A. Master plumber Rex Cauldwell responds: The small volume of flushing water that’s available to carry the waste down the pipes is a potential problem, and you are wise to be concerned. With a smooth pipe, such as PVC, there isn't a significant problem until you start using very long runs with large-diameter lines (4 inches or larger). With cast iron, you may have a problem from the onset, because the interior surface is rough and the water does not flow smoothly. Short runs usually present no problem, but long runs can.
You mention older pipes. Actually, if the cast iron has been in for a few years, it has built up a scum on the inside to help smooth out the flow. But there is still no doubt that 1.6 gallons may not be enough water to carry the waste for the entire length of the sewer line by itself.
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