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.
As you’ve guessed, there are some
tricks we plumbers use to keep ourselves out of
trouble. I still do what my Uncle Bud taught me in
the 1950s: I always design the system so the waste
line of the 1.6 is scoured by another fixture
upstream. This way the additional water from the
other fixture will always clean the line —
it's just common sense. If this is your situation,
you should be okay.
If you have a toilet by itself at the end of a
long run of cast iron, you may need a
pressure-assist toilet to increase the velocity of
the flush. In some cases, the pressurized tank will
still not be enough to solve the problem. For
example, in my own house, I have a run of more than
250 feet of PVC to the septic tank — 1.6
gallons, pressure-assist or not, simply won't work.
For such "big problem" cases, plumbers have been
known to modify the toilet to increase the water
flow. I never buy a 1.6 that can't be modified.
Talk to your plumber about this if you think
you’ve got a major problem situation.