maximum flow through a 1.5" herbie
 

What's the maximum flow through a 1.5" herbie overflow?

At least 1000gph.

Skimmin had 2 1.5" and with his Dart return figured ~2000gph.

I've learned that this is not a simple question. The reason that it is not is that it depends greatly on the amount of head pressure above the Herbie. (that's the whole reason that Herbies work as they do)

So....the greater the difference between the height of the water above the Herbie and the height of the water in the sump....the more flow that Herbie will take. At some point, with extremely high head pressure, there would be a maximum flow...but we don't generally get near that.

In my situation, I flow about 1000 gph through each of my 2 1.5" Herbies, and they are dialed well back. That's with nearly 5 feet of head pressure.

Down below...where one sump moves into the other...there is only maybe 10" height difference from behind the Herbie to below it. That is also a 1.5" Herbie and it is running almost wide open and just barely takes that flow.

So...the upper Herbie's take that flow easily..the lower Herbie takes it just barely because there is much less pressure lower down.

Sorry, I'm sure you wanted a simpler answer.

I had a snapper on my last tank and my 1.5" Herbie worked fine

Using the Drain Hole and Overflow size calculator on RC a 1.5" drain hole at 20" head (as per what Untamed was explaining) it will drain at 1325gph and anything higher than 20" will raise the gph.
Kevin

Where on the calculator do you get the height? If I put in 1325gph my understanding the 20" linear inches is the length of the weir.

Your right thanks Mark. Let me start over on that.

With a 1.5" hole you can drain 1325gph at 20" linear (which has nothing to do with what Untamed was talking about) but if you increase the linear you can as per RC calculator increase GPH.(not sure why as it becomes a side ways pressure instead of a down pressure) ??? LOL

Kevin

Believe the recommended weir length is just for designing a overflow box that will give you ~0.25" rise of water over the weir for good surface skimming.