Unstable siphon...help!

J

JingoFresh

New member
A few times I have left for 5 days at a time for work, and I come back and my siphon is barely functioning, with a lot of air in the U-tube. If I had come back later it seemed that my tank could have been overflowing.

What could have caused this?

The only thing I am thinking is that my overflow box is not securely fasted, as I couldn't get the wingnut on tight enough, and so perhaps it moved from a large snail or something being on it.

Are there any other potential causes of this happening that I can investigate?
 
One thing to check is that your return pump can keep up with your overflow. If the flow is too slow it allows air to build up in the u-tube and the siphon will break.
 
If it is a single u-tube then I would guess you don't have sufficient flow through the tube to flush out small bubbles and they are accumulating.
 
Yes it's just a single u-tube.

I didn't buy the overflow box new so I'm not sure what it is rated.

My return pump is a rio 1400 which does about ~380 GPH.

I have about 4 feet of head.

How can I determine the GPH for the overflow box?
 
RocketEngineer said:
What size plumbing?

First guess still seems like the right one, you need a stronger return pump.
Click to expand...

+1 Most overflows can handle 600gph easily. 380gph seems really low to me. A pump like the jebao I listed has adjustable flow so you can dial in the perfect flow rate for your tank. I really think this is your best bet.
 
Yeah it can handle it, Your flow with that head pressure will go from ~250gph to 500 and should solve the air gap problem if that is indeed what it is. I use Sicce pumps but Rio will get the job done.
 
How can I tell what flow my overflow box is rated for?

I took a picture but I don't know if that helps:

6dOkjp7l.jpg
 
That looks like the smallest eshopps overflow box which is rated at 300GPH.

I do think your return pump is too small as others have mentioned, with 4 feet of head I can see from the pic that pump isn't moving alot of water.

But I have to wonder if there's another issue because I can dial back my return pump to practically a trickle and I still don't get any air build up in my siphon tube like that.
 
Last edited:
Turn off the return pump and remove the bubble. Turn on the return and see if you can spot where bubbles are being generated. It may be coming from somewhere you can tweak.
 
The green hose is connected to vinyl tubing, although they have the same diameter.

I've bought the for 692GPH so will see if that fixes my problem. Hopefully it will not be too much for the overflow box to handle. According to the calculator on the site the Rio 2100 pump with 4.3 feet of vertical head results in 312GPH.

If my overflow is only rated for 300, will this be a problem?

The air gap forms very slowly...I don't see it while watching after removing the bubble, but checking back every few hours it increases.
 
JingoFresh said:
Is there a chance the DT could overflow with the overflow rated at 300gph and the return flow being 312gph?
Click to expand...

That's the inherent risk with an overflow box. For a difference of only 12, it might not ever get that far. Look again at the numbers though, because what I'm seeing on the chart that's on the amazon page you linked to, the Rio 2100 does 375 @4'.

Looking at that pump though, it comes with a valve to adjust the return flow. You might also consider adding a lift pump. That's a small air/water pump that would attach to the apex of the overflow tube, and pump out any air helping to maintain the siphon, and restart it when it is lost.

Something else to consider might be a water level switch that will shut down the return pump if the sump level falls lower than it should be. It won't stop an overflow, but will limit the size of the ensuing flood.
 
W1ngz said:
That's the inherent risk with an overflow box. For a difference of only 12, it might not ever get that far. Look again at the numbers though, because what I'm seeing on the chart that's on the amazon page you linked to, the Rio 2100 does 375 @4'.

Looking at that pump though, it comes with a valve to adjust the return flow. You might also consider adding a lift pump. That's a small air/water pump that would attach to the apex of the overflow tube, and pump out any air helping to maintain the siphon, and restart it when it is lost.

Something else to consider might be a water level switch that will shut down the return pump if the sump level falls lower than it should be. It won't stop an overflow, but will limit the size of the ensuing flood.
Click to expand...

It is 375 at 4'. but 312 at 4.3' which seems to be closer to the actual height, best I could measure.

Do you have examples of a lift pump and water level switch to shut off the return pump that you would recommend?
 
There is an easy way to know if it's your pump, the tank water level or whether you've got too much turbulence at the top that's creating irregular flow and bubbles. Turn off the pump giving you surface turbulence. If you're hand is small enough, cover one end of the YouTube with your fingers as best you can to slow the movement of water through it. Allow the overflow chamber to fill up a bit then let go. The rush of water will push out the bubble in the utube and you'll be able to see how fast that siphon should be pushing water through. Watch the overflow chamber for a minute. Turn on your surface pump. If the turbulence around the overflow creates more bubbles in the overflow chamber, that's your problem. If you can't lower your overflow chamber enough to overcome the difference, then you need a stronger return pump.
 
To find out what size your box is - Nano (3/4" bulkhead - 200 GPH), 300 (1" bulkhead - 300 GPH). Hope this helps.
 
MY thoughts here:
There is too much turbulence in the overflow box.
It should be lower in the water raising the water level & eliminating the bubble source.
The water flow through the u tube should & MUST pull away any air bubbles trapped in the pipe to work correctly!
I ran 2 for years that way & had no problems.
 
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