By Boiler Quotes Editorial Team · Updated 20 July 2026
What Is a Back Boiler?
A back boiler sits behind a fireplace or gas fire, usually in a living room, and heats water for radiators and a hot water cylinder. They were installed in large numbers in UK homes from the 1960s through to the 1980s and, remarkably, many are still running.
They are also, almost without exception, the least efficient heating appliance left in British homes. A back boiler commonly operates well below 70 percent efficiency, compared with 92 percent or more for a modern condensing boiler. If you still have one, replacing it is usually the single biggest efficiency improvement available to you.
What the Conversion Costs
| Scope | Typical cost | Duration |
|---|---|---|
| Back boiler removed, combi fitted in kitchen | £3,000 - £4,000 | 2 - 3 days |
| As above with fireplace made good and capped | £3,300 - £4,500 | 2 - 3 days |
| Complex cases: asbestos, difficult flue, radiator upgrades | £4,500+ | 3 - 4 days |
The cost sits above a straight boiler swap because this is really three jobs in one: removing the old appliance and its flue arrangement, taking out the cylinder and loft tanks, and installing and commissioning a new combi somewhere entirely different.
What Happens During the Work
A typical conversion runs roughly as follows.
- Day one: the system is drained, the gas fire and back boiler are disconnected and removed, and the cylinder and loft tanks are taken out. New gas and water pipework is run to the position chosen for the combi, usually the kitchen or a utility room.
- Day two: the combi is mounted and connected, the flue is fitted through an external wall, the condensate drain is routed, and the system is flushed and dosed with inhibitor.
- Day three: commissioning, controls set up, the old fireplace opening dealt with, pipework boxed in where needed, and the paperwork completed.
What Happens to the Fireplace
This is the question most homeowners forget to ask, and it can be the difference between a tidy finish and a hole in the living room wall.
Once the back boiler is out you have an opening and a redundant chimney or flue. Options usually include capping and sealing the chimney with appropriate ventilation, fitting a decorative fire surround, plastering over to leave a flat wall, or opening the recess up for a media wall or a new fire. Each has a different cost, and some involve a plasterer or builder rather than your heating engineer.
Agree in writing what "making good" includes before work starts. Many heating quotes cover capping and a basic plaster finish but not decorating or a new surround.
Things to Check Before You Commit
- Asbestos. Properties of this era can contain asbestos-based materials around old fires and flues. A competent installer will assess this and arrange specialist removal if needed, which affects both cost and timescale.
- Mains water flow. A combi is only as good as your incoming mains. Ask for a measured flow rate before committing to a combi, because in some older properties a system boiler with a cylinder is the better answer.
- Pipework condition. Systems of this vintage often have narrow microbore or heavily corroded pipework. Ask whether the existing radiators and pipework are fit to reuse.
- Radiator sizing. Radiators sized for an older, hotter system may need reviewing, particularly if you want the new system to run efficiently at lower flow temperatures.
- Where the combi will go. Position affects flue routing, condensate drainage and how far hot water has to travel to the taps you use most.
Is It Worth Doing?
For most households with a working back boiler, yes, on three grounds. The efficiency gain is the largest available from any single heating change. You reclaim the airing cupboard and the loft space taken by tanks. And you remove an appliance for which parts are increasingly scarce, so the risk of an unfixable mid-winter failure disappears.
The counter-argument is the disruption and the cost, which is real. If the back boiler is running well and money is tight, it is a reasonable decision to plan the work for a warmer month rather than doing it immediately. What is not advisable is waiting until it fails, because an emergency conversion in January costs more and leaves you without heating for several days.
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