Abstract: A method of controlling the ratio between the water flow rates in the primary and secondary circuits of hot water heating systems, particularly of low-temperature hot water heating systems, by mixing flow water with return water passing through an adjustable bypass by means of a multi-way mixing device. To make maximum use of the rangeability of said mixing device and to optimize overall heating system control, the bypass water flow rate is controlled as a function of the mixing device position and the bypass is substantially closed when the way from the primary circuit flow piping to the secondary circuit flow piping is fully opened and/or an appropriate signal is given by the heating system controller.