Back boilers can be repaired,it depends on the fault.If it is leaking from the heat exchanger then you will have to replace it due to the cost of supplying and fitting a replacement,but it is not that expensive to repair if all it needs is a new thermocoupling or gas valve which are the most common faults.In the end though,you will have to think about giving in to upgrading to a combination condensing system,this means no tanks in loft and no more wasting energy on heating up a full cylinder of hot water when the demand is not always there,also you will definately notice a reduction in your gas bill with the new system.You will be looking at around £2500 to £3000 to convert the system,and i would recommend a Vaillant eco tec condensing combination boiler.
Andy-APM Plumbing and Heating (north east london), January 2008
