It does have elbows with holes drilled in them for the outside though, which is what i meant. Putting any sort of normal plumbing fitting in their place means not being forced to use air assisted drains. I think the internal baffle plate would make the siphon pipe not draw air almost as good as a down-turned elbow, but it does make having the other pipe dry very hard... Not sure if there is a smart solution to that part of making a herbie.
I find the name (flow number) on the overflows a little strange, as i don't think most people would enjoy hearing 1500 gph running air assisted (if it even can do that).
The glass-holes 1500 overflow, stock out of the box will handle ~ 700gph before it drives the owner nuts... each 1.5" durso (that is all the drains are) will start acting up, just as any other open channel drain at ~ 350 gph. When we start talking about "Flexible Underwater Siphon Husher' ('F.L.U.S.H.') you have to sit back and say ok, somthing is not right here. Siphons are silent without a silencer, and the Glass-Holes overflow is not a siphon system. Glass-Holes is a lively fun company, but you can't beat the physics. I compliment you on taking a more considered view here.
The larger issue is the OP is not going to be able to do what he wants to do with this drain system, or any other drain system. Have to get past that first. Simply using Glass-Holes documentation tells us that their overflow system is not suitable for the OP's intended use. (Quoted so those who think I dream this stuff up, will see that I don't.
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"This plumbing should consist of the minimum of fittings needed, and no valves, or fittings smaller than the pipe size of the kit you purchased. If you have a nano series kit, this would be 1” PVC plumbing all the way to the sump, for 700 and 1500, this is 1 1/2”, and for the 3000 kits, this is 2” plumbing. Do not combine dual drains.
Avoid long horizontal pipe runs. Do not use insert (barb) adaptors in the elbows. Do not use anything other than PVC pipe or UltraFlex PVC pipe.
The kits will not work properly with smaller or restricted plumbing!!!!"
Incidentally, the laminar flow rate of a 1" open channel is around 50gph; on a real good day, maybe a bit more...
The OP wants to run a 4' horizontal run, and use a gravity fed skimmer. The horizontal run is self explanatory; the skimmer is a restriction in the drain line... plus a double gravity feed to get to the return pump... Taking this the next step further, you don't put valves in an open channel drain, it makes the problems with them WORSE, you don't want horizontal runs in a siphon system (OF EITHER TYPE,) and you don't want to feed a skimmer from a drain line because it messes with the physics of the drain. Double gravity feeds are a nightmare to balance and keep balanced especially with unstable drain systems. Not to mention that the boxes with the kits are over-rated and too small to do anything with.
So before the OP commits to any drain system, he needs to re-evaluate what he is intending to do. My very direct recommendation is the OP take this back to the drawing board.
