Lower intake/drain beneficial or not?

[chome360]

Hello, working out the details on a custom build. I have a decision to make on the drain/overflow box. Should I have the overflow box run the full height of the tank (400 gallon peninsula) and inlude a lower drain/intake about 2 inches from the bottom, or should I just stick with the top (true) overflow? I'm reading about the benefits of adding the lower drain, but I don't see it done a lot and I'm wondering why. Any input is greatly appreciated!

 

[Jekyl]

I'd rather it up top to help with surface skimming.

 

[UncommonSense]

If a lower intake is added, there is typically a secondary weir inside the overflow box at the tank’s operating water height!

Without this secondary weir, a leaking drain standpipe would drain the entire tank to the lower intake level, overflowing the sump onto the floor!

Personally, I’m not a fan of this, and don’t see it actually moving much water… I like to have my water overflow to the sump from the tank’s surface! — wave makers do 80_90% of the display mixing anyways!

 

[chome360]

I'd rather it up top to help with surface skimming.

technically I'd have both, up top and down lower, so in theory it would do both. I think the idea is skimming at the top detritous at the bottom, not sure how well it works in practice though.

 

[chome360]

If a lower intake is added, there is typically a secondary weir inside the overflow box at the tank’s operating water height!

Without this secondary weir, a leaking drain standpipe would drain the entire tank to the lower intake level, overflowing the sump onto the floor!

Personally, I’m not a fan of this, and don’t see it actually moving much water… I like to have my water overflow to the sump from the tank’s surface! — wave makers do 80_90% of the display mixing anyways!

Ah, good point on a leaking drain pipe, hadn't thought about that. Also on the wavemakers. Planning to have a coupel MP40's on that back wall, so what's the real benefit... thank you

 

[UncommonSense]

not sure how well it works in practice though.

I’m personally not super impressed with the handful of different bottom inlet designs I’ve worked with…

They don’t seem to actually move very much water, or make the tank drain an extra 1/2”+ of water during return pump shutdown if they are designed to actually move an appreciable amount of water (inner weir for bottom inlet lower than top overflow teeth)!

— this inner weir also means an additional set of seams that could fail on the overflow box, which is usually acrylic siliconed to glass! (Causing the overflow box to weep tank water in constantly; an issue if you ever need to replace a bulkhead fitting, or have a leaky standpipe!)

 

[TX_REEF]

If a lower intake is added, there is typically a secondary weir inside the overflow box at the tank’s operating water height!

Without this secondary weir, a leaking drain standpipe would drain the entire tank to the lower intake level, overflowing the sump onto the floor!

Personally, I’m not a fan of this, and don’t see it actually moving much water… I like to have my water overflow to the sump from the tank’s surface! — wave makers do 80_90% of the display mixing anyways!

agreed, the lower drain is potentially dangerous, because in the event the return pump fails and there is a leak in the overflow box, the volume drained will likely massively overflow your sump, and also potentially fill your tank with freshwater from ATO. Recipe for potential disaster, in my opinion. Risks outweigh the benefits.

 

[chome360]

On that note, since I'm removing the bottom drain and shortening up the height of the overflow box, what's the ideal size? The tank is 7 feet long, by 3.5 feet long, and 28 inches tall. 417 gallons total capacity. Is there a rule of thumb on this?

 

[Troylee]

On that note, since I'm removing the bottom drain and shortening up the height of the overflow box, what's the ideal size? The tank is 7 feet long, by 3.5 feet long, and 28 inches tall. 417 gallons total capacity. Is there a rule of thumb on this?

There is no rule.. typically they’re 6-8 inches tall with a short stand pipe in them.. the deeper you go the noisier they get.

 

[UncommonSense]

On that note, since I'm removing the bottom drain and shortening up the height of the overflow box, what's the ideal size? The tank is 7 feet long, by 3.5 feet long, and 28 inches tall. 417 gallons total capacity. Is there a rule of thumb on this?

The only big things to watch out for on the mechanical side of things are giving yourself sufficient space in the exterior box for drain bulkhead fittings, and making the weir inside the tank long enough to accommodate your flow requirements!

— synergy reef lists their 16” long overflow (technically ~14.75” of weir teeth) at 2,400gph maximum flow rate, for a point of reference!

 

[chome360]

The only big things to watch out for on the mechanical side of things are giving yourself sufficient space in the exterior box for drain bulkhead fittings, and making the weir inside the tank long enough to accommodate your flow requirements!

— synergy reef lists their 16” long overflow (technically ~14.75” of weir teeth) at 2,400gph maximum flow rate, for a point of reference!

Ah, I see. I'm told the shortest I can go on this custom build is 12 inches, which still seems bigger than I was expecting.

 

[UncommonSense]

Ah, I see. I'm told the shortest I can go on this custom build is 12 inches, which still seems bigger than I was expecting.

I would agree that 12” is pushing it… let’s remember that 1,200gph is 20gpm, or a gallon every three seconds! — look at it that way to help you wrap your head around it!

I’ve got a 19” span of teeth on my center overflow, they’re custom cut (see below); but they seem like they’d max out around 1,500gph in the real world, unless I made the slots physically larger/taller! (Note that just making the weir slots taller to achieve higher flow rates will increase the operating water height of the display, and cause more water to drain to the sump during a power outage/return pump shutdown!)

If I was to redo this design for a tank your size; I’d make dimension F ~.150”-.200”, plus dimension b and C .100” larger! — possibly increase dimension A to 1.250”, from the top of the V only (not increasing dimension D)!

 

[chome360]

I see, just to make sure I explaned it right, I'm referring to the vertical height of the external overflow box. Although to your point, now you've got me thinking about the correct width/length of the weir teeth. Currently the design has is sitting a roughly 17 inches...