# External Overflow Question



## BenM002 (6 mo ago)

Hello, I am planning on building an external Herbie overflow like the one below:








I'm confused on the bulkhead sizing, why is the tank to overflow bulkhead a larger diameter than the main drain? I would have thought they would want to be the same. Is it in case you want to run a trickle emergency which would therefore require more flow in to the box?

Thanks,
Ben


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## Dartmouthseven (Oct 13, 2021)

BenM002 said:


> Hello, I am planning on building an external Herbie overflow like the one below:
> View attachment 845261
> 
> I'm confused on the bulkhead sizing, why is the tank to overflow bulkhead a larger diameter than the main drain? I would have thought they would want to be the same. Is it in case you want to run a trickle emergency which would therefore require more flow in to the box?
> ...


I'm not exactly sure why they designed it that way, I'm sure they have their reasons. Could be a 
fail-safe of some sort. I wouldn't worry about it. Just build yours to your liking.

🙏


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## Lew93b (6 mo ago)

it is a fail safe to keep you from flowing over the top of your tank if it gets partially clogged similar to having an emergency drain.


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