What does Toto mean by "Ultra-low high efficiency toilets"?

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I was thinking about installing an Aquia IV one-piece toilet with an S500e washlet in my Mother's 1968 home. I thought I might as well go for the highest efficiency version, which is currently (Jan. 2019) the 1.0/0.8 GPF MW6463046CUMFG.

While comparing models, I noticed that under "INSTALLATION NOTES" in the spec sheet for the MW6463046CUMFG model it says,

Ultra-low high efficiency toilets require a system with adequate
supplemental flows from other water usage fixtures and a 4" ID
drainline with minimum 2% slope and maximum linear runs of 60ft.​

This sounds like the entire drain from the toilet to the point where it exits the house (at least) must be 4" pipe, without any reductions.

Mom's place has 3" copper toilet drain lines running to a 3" cast iron sanitary tee that exits the basement wall; so, apparently, the "ultra-low high efficiency" toilets are out.

The spec sheet for the MW6463046CEMFG 1.28/0.8 GPF model does not include this warning paragraph. However, both of these models spec sheets describe them as having the "ultra high-efficiency" DynaMax Tornado flush system and "ultra high-efficiency dual flush".

The problem is that I'm not sure what "ultra-low high efficiency" means (I'm guessing it's actually a mistake on Toto's part). Because this paragraph only appears in the 1.0/0.8 GPF model spec, I'm assuming it refers to only the 1.0/0.8 GPF toilets. However, if it really means, "ultra-high efficiency", then it would seem to apply to the 1.28/0.8 model as well. Can anybody shed some light on this?

More to the point, am I correct in assuming that the 1.28/0.8 model should work in my mother's home and the 1.0/0.8 model shouldn't (without replacing part/all of the drainline)?

Thanks for the help!



Retired Defense Industry Engineer xxx
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Going way back, a toilet might have used 7-10 gallons when flushing. Over the years, that kept getting whittled down. What people have found is that if your drain pipes are marginal, a toilet that uses so much less water may not have enough to move the waste well unless it has adequate slope and diameter. Note, it's not uncommon for the waste to not make it to the end of the line out of the house on the first flush. If there's enough slope, subsequent water uses will move the 'piles' a bit further along until they reach the main sewer. That isn't as reliable if the flush volume is lower or the lines aren't to spec.

Sounds like you really shouldn't use that toilet in that house! One of the 1.2 or 1.6g flush versions should work fine.

Note, this does not mean that the toilet doesn't empty itself...it's what may happen down the drain line with less water to help move things along.
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