A heating system comprising: a water tank for storage of domestic water; a solar collector associated with the water tank for heating the domestic water; a combustion device coupled with the water tank via a heat exchanger for heating the domestic water; a space heating apparatus adapted for being passed through by a hot transfer fluid thereby radiating heat into space, wherein said combustion device is coupled with the space heating apparatus to heat the transfer fluid for space heating, and wherein said space heating apparatus is coupled with the water tank via the heat exchanger for allowing the domestic water to heat the transfer fluid for space heating; characterised in that said water tank defines therein an upper combustion heating zone and a lower solar heating zone, and a domestic water pipe circuit is connected between the heat exchanger and the water tank and has a water outgoing line extending downwardly from top of the water tank to a predetermined distance; wherein said upper combustion heating zone and said lower solar heating zone are defined by the bottom of the water outgoing line, thus, the domestic water at the lower solar heating zone is mainly extracted to the heat exchanger via the water outgoing line to heat the transfer fluid when the domestic water is applied for space heating; and the heating system further comprises a controller operable to judge whether to enable the domestic water to heat the transfer fluid; if not, the controller continues to judge whether to enable the combustion device to heat the transfer fluid, and the controller is operable to judge whether to enable the combustion device to heat the domestic water by judging whether the temperature of the domestic water at the upper combustion heating zone is smaller than a first threshold. a water tank for storage of domestic water; a solar collector associated with the water tank for heating the domestic water; a combustion device coupled with the water tank via a heat exchanger for heating the domestic water; a space heating apparatus adapted for being passed through by a hot transfer fluid thereby radiating heat into space, wherein said combustion device is coupled with the space heating apparatus to heat the transfer fluid for space heating, and wherein said space heating apparatus is coupled with the water tank via the heat exchanger for allowing the domestic water to heat the transfer fluid for space heating; characterised in that said water tank defines therein an upper combustion heating zone and a lower solar heating zone, and a domestic water pipe circuit is connected between the heat exchanger and the water tank and has a water outgoing line extending downwardly from top of the water tank to a predetermined distance; wherein said upper combustion heating zone and said lower solar heating zone are defined by the bottom of the water outgoing line, thus, the domestic water at the lower solar heating zone is mainly extracted to the heat exchanger via the water outgoing line to heat the transfer fluid when the domestic water is applied for space heating; and the heating system further comprises a controller operable to judge whether to enable the domestic water to heat the transfer fluid; if not, the controller continues to judge whether to enable the combustion device to heat the transfer fluid, and the controller is operable to judge whether to enable the combustion device to heat the domestic water by judging whether the temperature of the domestic water at the upper combustion heating zone is smaller than a first threshold.