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Old 02/14/2019, 04:37 AM   #1
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Join Date: Jun 2011
Location: Switzerland, Zurich
Posts: 54
Herbie emergency drain hight

Hi guys

I'm planning a Herbie style overflow with a trickle down the emergency drain. I noticed that everyone seems to choose the emergency drain hight lower than the weir, so that the water drops down by about 1/2 to 1 inch.

What would be the cons of mounting the emergency drain a little higher, so that the water does not fall down in the overflow box but instead "just flows in".

As a pro, I see no noise at all.
A con could be less room to the top of the tank and therefore less room for the edrain to start a full syphon. That however wouldnt bea concern in my tank.

Thanks, Marco

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Old 02/14/2019, 04:09 PM   #2
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Join Date: Mar 2008
Location: sf bay area
Posts: 5,109
you will get noise on the drop if your flow is really high. so high that water shoots forward and drops instead of the intended design of water coming down attached to the inner wall of the box.

if you make the emergency higher, level might bounce, as water will try to exit the emergency first, then main gets less water, so level goes down, then fills up again to emergency and cycle repeats.

is your overflow box two layers?

on mine, there is an inner box and an out box about 1/4" or so apart. The outer box has slots near the bottom where water can get sucked in. The flow is induced by Bernoulli effect at the top hence the drop is necessary to create the fast water flow speed between the inner and outer box gap to create the sucking action so water flows in from the bottom slots. I think this is a genius design.

if your emergency is slightly lower, it will muffle the trickle sound more.

I made mine 2 or 3" lower, can't remember for sure, but definitely not 1" or less.

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