Sump overflow

[snookie]

Silly question but does a sump need an overflow ? 

[hovmoller]

No. not if it has enough room to take the initial back flow from the pipe. 

[snookie]

Can you explain a bit more I am a sump virgin....

[Shilo]

When you turn off your pump (or there is a power cut) the water will siphon from the return pipe in your tank down into your sump.  If your outlet of the return pipe is on or just below the water surface it will only siphon down to this level.  But if your outlet is near the bottom of the tank all the water above it could siphon out.  Can be easily stopped by using a check valve (not recommended) or a small hole drilled through the return pipe just below the water surface as a siphon break (will suck in air and stop the siphoning action). This will mean the water will only siphon out to the level of the hole you drilled.

But what ever method you use you need to allow room in the sump for the extra water that will flow into it before the siphon action breaks and stops.  If you use an overflow on the sump instead then when the pump starts up again the tank will fill back up to its normal level but the sump will be drained.

An overflow is only needed on the sump if you plan on using an automatic drip water change system (water continuously dripping into the tank or sump with the excess going out of the overflow to avoid having to do manual water changes).

[David R]

^ Nailed it. It mainly depends on how your overflow and drains are set up, but you need to make sure the return pipe also doesn't siphon too much back into the sump.

IMO it's critical to design the sump large enough to hold the "skim" when the pump switches off, even if it does have an overflow. Otherwise if the tank is unattended and the pump stops (eg power cut) you're not going to have enough water in the system for when the pump restarts.