Similarly, how much does it cost to replace a back boiler?
That's not an easy question to answer. The price will vary from property to property. A typical 3-4 bedroom property with 8-10 radiators, all pipework, and an A-Rated combination boiler, is likely to cost around 5,000.
Subsequently, question is, can a back boiler be replaced?
Back boilers are no longer being manufactured and are being replaced by modern units that are more efficient, compact and safer. If your property has a back boiler that's either given up or you simply want a more modern heating system, most professionals will advise you to move on to a combi boiler.
There are still replacement parts available for Baxi back boilers, so if you're determined to keep hold of yours then this is likely your best avenue for exploration. In most cases, however, your best option is to replace your back boiler with a new system.