A water heating system, in which water from a relatively low pressure closed-loop heated water circuit flows through a heat exchanger, through which a relatively high pressure mains cold water feed also flows, to supply both a demand for hot water from the high pressure circuit and/or a demand from heated waterconsumers such as radiators or showers, is characterised by the feature that the low pressure heated water always flows first through the heat exchanger before supplying any of the heated water consumers; or by the feature that flow restrictors are incorporated into the heat exchanger circuit in feed; or both.