Overflow box length - does it matter?

[BigTerp]

Planning a 40 gallon breeder mixed reef with 20 gallon long sump and am curious about the effect of the weir length/width. I'm planning a bean animal type overflow with the plumbing on the outside and the overflow/weir on the inside/wet side of the tank. Considering going with Modular Marine's 1600 gph overflow, and having them modify it for 1" bulkheads all the way around. The weir box is 16" in length, but I would prefer something smaller. More like 10" - 12". What effect or advantage would a smaller length weir box have over a longer length?

 

[T-J]

I assume the smaller the box, the less flow it can handle.

 

[csund]

The shorter the box the less area for water to pass through the weir. At higher flow rates this will cause the water level in the DT to raise further. No major issue but this water will add to the volume of water that drains back when the return pump is turned off. Make sure the sump can handle that and your good to go.

 

[Sean Clark]

I prefer as big as is reasonably possible.

 

[BigTerp]

Thanks all. I'm looking at probably 5-600 gph max of flow from my return pump. So with 1" bulkheads I would think the weir would have to be pretty small to make a noticeable effect in terms of water level in the DT. Just wasn't sure if there were any specific advantages to having the weir box cover more surface area or not.

 

[csund]

I use a smaller modular marine with 1/2” drains pushing about 500 gph on a frag system. However, the drain size has no relation to water level in the tank as long as the water level in the exterior box is lower than the DT. This overflow is probably about 8” long and with 500gph the water level is just over 3/4 of the weir height.

I actually wanted the water level in this tank lower to accommodate a wave box but I didn’t take into consideration the additional increase of water level. I could probably have them make a custom internal box with a lower weir but it hasn’t bothered me too much.

 

[KStatefan]

As mentioned before the height of the water in the DT will go up with less weir teeth. A wider box will also skim more surface area but with good water movement not sure that matters.

 

[BigTerp]

As mentioned before the height of the water in the DT will go up with less weir teeth. A wider box will also skim more surface area but with good water movement not sure that matters.

The bolded was exactly what I was wondering if it made any sort of difference. I plan to have appropriate powerheads/wavemakers to accommodate a mixed reef, including some SPS (eventually). So water movement won't be an issue.