Toilet waste running going the wrong way UPhill

Hi Everyone.
I am plumbing my house from scratch. The stack pipe is at one end of the house and I have run a wast pipe horizontaly along the floor upstairs 16 meters, it has a fall of 25mm per meter. There is a toilet connected with a swept t at each end and one in the middle.
I meter uphill from the middle toilet connection to the main pipe I have a T with a cap for rodding. I got my wife to flush loo paper down the middle toilet whilst I looked in the rodding point. The water started going the right way and about 2 seconds into the flush I noticed water and paper flowing the wrong way past the rodding point then after a few seconds it turned and all went the right way. the paper got hung up on the connection of the rodding point.
I can see what is happening, as the huge torrent of water from the toilet goes into the swept T it's swept down the pipe, as the pipe is so long and the fall slow the water is flowing away slower than the water entering the pipe, at the point of the T the resistance of the water flowing downhill is greater than the 0 resistance of the air behind it so the water flows backwards uphill untill, there is a point where the forward flowing water is at the same level as the backward flow then the direction changes so it all goes downhill. Phew.
My question is On such a long run with a slow fall should I have used Y connectors with a 45 on the Y so as to make the wast pick up speed and be slung more directly in the direction of flow rather than with the swept T which is more like a very steep bend with a flat wall in front.
Any advice much appreciated.
regards Mark.

Your explanation is a little confusing Can you upload a photo or diagram of how the middle swept T is connected. Also 25mm/metre seems a little excessive. Waste pipes need to be only slightly above horizontal although I forget the exact mm/metre that is required.


One thing is certain though, sht doesn't flow uphill!

Hi Htgeng,
Thank you for the reply,
Ill get a photo tomorrow.
Here is a diagram, the T is swept t toilet connectors, the R's are rodding points and the I is the vertical pipe to the septic tank and upto the roof, the o is the cap ontop of roof. The pipe that drops to the septic tank verticaly is under the last R


o
I
I
I
I--------R---T-------------------------R----T-------------------------------T-R
FLOW LEFT TO RIGHT I
Thank you for the reply, regards Mark

Hi Everyone,
I took 3 pics this morning.
1 is pipe to the middle toilet.
2 is a view from the floor looking at the toilet so you can see the angle of the toilet pipe and the T.
3 is a top view of the toilet and swept T.
I',m wondering if the connector at the back of the toilet should have been a T going to a air admittance vale , just wondering if its possible for suction in the toilet exit bend to be trying to drag the wast back before it gets to the swept T.
Any advice would be great please.
reagrds Mark
LINK TO PICS BELOW

//www.diynot.com/network/Markymilarky/albums/

Fall on 110mm soil pipe should be 18mm/m minimum; most are run at 25mm/m & this is usually considered the norm. Maximum unvented branch for 110mm is 6m; not sure how long that run is but, assuming I’ve understood your diagram correctly, an open vented stack at the end of the run the other side of that wall should provide enough ventilation; the swept tee should work OK & I’m not sure an additional AAAV would help much. If the water is flowing fast enough initially & then back flowing, either air is not getting in via the stack, you haven’t got enough fall on the run or you have a partial blockage in there somewhere.

Duplicate post; either something is wrong with DIYnot server this morning or my browser is playing up

Richard C said:

Fall on 110mm soil pipe should be 18mm/m minimum; most are run at 25mm/m & this is usually considered the norm. Maximum unvented branch for 110mm is 6m; not sure how long that run is but, assuming I’ve understood your diagram correctly, an open vented stack at the end of the run the other side of that wall should provide enough ventilation; the swept tee should work OK & I’m not sure an additional AAAV would help much. If the water is flowing fast enough initially & then back flowing, either air is not getting in via the stack, you haven’t got enough fall on the run or you have a partial blockage in there somewhere.

Click to expand...

I`m with you there " the o is a CAP on the pipe ...roof "

Hi Richard,
Thank you for that great info, I can eliminate some things now. just checked the fall again and it looks like the pipe has drooped in the middle making the drop from the middle loo more like 11mm per meter for the 2 meters after the loo.
Very much appreciated your help as its been driving me crazy here.
Hi Nige, thank you, the o should be a vent on the roof, misprint.
Thanks a bundle Guys. Areal help.
Regards Mark