Evacuated heat pipe solar collectors (heat exchangers); solar collectors (heating); solar collectors for heating; solar energy collectors for heating; solar energy powered heating apparatus; solar energy powered heating installations; solar heating apparatus; solar heating installations; solar heating panels; solar panels for use in heating; apparatus for producing hot water; apparatus for the supply of hot water; boilers for hot water supply installations; electrically heated hot water tanks; gas fired boilers for producing domestic hot water; hot water apparatus; hot water boilers; hot water installations; hot water supply heaters; heat pumps