I hadn't even considered a straight down tailpiece, as I didn't think there was room to clear the tank. I just checked and there is, but barely! A single!
Also, I didn't know the that a kitchen sink tailpiece and a shower drain were the same size and thread, but they are! Two runners on!
And getting rid of the tub drain elbow frees up extra length for the drain pipe and trap also allowing drain pipe alignment by rotating the trap slightly. Bases loaded!
Agreed, using a water p-trap instead of a moving part (flapper valve in the waterless) removes potential mechanical failure. Home Run!
I think that may well do it, and solves my other issue of how much of a notch to cut out of the shower pan support board. Nada, none. Just wide enough for the flange nut. Now where to find a flange nut? Oh there's one, right on the bench, the tub elbow!
In these pictures there is a combination of parts between a kitchen sink system and a tub drain.
Here you see the brass tub drain, and directly behind it is the tub underside flange nut elbow (as opposed to using its official name because I don't know what it is). The tub drain can thread onto the straight tailpiece to the left, using the brass nut and flat tailpiece plastic washer. (Just can't seem to get away from these washers in SOME places.) But that will seal the drain to the tailpiece.
Here it is assembled. And note the elbow behind, I'll have to cut the elbow off, retaining enough flat flange and thread to draw the rubber washer up against the underside of the shower pan, using plumber's putty on the top surface under the brass drain edge. That'll seal the drain to the pan.
Here you see the o-ring feature of the tub/shower drain. Movable with a finger or toe. ticki2 this sort of drain stopper might work in your scenario as well, and I think it will also aid greatly in the slosh back thing.
The Ferco coupler was in the pictures to show its wall thickness - 1/4". It is that 1/4" I took into account to verify just now, a straight down tailpiece would indeed fit - and I believe a Fernco trap is slightly thinner walled, if my eyes are remembering correctly from my last several visits to the store.
Regarding the 1.25" reduction thing. I can't even remember now if I was trying to better use the short available distance by reducing (smaller trap radius?) or if I was just staging with parts on hand. But it does me good to hear you ALSO don't think it would help appreciably on slosh back, and there's one thing for darn certain, the reduction LOOKS stupid and that's not good in anybody's book :)
Between the p-trap and the drain seal, maybe we'll control slosh back. We'll keep fingers crossed.
Thanks for the valued input ticki2.
