(1) Except as permitted in Sentence (3), no drainage pipe that is of minimum size required by this Part for the purpose for which it is installed shall be so connected as to drain to other drainage pipe of lesser size.
(2) Where a building drain connects to a stack through a wall or floor, the drain shall retain its full size through the wall or floor.
(3) A sanitary drainage pipe may be connected to a pre-engineered waste water heat recovery system that incorporates piping of a lesser size than required by Sentence (1) provided that it does not convey sewage,
(a) from a sanitary unit, or
(b) that contains solids.