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OK so i built an over flow that looks exactly like the one in this post
http://www.plantedtank.net/forums/diy/13788-diy-pvc-overflow-retrofit.html

I didn't want to take over their post i'm trying to figure out what i did wrong here is a diagram of what mine looks like



Red is elbo green is cap with 7/64 hole in it yellow is T first pipe 8 1/2" second pipe 8 1/2"' 3rd pipe 4 1/2" bottem pipe below T (yellow) 28" and pipe with cap on it above T (yellow) is 6 1/2" i really need help because i cant get it to siphon any suggestions as to what i did wrong i did have it working when the 4 1/2" pipe was a little bit longer about 1 to 2" but it didnt have a full flow just a half flow
Thanks in advance
 

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This overflow, the part on the right, is working perfectly for me. I think you have to have the extra "trap" portion leading up to the inlet to the overflow. If you imagine the water in this, then let the water level drop, in your imagination, you can see that this design traps the siphon so you never lose it. Starting this the first time was some trouble, but I did it by temporarily hooking up a water hose to the outlet and back flowing it (with the vent hole outside the tank plugged). That filled the siphon the first time. It has worked for a couple of months since then, with no problems.
 

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the design you have uses a horizontal flow rate vs the max vertical flowrate of a standard PVC overflow
seen here:


as mentioned above. an overflow like the one you have built (referred to as a "bottom siphon" overflow) offers a much slower flow rate.
while a standard PVC overflow built out of 1" PVC will give you a max of 600gph, the "bottom siphon" overflow will only allow around half of that.

also, you should check for cavitation.
any air built up in the bends of the overflow will make it flow even slowe. this is where your check valve comes into play.
an inline check valve (placed in the green cap portion of your drawing) with a section of airline will allow you to suck out any excess air from your overflow.
 

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Good point on the check valve and accumulated air. BTW, how do you suck the air out?

Be careful with making F as low as you have it in the diagram. I believe the water level in B (on the left side of drawing) will drop to be the same height as the bottom of the outlet of F. This means water falls down B quite a distance, possibly resulting in air bubbles being dragged along and up to the top trap.
 

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you drill a small hole in the pipe and glue in the check valve (pointing out from the pipe). then simply attach a piece of airline tubing and suck until only water comes out. this will need to be check ed periodically to ensure the overflow is operating properly.
alternatively, you can attach the airline to an "aqualifter" which will pull the excess air out for you.

the placement of piece F in the diagram is just fine, and will not result in any such problems. you will get the same amount of air in the pipe regardless how high the T fitting is placed.

heres a quick slide explaining how the PVC overflow works.

 
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