Aiming at the central mechanical circulated hot water supply system recommended by the current code, the criteria of water flow direction in the pipes of transport and distribution is brought forward based on the hydraulic principle, followed by the analysing with charts of flow directions in the reversed return hot water system consisting of two, three and several vertical pipes in the working conditions of simple return and a little water distributing. There are several directions of circular flow, the only working condition of simple return and single water flow direction which applied in current hydraulic calculation of reversed return hot water supply system is incomplete, certain hydraulic precondition is necessary to ensure circular flow flowing in each vertical pipe as the assumed direction in the course of hydraulic calculation.