A.Jim Stack, a plumbing contractor in Kirkland, Wash., responds: The drain-line requirements in today's plumbing codes were developed decades ago, and were based on the number of fixtures needed to reliably clean and scour lines of various sizes. At the time, toilets used 3.5 to 4 gallons per flush.

Fast-forward to today, when water use has been reduced by more than half, to an average of 1.6 gallons per flush. The newest generation of high-efficiency toilets, or HETs, use just 1.28 gallons per flush. Most of these fixtures flush well enough to clear the bowl with that amount of water - but once the waste leaves the fixture itself, the...

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