Anti-siphon Hole questions

[Spridle]

I want to drill 1/8" anti-siphon holes in my PVC pump return lines to prevent reverse-siphon. Several posts on TRT state to drill the holes "just below the water line." Do the holes really need to be below the water line and, if so, why?

Since the return lines I will be drilling are in my overflow box, there is a chance that the water level in there will drop below the anti-siphon holes sometimes.

I've tested a 1/8" hole in a piece of tubing about the same diameter as my return lines by attaching it to my water outlet into the tank and bending the tube such that the 1/8" holes are above the water. It does not suck air into the line when the pump is on. In fact it shoots out a strong thin line of water.

Also, how prone are anti-siphon holes to clogging? How often do they clog? Are there ways to prevent it?

FYI - I have a Sequence Barracuda pump that pumps water about 6' up. The output is split into 3 (2 return lines and one to feed the skimmer).

Any other advice would be helpful too. Thanks.

[JennM]

If it's above the water line, water will spit out of it. Mine are in an AGA standard overflow, above the water line (in the 90-degree elbow, underneath) and water dribbles back into the overflow box - not an issue in that instance.

If it's just below the water line, then if/when water back-siphons, it will do so until that hole is exposed to air, then the air will break the siphon.

IMO submerged holes are more prone to clogging. I use a bent paper clip or a set of dental instruments (purchased for ~$5 at Harbor Freight tools) to clear them periodically as part of routine tank maintenance.

Even exposed holes can get salt creep or algae build up on them, so be sure to inspect and clear them regularly.

Another option is to install a clear true-union check valve on your return.. I use these in several applications - easy to clean and service as they are clear and you can see inside them, and they have unions on either side so you can remove and clean them easily.

HTH

Jenn

[TimmyG]

To add to what Jenn said, If the hole is above the water line it may sprey onto your lights or just splash alot and cause even more salt creep.

[yardboy]

That's been my experience too. Above water level they spray and make bubbles that pop and throw salt onto your lights. They also clog with algae and salt (I think they're worse than submerged, but maybe not). I have lockline returns on one tank so we just adjust the water level in the tank until the anti-siphon hole is just barely below the surface and it reduces most of the hassles.

[Doug1]

the anti siphon hole can be above the water line but as mentioned it will squirt out, it's recommended to put it just below the normal water level so that if the power goes out and water drains to the sump, the overflow box or drilled overflow will stop sucking water , due to their design , the return lines are usually below the water surface and without a siphon break hole, the return line can keep siphoning, and if its very far baelow the surface it can drain a lot of water into the sump

[FastFish720]

Agreed with everyone else. I drilled above my water line and corrected the problem by shoving a small piece of ro/di tubing into the hole.