The Reef Tank banner

Help!! Overflow cycles up and down

2412 Views 5 Replies 3 Participants Last post by  kjdeut
In an effort to reduce the water noise in our living room I recently moved the sump from under the tank (Oceanic 92 gal bow front corner w/built in overflow) to an adjoining room. I have reduced the noise but created a new problem, the overflow cycles up and down, sometimes to the point where I’m afraid the tank will overflow.

Using flex PVC the line drops maybe 6-8”from the bottom of the tank and turns (bends) 90 degrees into the wall. On the other side of the wall it takes another 90 degree turn (PVC elbow) and follows the wall down to the sump (sitting on the floor). In total it is about a 6’ horizontal and 18”-20” vertical (drop) run.

Since I made the change the sump has become very quiet, but now the overflow in the tank cycles up and down. I do not have the gurgle or toilet flushing noise, but just cycles up and down. I wouldn’t be that concerned but like I said, at times the level in the tank gets so high I’m afraid it is going to overflow.

I have tried throttling back the return pump (CSL T3) and that has slowed down the cycling but hasn’t eliminated it. Looking back I’m wondering if flex PVC was the way to go. With such gradual slope into the sump, could a sag, however slight, be causing back pressure which is making the overflow go up and down? I did support the line as much as possible so it is a straight gradual slope. I did have a vent in the line when it was under the tank to reduce air in the line, but I did not install one in the new run, should I have?. With the way the line runs and the gradual slope I’m not sure where along the line I would put it.

Any suggestions would be greatly appreciated.

1 - 6 of 6 Posts
The cycling is the result of the diameter of the drain pvc pipe you are using. As the water is draining down the pvc pipe, it starts building up back pressure due to the 90 degree elbows and the 6’ horizontal run. This is causing the over flow to not be able to keep up with the water that is returning to your tank by the T3. As the water level rises in your overflow, the water weight increases the preasure enough to overcome the back pressure and the overflow water level drops. This cycle keeps repeating. Is there any way you could add a second drain line or increase the diameter of the original drain line? Also, is the drain line output submerged?
The end of the run is not submerged, it sits more or less at surface level of the sump. I should clarify that the 6' horizontal run is sloping down the entire run to the sump, but it isnt a very steep slope.
When you suggest a larger dia drain line are you talking just from where it connects to the overflow or do you mean increase the dia of the standpipe also?

Thanks for the help,
Use the largest stand pipe and drain line possible. If you have a 1 inch bulkhead then use a 1 inch standpipe and drain line. Depending on the bulkhead size, you may have to valve down the output of you T3 in order for your overflow to be able to keep up with it. Another thing you could try if possible is to run a continuous drain pipe that does not contain the 90 degree pvc fittings. This would reduce the back pressure considerably. Hope this helps.:)
As Resplendens suggests,

I would try using a larger diameter pvc from the drainpipe to the sump. If your using 1" now go with 1.5" PVC. From the sounds of it there too much backpressure and it isn't until the drainpipe is summerged deep enough that a true siphons starts and flushes it down the line and then builds back up again.....
Thanks everyone. I will give the larger drain line a try frist and then if needed I'll make the standpipe larger also.

1 - 6 of 6 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.