have a problem with the unvented hot water cylinder on system, problem being is it does not produce enough hot water, the cylinder has 2 emersion heaters on it also flow and return to boiler and flow and return for heating, it also has a pump at back of cylinder which i guess is for a higher flow rate. the tank is hot and so are the pipes but when you turn tap on you only get a hand full of hot water then goes cold any suggestions would be great help the system is about 16 years old it also has flow switch on pipe work which has been renewed, its baffling seems to have small pockets of hot water in tank.