Switching from a conventional boiler, system boiler, or back boiler to a combi is one of the most common heating upgrades in the UK. For most homes it’s an excellent decision — but it’s not right for everyone. Here’s what you need to know before committing.
Is Converting to a Combi Boiler Worth It?
For the majority of UK homes, yes. A combi boiler eliminates the need for a hot water cylinder and loft tanks, heats water on demand, and runs at over 90% efficiency. Older conventional boilers can be as low as 50–70% efficient — meaning up to half your gas spend is wasted.
The average UK household saves between £300 and £500 per year on energy bills after switching from an older G-rated boiler to a modern A-rated condensing combi boiler, according to the Energy Saving Trust. A new boiler installation also needs to be fit correctly to get the benefits, so let’s see how to decide and what to do next.
When Is Changing to a Combi Boiler the Right Decision?
Converting to a Combi Is Likely Right for You If:
- Your home has one bathroom and up to four bedrooms
- You want to free up the space taken by a cylinder or loft tanks
- You’re replacing a back boiler — a combi is almost always the natural swap
- You live in a flat, terraced house, or smaller semi-detached
- Your mains water pressure is good
When Switching to a Combi May Not Be the Best Fit
- You have two or more bathrooms regularly used at the same time
- Your household has consistently high hot water demand
- Your mains water pressure is poor
- You have a large detached property with four or more bedrooms
If any of the above apply, a system boiler with a pressurised unvented cylinder will likely serve you better. Our engineers will advise during your free home survey.
Pros and Cons of Changing to a Combi Boiler
Pros
- Frees up significant space — no cylinder in the airing cupboard, no tanks in the loft
- Instant hot water on demand — no waiting for a cylinder to reheat
- Lower energy bills — you only heat water when you actually need it
- Over 90% efficient — A-rated combis vs 50–70% on older conventional boilers
- Simpler system — fewer components, fewer potential faults, lower long-term maintenance
- Smart thermostat compatible — control your heating from your phone
- No risk of running out of stored hot water
Cons
- Flow rate is fixed by mains pressure — running two showers simultaneously can reduce performance
- No immersion heater backup — if the boiler fails, you lose heating and hot water together
- Not suited to all large homes — very high hot water demand is better met by a system boiler
- Mains pressure dependent — low mains pressure can affect performance
What Does Converting to a Combi Boiler Actually Involve?
Switching from a conventional or back boiler to a combi is more involved than a straight boiler swap. Here’s exactly what the changeover includes:
Full Combi Conversion Process
- Drain down the entire heating system
- Remove the old boiler — disconnected, made safe, and taken away
- Remove the hot water cylinder — decommissioned and removed from the airing cupboard
- Remove cold water feed tanks — header tanks in the loft drained and taken away
- Upgrade pipework where required — from 15mm to 22mm to meet current installation regulations
- Power flush the system — removes sludge, rust, and debris to protect the new boiler
- Install the new combi boiler — fitted in its new location, typically a kitchen cupboard
- Fit a magnetic system filter — captures ongoing debris and protects the heat exchanger; required by most manufacturers to validate the warranty
- Install smart thermostat and controls — configured and tested before handover
- Commission and test the boiler — full safety and performance checks carried out
- Register the warranty — activated with the manufacturer on the day
- Issue Building Regulations Notification Certificate — handled by your Gas Safe engineer as part of the job
- Full handover — engineer walks you through the controls before leaving
How Long Does the Changeover Take?
| Existing System | Estimated Changeover Time |
|---|---|
| Like-for-like combi swap | 1 day |
| Converting from a system boiler to combi | 1–2 days |
| Changing from a conventional boiler to combi | 2 days |
| Converting from a back boiler to combi | 2 days |
The additional time on conventional and back boiler conversions comes from removing the cylinder, clearing the loft tanks, upgrading pipework, and routing a new flue. It’s a bigger job — but a one-off disruption that pays dividends long term.
How Much Does Converting to a Combi Boiler Cost?
| Conversion Type | Estimated Cost |
|---|---|
| Like-for-like combi swap | From £2,000 |
| System boiler to combi conversion | £2,500–£3,500 |
| Conventional boiler to combi changeover | £2,500–£4,000 |
| Back boiler to combi conversion | £2,500–£4,000+ |
All prices include supply and installation of the boiler, removal of the old system, power flush, magnetic filter, and new controls. The exact cost depends on the extent of pipework upgrades required, flue routing, and your chosen boiler model.
We provide fixed quotes after a free home survey — no hidden costs, no surprises on the day. Finance options are available if you’d prefer to spread the cost.
FAQs About Changing to a Combi Boiler
Will my cylinder be removed as part of the conversion?
Yes — it’s included in the changeover. The cylinder is no longer needed once a combi is fitted, and removing it is part of the process.
Is the pipework upgrade always needed when converting?
Not always, but often. Where existing pipework is 15mm it must be upgraded to 22mm under current regulations. Your engineer will confirm what’s required during the home survey.
Does converting to a combi require Building Regulations approval?
Yes — all boiler installations in the UK must be notified to Building Control. Your Gas Safe registered engineer handles this and provides you with a completion certificate.
Can I spread the cost of the switch?
Yes. Clever Energy Boilers offers flexible finance options to spread the cost of your combi conversion. Ask our team for details when you book your free survey.
What happens if the boiler breaks down after I’ve switched?
With a combi, a boiler fault affects both heating and hot water simultaneously. This is worth factoring in for larger households. Our engineers can advise on the best setup for your circumstances.
How much will I save on bills after converting?
Switching from a G-rated boiler to an A-rated combi typically saves between £300 and £500 per year, according to the Energy Saving Trust. Exact savings depend on your current boiler’s efficiency and your usage.
Book Your Free Combi Conversion Survey
Clever Energy Boilers carries out free home surveys across the North West, Yorkshire, and the Midlands. Our Gas Safe registered engineers will assess your property, confirm whether switching to a combi is right for you, and provide a fixed quote with no obligation.
Call us on 0330 055 2210 or get in touch via our website to book your conversion survey today.
