Help needed with overflow box

[Bob]

For some reason about a week ago my overflow box starting draining completely. It fills up and drains, fills up and drains. Skip ahead to about 1 minute in the video. Nothing in this setup has changed. I tried taking the return pump out and cleaning it but that didnt help.

https://youtu.be/GSM4HvhWmDI

 

[Cory]

Here is my theory on what's going on. Basically what's going on is it's starting a siphon. This happens when more water is going in than is free flow draining. Now I realize nothing has changed, but in general the slime layer that builds up inside of PVC, that brown slime like stuff changes the diameter of the pipe by a little bit. This causes less free flowing water. You must have been right on the edge.

To fix it you could clean your pipes... I don't do this because the problem just comes back. You could put a ball valve on the return pump and dial it back a tiny bit. You could improve plumbing if there are 90 degree bends you could use 45 degree bends etc. This can sometimes fix it as well. Sometimes creating less back pressure could fix it also. If the end of the pipe is submerged in water, try it without being submerged and see if the problem is solved.

 

[Caseyf]

I'm thinking maybe this is a decent explination. https://youtu.be/ISfIT3B4y6E

 

[GoGo98102]

Add a 2nd intake bulkhead/hose to make it a herbie system. Durso is not really a good/quiet system.

 

[jrygel]

Add a 2nd intake bulkhead/hose to make it a herbie system. Durso is not really a good/quiet system.

This is basically my plan - going to get an oversized (for my tank) dual inlet box and turn it into a herbie style system. The idea with a herbie style system is that most of your flow goes through a pipe that can form a full siphon - you achieve this by adjusting it with a valve, you then have second overflow pipe that takes whatever doesn't make it down the full siphon pipe partly as a backup and partly just to take up the slack as your siphon pipe gets clogged and doesn't flow as well. I think Cory's explanation is correct - I wouldn't recommend dialing back your flow with a ball valve to get a more consistent siphon though because there will be no margin left in the system - if you are already running a full siphon with no backup of any kind, it will slow down as it gets dirty, and this can result in your return pump outrunning the overflow.

 

[Cory]

If you are slowing flow down on the pump side a ball valve is fine. If you are controlling the siphon flow on the bulkhead side, then you need a gate valve.

 

[GoGo98102]

And if your pump is more powerful than your bulkheads can ever handle, you only need the gate valve on the pump side. Just let the siphon be fully open.