View Full Version : rear discharge on negative slope waste line

01-01-2006, 11:39 AM
i have a rear discharge toilet that isn't flushing properly. bowl fills up to below rim, and "flushes", after tank fills, the bowl water level drip a few inches, as if the water is siphoning out. i tried rodding the toilet and found nothing. pulled the toilet off the wall and water came out of closet bend. there was, i am guessing, less than 1" of water sitting in it. the toilet doesn't seem to be clogged, and the waste line has been rodded. but obviously the horizontal line is sloped the wrong way. i am presuming that this is what is causing the flushing and siphoning problem.

this toilet is at the end of a ap. 8' horizontal run, before entering the vertical stack. 6' away is another rear discharge in the adjoining bathroom. so: toilet, 6', another toilet, 2', vertical stack. flushing the other toilet causes some water to come out the closet bend of the none flushing toilet. it is all pvc. i could try opening the wall and see if i can raise the horizontal a bit, and then i would have to have the toilet sitting on a 1 1/4" or so platform.

another option i am considering, is installing a power flush rear discharge.

what do you think?


01-01-2006, 12:10 PM
S*** doesn't flow uphill. If that is in fact your problem, you need to restructure the drains so that they have 1/4" per foot drop to the stack. Also, I think (I'm not a pro) that your stack is too far away to act as the vent. Maybe it is okay with the 3 or 4" drain line you have. Ideally, the vent would be closer to the toilets. A power discharge (pressure assist) won't make it better.

01-01-2006, 12:44 PM
i know, but it does get pumped up hill. which a power flush, to some sense, would do. i know that ideally the waste lines would be correctly slanted, involving opening up walls and perhaps affecting other lines that are installed near the the waste lines. using a power flush might depend on just how much the current line is *off*, which i wouldn't know without opening the wall and putting a level on it.

01-01-2006, 01:09 PM
I think you are going to have to bite the bullet here and fix the slope. The pressure assist is just one way to dump a lot of water fast. The velocity of that water will quickly diminish once it is over the trap in the toilet and into the much larger drain line...it won't make much difference at all. Plus, they are noisy and more trouble prone than a standard gravity assist toilet. I fear patching this will not make you happy. My unprofessional opinion.

01-01-2006, 02:13 PM
The manufacturers of pressure assisted toilets claim that there is a further "carry" of waste into the line on flush. I would not count on this at all. A pressure assist toilet in no way can be compared to a pump, and if there is any back-pressure on the discharge, it would likely cause an incomplete flush. It is not a closed system: the pressurized water is aimed at the inlet of the trapway.....any resistance in the drain line would just cause that water to fill up the bowl.

01-03-2006, 06:35 PM
ok. i've given up my dream of pressure assisted resolving the problem. i am trying to figure out the simplest way to fix the waste line slope.

i have to admit that i am a bit puzzled as to how this system could have ever worked. it's concrete slab construction. how do you install two toilets, 5' apart, connected to the same horizontal, with another 2' to the vertical waste line and get a proper slope? all pvc. if the slab is level, either one toilet has to sit below the top of the slab or the other has to sit 1.75" higher than slab. or can you simply rotate fittings to the waste 't' to get from below slab horizontal, to locate the closet bend 4" over slab top? the wall only seems to have about 6" or so of space.


01-03-2006, 06:57 PM
When you put on the pvc cement, it basically disolves some of the surface layer...there is a little give, and you can tweak the angles a little to allow for the slope. When they used (and still do occassionally) cast iron and lead/oakum, you had a lot more leaway to tweak angles. At least, that is what appears to happen to me! It's done all of the time, so something is at work here...

01-12-2006, 07:11 PM
well, i gave up the dream, but the owner didn't. redoing the plumbing was looking to be prohibitive, so shutting down the powder room, closing off the toilet and lav plumbing, and expanding the adjoining bathroom was the next plan. just on spec, and with the idea that we would be getting a new toilet anyway, a pressure assist was purchased and tried out in the powder room. seems to work based on a few flushes. toilet flushes normally, and doesn't siphon anymore. so, out it comes again, to tile the floor and paint.

i realize that fixing the waste line would have been the best idea, and it is possible that problems may develop later, but maybe it will be ok.

thanks for the feedback.