Why Does My 3/4 Inch Overflow Prime But Not My 1 Inch?

[watertown28]

So I took the step to make a sump and did. Still work in progress but the I wanted to get it pumping first and then fine tune everything as it goes on.
 
Anyways, I made a smaller 3/4 inch pipe overflow, and that could not keep up with the 800gph pump. So I decided to try a 1 1/2 inch pipe one, and that one failed miserably so to say. Sucked out the water to fast....so I went and did a 1 inch one... which while its running, does fine.
 
Now the 3/4 inch pipe one stays primed. I can shut off the pump, let it all drain where it stops coming out of the tank, turn the pump back on and withen a minute or 2 the 3/4 inch starts full flow.
 
Not so much on the 1 inch. The layout is exactly the same.as you can see in the pic, they pretty much are even in length and layout....but it just wont start up on its own. Some water comes out, but it does not seem to fully kick in at all after a few minutes.
 
Now, I will say I used street elbows on the 1 inch layout in some spots, where as the 3/4 one is elbows and straight pipe connecting. Neither overflow is glued. Do you think that the street elbows mess up keeping prime? I see no leakage from them.
 
So , looking for ideas on this.
 
The one inch pipe is in the front, 3/4 inch is behind the pump return line. Also, this is not a final layout, for I would love to move the overflow pipe to the other end and bring the pump closer to the tank.

 
PS: for a bit of a laugh, when i first set this up with the 3/4 inch pipe a few days ago, I had no cap only a pipe..... let it run a half hour, noticed something strange in the sump part... here my dino decided to get sucked up. Took him out, put back in the tank only to watch him swim right back to the other end to get sucked into it again. I think it thought this was Wisconsin Dells, the water park of America lol.
Needless to say that cap got put on rather quick after, and I can say dinos are sure hardy fish, for that ride for sure would have killed 90% of the fish that went for a ride, but this one is still alive with no ill effect it looks like after a few days.

 

[Chad]

It's probable that the larger pipe still had air in it. You can use an aqua lifter or power head to get it out and auto start it. 
 
https/youtu.be/Q7hRzaAaxi0

 

[watertown28]

It's probable that the larger pipe still had air in it. You can use an aqua lifter or power head to get it out and auto start it. 
 
https/youtu.be/Q7hRzaAaxi0

Hrm.... would it matter even after it runs? What I did to test it was to stop the pump and let it all drain till nothing was coming out of both drains, and then fired up the pump.....
 
Now I have noticed that the water level after watching this, does not go down to the top of the pipe on the 1 inch. its a little bit above.....
 
What I am going to do is get the pipe longer inside the tank, I got a good 2 more inches at least I can bring it down and raise up the inlet pipe a bit more ....
 
The other option I was gonna try is raising the T up a inch or 2.
 
I will try the hole on the back of the elbow like that. I used that for the 1 1/2 inch pipe and worked well, but I could not figure out how to get the air from sucking back in via the tube....I do have a few air line valves somewhere where I can close it off I need to find them though.
I just did not do this because one the 1 1/2 when I put it back together gluing it, I was careless and that elbow with the hole of course ended up on the bottom outside of the tank... and being glued there was nothing I could do lol.
 
BTW dude that is brilliant! Powerhead like that NICE!
 
Okay I can run that into the sump also... I kinda wanted to do my DIY co2 inside the sump also instead of the tank, but I have not figured out yet if that would hurt the co2 levels in the tank ...but that is a topic for another discussion.

 

[Chad]

I've found that my over the side overflows tended to build up air over time that required me to remove it every so often. If the bubble of air gets too large the siphon breaks so it's important. This method prevents that from happening. Now i use only drilled tanks so this isn't an issue, but previously it was something I was very concerned about.

 

[watertown28]

I've found that my over the side overflows tended to build up air over time that required me to remove it every so often. If the bubble of air gets too large the siphon breaks so it's important. This method prevents that from happening. Now i use only drilled tanks so this isn't an issue, but previously it was something I was very concerned about.

Well that is drilled issomething I might look into down the road. I want salt eventually, Its just the cost of tanks is kinda comical for me around here for used tanks. People want 150-200 bucks for used 55-75 gallon tanks. Plus the tanks being already stocked makes it even harder to drill.
 
Im starting to think that the air builds up easy in that 1 inch pipe for whatever reason. I drilled the hole and put in the air line and stuff, but I dont have a power head that takes the air line. I mean they fit inside the hole and thats it lol.
 
Anyways, I did manage to get the 1 inch to auto start, but I as the day moved on it seemed to not want to after a while. I have been in the middle of putting my PO4 bottle in and had to turn off the pump a few times.
 
Do you think that maybe some air is leaking in in the joints not being glued?

 

[Chad]

I'm not sure of the physics behind it. It happens in non-homemade overflows as well, and single piece designs that don't have joints so it's clearly a byproduct of the siphon. 

 

[watertown28]

Yeah , I think its the pipe size that is doing it. I put in another 3/4 inch overflow and it keeps ticking just fine. Thanks for your help... this is my first real big DIY project I have done and its learning for me. Even with all the PVC laying around not used and the joints, a 50 gallon OF would have cost me at least 2x more.... I have seen them for 300+ and that is not counting all the plumbing anyways.
 
Now to get some polypipe... this other stuff is so stiff to use and because it was curled I cant get it straight... keeps on trying to curl up on me and making my pump sit sideways. Need something more flexible.

 

[Chad]

.. this other stuff is so stiff to use and because it was curled I cant get it straight... keeps on trying to curl up on me and making my pump sit sideways. Need something more flexible.

Try heating it just a bit. Not so much to blacken or melt it, but a little heat works. I've used a hair dryer on the thinner walled tubing but for thicker you would likely need a stronger heat source like a paint remover type thing. 

 

[watertown28]

 

.. this other stuff is so stiff to use and because it was curled I cant get it straight... keeps on trying to curl up on me and making my pump sit sideways. Need something more flexible.

Try heating it just a bit. Not so much to blacken or melt it, but a little heat works. I've used a hair dryer on the thinner walled tubing but for thicker you would likely need a stronger heat source like a paint remover type thing. 
 

Yeah there was this black type tubing, vinyl that I got that is much more flexible, though it curls a bit still.
 
I got rid of the pvc pipe on the return, went straight vinyl on that and it seems to have kicked up the return flow. I was reading that the 90degree elbows restrict flow, though being few gallons per minute, but adds up over the hour let alone the day.
 
Now with the 2x 3/4 inch pipes for the over flow, the amount taking out is wayyyy more than the pump can handle. Its this pump:
http/www.amazon.com/gp/product/B006M6MSHE?psc=1&redirect=true&ref_=oh_aui_detailpage_o01_s01
 
So I figure its a 3 foot lift... im getting over 600gph from it ( though how can I measure that is beyond me lol ) which would make it a 8x turnover rate in the tank.

 

[Chad]

I always put a valve on my return and a valve on my drain so I can tweak them as needed.