NOVELTY - The apparatus has a transparent tube (14) for conducting liquid or aerosol medium i.e. water, and a porous optical absorber (18) arranged within the tube so that the liquid or aerosol medium is permitted to percolate through the tube. A transparent liquid-retaining top covers a reflective collector unit, and seals side walls (11) of the collector unit along a bottom surface (24), so that another liquid medium (20) is contained within a volume that is defined between an outer surface of the tube, a concave surface of the collector unit and the bottom surface of the top.    USE - Solar heating apparatus i.e. solar heater, for heating liquid or aerosol medium i.e. water (claimed), in a building. Can also be used for heating water in a swimming pool and providing heat for thermo-chemical reactions.    ADVANTAGE - The apparatus is optically efficient, thermally stable, can be manufactured and installed economically, and has fast heating response and reduced environmental impact. The collector, top and the transparent tubes are extruded and made from same recyclable plastic material, thus reducing weight of the apparatus, and hence enabling installation and structural integration of the apparatus easily while facilitating recyclability of the collector. The liquid medium contained within the volume that is defined between the outer surface of the tube, the concave surface of the collector unit and the bottom surface of the top is drained, thus protecting the apparatus against freezing and overheating. The shape of the transparent tubes becomes more circular due to boiling or line over-pressure conditions, thus absorbing the additional stress without bursting while recovering nominal shape of the tube after any freezing or over-pressure conditions due to the elastic properties of the tube material.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a method for manufacturing a solar heating apparatus    (2) a method for heating liquid or aerosol medium.    DESCRIPTION OF DRAWING(S) - The drawing shows a partial sectional view of a solar collector.    Collector unit's side walls (11)    Collector channels (12)    Reflective surface (13)    Transparent tube (14)    Top cover (15)    Air gap channel (16)    Liquid-retaining top (17)    Absorber (18)    Side air gap channels (19)    Liquid medium (20)    Collector unit's outer top surface (21)    Liquid-retaining top's bottom surface (24)    Insulating foam (26)