The invention provides a solar hot water heating system including one or more solar energy absorbers (102) having at least a first fluid circulation path wherein, an over temperature path (119) is provided, the over temperature path including a pressure vessel (120) which is normally closed to atmosphere, the over temperature path being connected to the first fluid circulation path (108) so that, in the event that fluid in the solar energy absorber vaporizes, the fluid is forced out of the solar energy absorber and into the pressure vessel. The present invention also provides a temperature sensitive valve leaving a flow path and a valve member operated by a thermal element having thermal expansion characteristic the valve including a support member against which the thermal element expands to force the valve element to close the flow path.