Twin 1200 Modular Marine Overflows... how to get them running correctly.... any idea?

[Dukereefnz]

My 200G tank has twin 1200 modular marine overflows.
They are set up with bean animals....

2 x 8000 l/hr pumps running at 50%... so after losses on average 4000l/hr?

To get it working nice and quiet I blocked off one partial flow.
So I have the following..

2 x Full syphon
1 x Partial flow
2 x Emergency

I had to block off one partial to get both full syphon's going on both, if I did not block one partial I could not get the both syphoning...
So now it is working well... except I noticed that the water is not skimming at the top.
Both the overflow box and the tank water levels are about the same.

I have tried all sorts of thing to have the water pouring in to skim the top.. nope.
I am actually thinking this is not needed... water will be flowing in through the skimmer box in to the overflow box....what does it matter if the water is not flowing down.. its still being sucked in...

Could I have
2 x Full syphon
2 x Emergency

No partials???
Water levels equal in both boxes... I think that is how the modular Marine are designed.. both boxes are the same height..... ?

Not running...

 

[blaxsun]

Do you have gate valves on your overflow piping?

 

[Dukereefnz]

Yes a gate valve on each full .syphon

it has changed a little.
2 x pump returns with non return valves.
2 x full syphon with gate valves
1 x partial closed off - not shown
1 x partial now goes to the left.

 

[DCR]

I cannot tell for sure, but it appears that your partial overflows are set too high and that is causing the lack of surface skimming. The overflows could also be undersized for your return flow which is causing the level in the tank to run high above the overflow weirs, but I would not think that should be the case with two overflows. You could try reducing your return rate, but I would try lowering your partial overflows.

I do not understand how a partial overflow would prevent a full siphon from working unless the flow through the full siphon is too low to keep air from getting trapped in it.