Potable water shall be protected from backflow in accordance
with the provisions of the New York Uniform Code and this chapter.
A hot-water-return circulation system shall be installed in
all buildings of more than four stories in height supplied with hot
water and in all other buildings where the developed length of hot-water
piping from the source of hot-water supply to the extreme fixture
supplied exceeds 100 feet. The circulation return shall in all cases
be 1/2 inch or more in diameter.