the hot water is not being heated constantly when the tap is open . The boiler starts to heat the water and switches off after few seconds and after few seconds starts again so the water is cold and warm every few seconds . It helps when the central heating is on ,then the flame stays longer and the water is heated more stable (not perfect though)

Paul, May 2011

depending on make & model of your ravenheat boiler
it sounds like the plate heat exchanger blocked
needs replacing

manwithnoname, May 2011