Having some trouble determining the best size for an external overflow box.
Tank is being custom made. 98% certain it will be peninsula overflow.
72" L x 26" W x 23" H
Overflow about 22" slot on one side, leading to external box. Just not sure what the width and height of the external box should be.
1) Just standard sizing, so long as the plumbing is done correctly? Has it been determined that a certain size works better?
2) I see some say 1" drains and some with 1.5"...
3) I also just came across a post that mentioned upgrading the open channel to 1.25", to reduce the air/gurgle noise. Is this the elbow fitting or the entire drain at 1.25?
When I made my own in years past, I just made it big enough to work with and to get plumbing in. However, having a deeper overflow would allow for the open channel to be physically higher than the siphon, as opposed to the same height with an airline tube attached to the top - correct?
I read that there needs to be a certain amount of space if the open channel is higher than the siphon, so the siphon has time to purge itself of air before the water level recedes again... otherwise it's a gurgling mess.
So...
4) Do we know how much "higher" the open channel would have to be, if it's physically placed above the full siphon?
I've read more now that I have time. Here are my thoughts. Please verify.
1) Overflow size of function before asthetics. Enough room to work with. Deeper/taller may be nicer as more height variation between open/full siphons. Helps give time for flush.
2) Anything over 1500 GPH is best served with 1.5" bulkheads and 1.5" pipe. Original 1.5" pipe with smaller bulkhead was just to make water trickles quiter and due to build up/already drilled holes.
3) Above
So now I have two questions. In the original design it goes full siphon --> open channel trickle --> emergency --> airline on open submerges for full siphon.
1) Is there anything wrong with having the emergency drain kick in AFTER the airline tube is submerged on open siphon? So instead of airline tubing last, it is emergency last?
2) How much vertical height should there be between open channel air opening and main siphon, so that it has time to self flush? My overflow may use height difference to achieve this with a hole drilled in pvc elbow., so want to be sure.
Sizing is determined by your hole spacing, plumbing, and choice of material (acrylic vs glass). If it's made out of glass, you should have 1 hole diameter between glass edges, so it needs to be a minimum of 3 diameters wide. Beyond that, figure out your plumbing and make sure you've got enough extra room to slip your fingers in to clean.
As far as sizing the plumbing, in his original writeup, Bean used 1" bulkheads with 1.5" plumbing and ran 2000 gph easily. If you do the calculations, a 1" pipe with a 4' drop can run about 2300 gph, and a 1.5" pipe over 6000 gph. Using smaller bulkheads has the advantage of allowing smaller holes (1.65" holes vs 2.3" holes for 1 vs 1.5") I would probably use 1" bulkheads with 1.5" pipe like bean did, or at least 1.25Ä pipes. 1" would be cutting it a bit close.
For the open channel, the size of the pipe has a direct effect on how much water it can carry quietly. Larger pipes carry significantly more water before they start gurgling, so even if you're running 1" pipe on your siphon there is an advantage to increasing the size of the open channel. For you, I would just keep them all at 1.5".
As far as the pipe positioning goes, the open channel should be at the same level or higher than the siphon channel. A larger difference allows for a higher head height over the siphon as it's starting up, making it a bit easier to clear the air, but it should be fine either way.
With an external overflow, most people put an upside down 'U' on the siphon to mimic Bean's setup. Doing so isn't required, but has two advantages - first it reduces the risk of vortices forming and the siphon entraining air from the surface. Second, you can position the pipe somewhat close to the floor to reduce the risk of snails getting into the overflow pipe. If you're running 2000 gph you won't be able to have it too close, though. Regardless, if you do this I would not glue it in place to make cleaning easier.
The normal configuration is to have the air tubing for the open channel above the opening for the emergency drain. This is because once the open channel converts over to a siphon, it will rapidly drain the overflow box, dropping the level to the point that the siphon pipe entrains air. This often leads to a cyclic flushing scenario where the siphon never fully purges. Having the air tubing above the emergency drain means the water level rises to the level of the emergency drain and stays there until the air is purged, at which point the siphon capacity increases and the water level drops to its steady state.
In your setup, if you have a significant height difference between the siphon inlet and the open channel inlet, it may work. You can play around with it and see easily enough. Just make sure the height of your dry emergency drain is low enough that it can handle the entire flow from your return pump without overflowing the tank or overflow box.
