[untamed]

Originally Posted by Chin_Lee

alternatively you can use one overflow as the primary drain and leave the second overflow as the emergency drain. then only put one gate valve on the primary drain and leave the emergency drain open.

I'm not sure that will work with dual overflow boxes..but I may be misunderstanding your suggestion. Using one box as primary and the other box as emergency would make the emergency one run all the time unless the two boxes were at different heights in the tank...which I've never seen done.

Anyway....I can attest that with two overflow boxes, things work perfectly well with two gate valves. Each box has a primary and an emergency. This is exactly what I have running. They do interact with each other...that is...altering one does have an impact on the other.

You could join the two primary drains, and put a single gate valve below the joint. (You would need a single, larger, gate valve) That would probably be the best solution because any adjustment would have an exactly equal effect on both boxes. For that matter, you could also merge the two emergency lines as well. In my case, I chose not to do this for reasons related to feeding twin sumps.