size of hole in the return line?

[guitarded]

i just finished plumbing my sump. to break siphon in the event of a power failure, most of the people on this site have recommended drilling a hole just below the waterline on the return tube. what size hole are most of you drilling.

also, i understand that the hole will break siphon on a power outage, however, what about when the power comes back on. the overflow will begin filling up again, but the it will have lost siphon when the return pump went off and when the pump restarts it will not return water to the sump, thereby overflowing the main tank.

 

[theatrus]

In theory, the overflow should be designed to not break the siphon when the water in the tank gets under its input.

I would also recommend hooking up some airline from the top of the overflow siphon to a powerhead in the main tank. Some are better than others at sucking out the air (venturi types you can put in front of your Maxijet are fine, but the types designed for venturi skimmers are best), but its another catch in case the siphon does break, or slows down since air collects at the top.

 

[fishfreek]

Small holes. The exact size im not sure on but its one of the smaller drill bits in my collection.

The holes are drilled just below the water line on the RETURN plumbing. This is the plumbing going from the pump to your tank. This syphon break will have zero impact on your overflow syphon.

When the power goes out the water will continue to flow into the sump from the overflow and the pump will power off. Water will start to backflow down the return lines until air can enter the system via the holes. Once air is in the reutrn lines the syphon will break on the return line and the plumbing on the return will empty into the sump but no longer pull water out of the tank. The overflow will stop draning the tank when the tanks water level falls below the inside tank portion of the overflow.

When power comes back on the pump will start to pump water back to the tank filling the return plumbing and pushing air out of the return. You will see bubbles comming out into the tank from this. As the pump continues to put water into the tank the water level in the tank will start to rise and it will get above the intank overflow box. Once the water level is above this point it will start flowing into the overflow and since that syphon has not been broken it will go back down into the sump keeping your tank from overflowing.