Upgrading from an old back boiler to a modern combi boiler (known as a back boiler conversion to combi) is one of the most popular heating upgrades in the UK right now. Thousands of homeowners and landlords every year replace their outdated, inefficient back boiler behind the fire with a compact, energy-saving combination boiler – saving money on bills, gaining space, and bringing the property up to current building regulations.
In this guide, I’ll explain everything you need to know: what the conversion involves, average costs in 2025, benefits vs drawbacks, the full removal and installation process, grants available, and how to choose the right Gas Safe engineer.
What Are the Main Benefits of Converting a Back Boiler to a Combi Boiler?
Converting from a back boiler to a combi delivers multiple advantages for UK properties:
- Average energy bill savings of £300–£580 per year (Energy Saving Trust figures 2025)
- Instant unlimited hot water – no waiting for a cylinder to heat
- More living space – removes the cylinder, cold water tanks and the old fireplace unit
- Increases property value and makes the home easier to sell or let
- Brings the heating system up to current Part L building regulations
- Removes the risk of asbestos exposure from old back boiler units
- Quieter operation and far fewer breakdowns
How Much Does a Back Boiler to Combi Conversion Cost?
A full back boiler conversion to combi in the UK typically costs between £3,800 and £6,500 (including parts, labour, new flue, system flush and Gas Safe certification.
| Property Type | Average Cost (2025) | Typical Installation Time |
| 2–3 bed terraced/semi | £3,800 – £4,800 | 3–4 days |
| 3–4 bed detached | £4,500 – £5,800 | 4–5 days |
| Flat with difficult access | £4,200 – £6,500 | 4–6 days |
You may qualify for the Boiler Upgrade Scheme grant (£7,500 in England & Wales until 2028) if you install a heat pump instead of a gas combi, or the ECO4 scheme if you’re on certain benefits.
What Does the Back Boiler to Combi Conversion Process Involve?
A professional back boiler removal and combi installation usually follows these steps:
- Initial survey by a Gas Safe engineer
- Asbestos check and safe removal if required (mandatory for pre-2000 BBUs)
- Removal of the old gas fire and back boiler unit
- Capping off or removing the old cold-water tank and hot-water cylinder
- Powerflush of the entire heating system
- Installation of the new combi boiler (usually in the kitchen or utility)
- New flue, condensate pipe and pipework modifications
- Full system testing and Gas Safe certification
Check if our guide on how messy removing a back boiler is for more on what to expect.
Can Every House Have a Back Boiler Converted to a Combi?
Not every property is suitable for a direct combi swap. Common issues include:
- Low mains water pressure/flow rate (combi boilers need at least 1.0–1.5 bar and 10–12 litres/min)
- Very large homes with 15+ radiators and multiple bathrooms (a system or regular boiler may be better)
- Listed buildings or conservation areas (planning permission sometimes needed for external flues)
A good installer will carry out a full heat-loss calculation and water-pressure test before quoting.
How Long Does a Back Boiler to Combi Conversion Take?
Most standard conversions take 3–5 working days from start to Gas Safe sign-off. You will be without heating and hot water during this time, so many people book the work for spring or summer.
How to Prevent Problems After a Back Boiler to Combi Conversion
To keep your new combi boiler running perfectly for 12–15 years:
- Have it serviced every 12 months by a Gas Safe engineer
- Install a magnetic system filter and limescale reducer (especially in hard-water areas)
- Keep the pressure between 1–1.5 bar
- Bleed radiators annually
- Consider a maintenance & breakdown plan (£10–£20/month)
FAQs
Will I lose my living-room fireplace when I remove the back boiler?
Most people replace the old gas fire with an electric or bioethanol fire, or board it up and install a TV/feature wall. A decorative fireplace can still be kept if desired.
Can I keep my existing radiators after a back boiler to combi conversion?
Yes – in 90% of the time your current radiators can stay. The installer will usually upgrade the old 8mm microbore pipework to 15mm where needed for better flow.
Is it worth converting if I’m planning to sell the house in 1–2 years?
Absolutely. An Energy Performance Certificate (EPC) rating usually jumps from D/E to B/C after a new combi, which can add £8,000–£15,000 to the sale price and make the property much more attractive to buyers.
Do landlords have to upgrade from a back boiler by law?
No legal date yet, but from 2028 all privately rented properties must have a minimum EPC rating of C. Most homes with back boilers are currently D or E, so landlords are upgrading now to future-proof.
Can I spread the cost of a back boiler to combi conversion?
Yes – many companies offer 0% finance over 2–10 years, or Buy Now Pay Later (nothing to pay for 12–24 months). Typical monthly cost for a £4,800 conversion is £45–£80/month over 10 years.
Stay warm, The Boilers2Go Team
I’m Penny North, a home energy heating expert. My mission is to demystify new boilers and complex heating systems to help you achieve a warm, cosy home with lower energy bills and a smaller environmental footprint.