How do you prevent backflow?

you make the hole in the return pipes where it sits about an inch below the water level i think it is, dont quote me on that though

 

Yes... make the hole slightly below your normal water level.

When the power goes out, the returns will syphon back until it hits the syphon break hole. That will suck air and break the syphon and stop the flow back into your sump.

M

 

Siphon breaks aren't foolproof, they can get clogged or a snail could wander over one. Can you raise your return up so it's closer to the surface of the water? That's the more foolproof way to prevent the sump from overflowing. If not then siphon breaks are probably your best option, I would put it just at the water line, maybe 1/4" to 1/2" under and use at least 2.

 

How big of a hole do you all recommend drilling? Thank you for all of the great advice

 

3/16-1/4'' would be about right imo.

 

NEVER rely on drilled holes, they can and will fail.
The only foolproof method of backflow prevention is an air gap. This gap is provided by keeping your returns close to the surface so they are exposed to atmosphere when a small easily calculated amount of water backflows to the sump. This amount of water is minimal and you should always maintain that amount of freeboard or extra capacity in your sump at all times. In my case I have a 100G display and a 30G sump, my backflow is a maximum of 3.5 gallons so I keep at least that much room in the sump at all times.
Drilled holes just like check valves are a false sense of security. they require cleaning and mainetance and even then could plug 1 minute after cleaning. Whats to stop a piece of frozen food, a snail, an anemone, a small fish, a chunk of surface algae etc from covering the hole?

Air gaps require zero maintenance and do not fail, water cannot jump up.
How big is your tank? measure the length times width and maybe 1" high, mine is actually 3/4" that flows back but 1" worst case then divide that number by 231, thats how many gallons a properly designed return will flow back if its 1" below the surface.

Mine calculates out like this 60"x18"x1"=1080 cubic inches /231 = 4.67 gallons at 1" depth but mine is 3/4" so 4.67x.75=3.5 gallons of backflow.
Not much at all and easily contained in my sump.

 

I installed a check valve on my return plumbing. When the pump goes off, the check valve immediately slams shut by the weight of the water column trying to flow back.

 

thanks I didnt know what that hole was for. I had duct tape over it.. and have had the same problem with backwards siphon = sump overflowing.. now i dont need to buy a check value. if the power goes out no more messes! thanks K+

I also raised my return line outlet with some plastic spacers w/ rubber bands... my hole is actually 1/4" above my water line now..

 

Holes and check valves fail, they are a false sense of security and will sneak up on you. We avoid check valves like the plague in the wastewater treatment industry and thats exactly what a reef system is, a miniature treatment plant. It does not have to be a catastrophic failure, even a trickle will flood given time.
Take the time to properly design the overflow and return systems and a flood is a thing of the past and you can sleep soundly at night.

 

Very true. A trickle over several hours amounts to a lot of water. I have locline splitting into 3 outlets in the display tank. I plugged the siphon break because I was getting too much salt creep out of it. I set the locline nozzles in such a way that they will all suck air before I get a flood (which goes back to your point about properly sizing it to begin with). In the event the check valve fails, then all 3 wide flare nozzles would easily fix the suction.