to 62 5 ml min such a wide variation would be noticeable to the user but the presence of the pressure compensating flow valve s ensures that water is supplied to the boiler at a uniform flow rate e g 40 ml min regardless of variations in pump performance between appliances and regardless of pressure variations experienced in an appliance as a result there is less burden on the appliance manufacturer to implement a pump with tight tolerances and no need for a more complicated control system to alter the output from the pump or to regulate the flow the pressure compensating constant flow valve s described above may be set at a constant flow rate or they may allow a user to adjust the flow rate the pressure compensating constant flow valve s may include an integral non return valve so as to reduce the effect of variations in back pressure and thus enable a smoother flow through the boiler examples of a suitable pressure compensating constant flow valve are available from netafim www netafim com and sold as on line pressure compensating u201c drippers u201d such irrigation drippers or emitters are also available from a number of other manufacturers while the invention has been described above with respect to two preferred embodiments it will be appreciated that the aspects and features of the invention may be applied to many different types of appliance especially other appliances which generate steam such as wallpaper strippers and other hand held steam generating appliances features.