How Low Does Your Overflow Go

[Mr. Fish]

So I am trying to figure out how big my sump needs to be to hold the water when I lose power. One of the key parts of that is knowing how much water my overflow will drain. On average how far down do the slots on overflows go down? My tank being made will be 220 gallons and I was using 2" as worst case for the amount I water I will lose, does this sound right?

[Psirex]

That depends on the make\model of the over flow mine drains less than maybe an inch of water but the bigger question is the placement of the return which will back siphon into the sump as well

[Mr. Fish]

That depends on the make\model of the over flow mine drains less than maybe an inch of water but the bigger question is the placement of the return which will back siphon into the sump as well

I'm planning on drilling a small hole in the return lines to break the syphon, so that doesn't happen.

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[SNAKEMANVET]

Shouldn't be much from the overflow,but the return line will siphon water back down until it breaks the water surface.Drill a hole in the return line about a 1/4'' below the water surface where the water level is at while running,or have your return barely under the water surface.

[gone fishin]

I used 3 inches on mine when figuring it out. once I had everything plumbed in I filled the tank and sump enough to start flow. I marked the the sump at this point then turned the pump off and let everything flow back then measured the level and left that much room in my sump. I ususlly drill a couple holes for a syphon break just in case one clogs (experienced this).Hope this helps