Originally Posted by mono
how you balance the amount of water being returned with the amount being siphoned
a properly working overflow does not siphon water, it just drains off "excess water". Yes, external overflows use a siphon, but only
to move the excess water from inside the tank to the drain-area in the overflow. from there, gravity takes care of draining the water down.
this way, the overflow can only drain "excess" water, or water that arrives in the tank in excess of the water level set on the overflow... hmm... try this explination
you have a tank with overflow, and a sump with a pump in it. you fill your sump with water and turn the pump on. the pump is now trying to overfill your tank, using water from the sump. however, the overflow is designed to handle this by allowing water from the tank to do a controlled "spill" into the drain. so water the pump sends up causes water to spill out of the tank (causes the tank to overflow
), but because you have an overflow, that spilled water is directed back into the sump.
This system is almosts fool-proof - the achillies heal of it is the capacity of the overflow must exceed the capacity of the pump. otherwise, as the overflow collects debris, it will slow down. once it slows down to the point where it can no longer accept water as fast as the pump provides it, your tank will flood
therefore, it's a good idea to over-buy your overflow so an occasional lapse in cleaning is not going to cause a flood in your home!
I've been toying with building something, but had just about given up on making below my tank, I was going to put it above, pump dirty water up to it, and let it flow through media and back into tank.
This system works on the same principals as the under-tank sump, however, it is the sump that is "overflowing" and draining into the tank.