My heating system has a manual balancing valve on the flow to the hot water cylinder which is only just open, hence the water takes a long time to heat and only gets to approx 55 degrees C (the radiators heat up fine). The engineer said it needs to be set that way to ensure there is sufficient flow to the radiators when both radiator and hot water circuits are in use. Is there a correct procedure to determine the correct position of the valve or is it just trial and error? Also if the flow is restricted too much when only the water circuit is on, is there a danger of damage to the system?