A manifold (15) comprising a flow chamber (35) for receiving flow heat exchange water from respective heat sources (3, 5, 7) through first inlet ports (47, 48) and from which the flow heat exchange water is delivered to heat exchange circuits (8, 9) through flow ports (57, 58). A return chamber (36) in the manifold (15) for receiving return heat exchange water from the heat exchange circuits (8, 9) through return ports (57, 58), and from which the return heat exchange water is returned to some of the heat sources (3, 5, 7) through first outlet ports (53, 54). A bypass chamber (37) located in the manifold (15) between the flow chamber (35) and the return chamber (36) receives flow water from the flow chamber (35), which has not been drawn off by the heat exchange circuits (8, 9), through a communicating passageway (40). Heat exchange water from the bypass chamber (37) is returned through second outlet ports (55, 56) to others of the heat sources (3, 5, 7).