A water heater/air heater system comprising: a solar collector (100") comprising a heat exchanger comprising: a first and a second transparent polymeric sheeting material (2, 2') defining a cavity therebetween suitable for receiving water and also suitable for receiving air; and at least one non-transparent heat conductor located within the cavity so as to be submerged inside the water or air within the cavity in use, in which the at least one heat conductor is an elongate portion composed of metal, alloy, or a combination of metals and/or alloys, and wherein, in use, at least the first transparent polymeric sheeting material (2, 2') is exposed to solar radiation, such that infra-red radiation of such solar radiation passes through at least the first transparent polymeric sheeting material to the heat conductor located within the cavity, and the heat conductor releases the radiation as heat directly to the water or air within the cavity; a cold water inlet valve and a hot water outlet valve in communication with the cavity of the heat exchanger, in which the valves are selectively operable to allow water to flow through the solar collector (100"); a hot water tank (25") in communication with the hot water outlet valve; a pump (20") selectively operable to pump water through the solar collector (100") and onwards to the hot water tank (25"); a spigot valve (21") selectively operable to drain water from the solar collector (100"); an air inlet valve and an air outlet valve in communication with the cavity of the heat exchanger, in which the valves (30, 32) are selectively operable to allow air to flow through the solar collector (100"); and a low pressure fan or pump selectively operable to pump air around the solar collector (100"). a solar collector (100") comprising a heat exchanger comprising: a first and a second transparent polymeric sheeting material (2, 2') defining a cavity therebetween suitable for receiving water and also suitable for receiving air; and at least one non-transparent heat conductor located within the cavity so as to be submerged inside the water or air within the cavity in use, in which the at least one heat conductor is an elongate portion composed of metal, alloy, or a combination of metals and/or alloys, and wherein, in use, at least the first transparent polymeric sheeting material (2, 2') is exposed to solar radiation, such that infra-red radiation of such solar radiation passes through at least the first transparent polymeric sheeting material to the heat conductor located within the cavity, and the heat conductor releases the radiation as heat directly to the water or air within the cavity; a first and a second transparent polymeric sheeting material (2, 2') defining a cavity therebetween suitable for receiving water and also suitable for receiving air; and at least one non-transparent heat conductor located within the cavity so as to be submerged inside the water or air within the cavity in use, in which the at least one heat conductor is an elongate portion composed of metal, alloy, or a combination of metals and/or alloys, and wherein, in use, at least the first transparent polymeric sheeting material (2, 2') is exposed to solar radiation, such that infra-red radiation of such solar radiation passes through at least the first transparent polymeric sheeting material to the heat conductor located within the cavity, and the heat conductor releases the radiation as heat directly to the water or air within the cavity; a cold water inlet valve and a hot water outlet valve in communication with the cavity of the heat exchanger, in which the valves are selectively operable to allow water to flow through the solar collector (100"); a hot water tank (25") in communication with the hot water outlet valve; a pump (20") selectively operable to pump water through the solar collector (100") and onwards to the hot water tank (25"); a spigot valve (21") selectively operable to drain water from the solar collector (100"); an air inlet valve and an air outlet valve in communication with the cavity of the heat exchanger, in which the valves (30, 32) are selectively operable to allow air to flow through the solar collector (100"); and a low pressure fan or pump selectively operable to pump air around the solar collector (100").