My boiler is approx 6yrs old and has now stopped supplying hot water after an E7 error code. The central heating works fine. I have been told by an nPower hometeam report that it needs a new heat exchanger at a cost of over £1100 due to sludge and limescale build up. They have already done a de-sludger and put inhibitor in but no joy. Can anyone advise if this is the only route? Thanks