Sump pump size...

[sphelps]

Originally Posted by mike31154

The way he described his overflows (post #8), the extra, or return holes are drilled outside of them. He'd have modify or move the overflows to include these outside holes (or plug them?) to do the Herbie using all drilled holes.

IC, in this case I would personally wouldn't use a standard herbie design, I think using a second overflow with the purpose of emergency only will lead to problems as already stated such as stagnant water even if it's higher as water will still sit around the bulkhead which could lead to mold growth and that algae smell. What I would do is use the one overflow as a primary to take the majority of the flow, you can still set it up with a gate valve but adjust it so a small amount of flow still goes through the second overflow which would have a durso standpipe. Setup the durso so it has a small siphon break hole and a longer downspout, if the primary clogs up the durso will siphon under the higher load to prevent tank overflow. The second overflow will need to be slightly higher but nothing significant, a layer of silicone should be sufficient.