What is a back boiler?
A back boiler is a boiler fitted behind a gas fire, heating both the room and the central heating/hot water system. They were common in UK homes from the 1960s to 1980s.
Efficiency
Modern combi: 90–96% efficiency. Back boiler: 55–70% efficiency. The efficiency difference alone — typically 25–35 percentage points — costs £200–£350/year in wasted gas.
Safety
Older back boilers are significantly more likely to have carbon monoxide issues due to deteriorating heat exchangers and flue systems. Many are now too old to be safely serviced.
Availability of parts
Parts for back boilers are increasingly unavailable. When a back boiler fails, replacement is often the only option.
The cost of replacement
Replacing a back boiler with a modern combi is slightly more complex than a standard replacement — typically adding £200–£300 to the job for removal of the back boiler and fire unit. But the energy savings of £200–£350/year mean the job pays for itself very quickly.
Verdict
Back boilers are obsolete technology with no redeeming qualities compared to a modern condensing combi. If you have one, replacing it should be a priority.