the pipe that is out of the water so that a back siphon can't be established and the water stops when the pump stops.

    The valve is good, just make sure it will hold the pressure. A siphon would be started because the tube will be filled with water all the way from the return section of the sump to the tank. Break this anywhere with air and you stop the siphon. Or like you said, with a valve.

    The only reason you would have a siphon there would be the pump not putting pressure through that pipe therefore the water will just drain back down due to gravity and if it's submerged it will keep draining until it can't drain anymore.
  9. johnny872005New MemberMember

    Thanks for all the helpful info! Then I guess the remaining question is - if it is barely below the waterline as advised, is that enough to get proper water circulation? It feels like gravity would drain it in too slowly, but I may be wrong. Also, to confirm, the water should be just above the top of the 90 degree corner of my intake, and NOT just above the part where water first comes in, correct?

    Also, is it OK if the tube going from the tank to the sump just drops the water into the sump? Or should this be submerged?
  10. LiterallyHydroWell Known MemberMember

    Since the overflow is acting like an overflow, and not as a siphon, the amount of water that flows down into the sump will be based on how much water your return pump is moving. The bigger the return pump the more water will flow into the sump. But too much flow on the return pump and it may pump water into the tank faster than the overflow can move water.

    And when I was running a sump, I let the PVC from the overflow go straight down into the water to cut down on noise, that should be fine.
  11. johnny872005New MemberMember

    ok that was my thought, but wasnt sure.

    and ok, i wasnt sure if there would be a siphon or some problem if I let the pvc from the overflow go straight into the water or not.

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