the invention concerns a water - saving mixer tap with a first water inlet , a second inlet for thermally conditioned water , such as a hot water inlet , at least one service water outlet for water coming from the first water inlet , from the second water inlet or from both , and remarkable in that it further comprises an auxiliary water outlet , and means for interconnection between the conditioned water inlet and the auxiliary outlet with a view to preparing conditioned water without losing water via the service outlet . the invention also concerns a flowing water system comprising such a mixer tap . the auxiliary outlet of the tap is connected to an auxiliary pipe provided with means for storing and / or means for discharging the water from the pipe for water which has lost the conditioning of same and flows towards the auxiliary pipe when the tap is in the position for preparing conditioned water .