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01 · Ask
Repair, replace or upgrade?
Cold rads, no hot water, F22 fault code? Describe what's happening — the AI tells you if it's a £150 repair or a £3k replacement.
Compare combi vs system.
Combi for compact homes, system for high hot-water demand — which is right for your bathrooms.
✓ Updated May 2026✓ Free quotes✓ Vetted pros
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Read the brief on sizing, brands and warranty rules — then let Three local engineers quote on the right boiler for your house.
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02 · Read the brief
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kW sizing, modulation ratios, Benchmark cert, manufacturer warranty terms. Know what to ask before they recommend the most expensive box on the van.
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03 · Compare
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Three local engineers come back with itemised quotes — boiler, flue, controls, power-flush, commissioning — so the like-for-like vs upgrade choice is yours.
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Top tips
Hiring a boiler engineer, without the regret.
Four moves that separate a smooth job from a nightmare.
Get a Gas Safe ID before letting them in.
Boilers are gas appliances. Anyone working on yours must be Gas Safe registered — and on the right scheme for your fuel (LPG, oil, natural gas are separate).
Demand the Benchmark certificate.
Every new boiler comes with a Benchmark commissioning checklist. Without it, the manufacturer warranty is void — and the certificate goes in your seller's pack later.
Size the boiler to the radiators, not the house.
Oversized combis short-cycle, wear out fast and cost more to run. A heat-loss calc — not a 'same as before' swap — is the sign of a careful installer.
Power-flush before fitting.
On a 10-year-old system, sludge will write off your new boiler's heat exchanger in 18 months. A power-flush should be quoted; if it's not, ask why.
Costs & timeline
Know what it costs. Know when it ends.
Indicative UK ranges and what affects price.
Cost range
By job type
Inc. VAT · 2026
Source: NMT quotes
Source: NMT quotes
Boiler service (annual)
£80–£150
Combi boiler swap (like-for-like)
£1.8k–£3k
Combi boiler upgrade (premium)Vaillant/Viessmann tier
£2.8k–£4.5k
System boiler + cylinder
£3k–£5.5k
Power flush
£450–£900
Conversion from back-boilerInc. removal + replumb
£3.5k–£6k
!
Quote spread is typically ± 18% — always get 3 quotes.
At a glance
Infographic · VideoThe Boiler Engineer briefing.
Infographic
Boiler Engineer quick-view
Video guide
Boiler Engineer explained
The combi vs system boiler choice is the most consequential decision in any UK heating install. Get it right and you have years of trouble-free hot water. Get it wrong and you live with limitations. This guide compares the two for UK homes and helps you pick.
Key Differences
| Feature | Combi | System |
|---|---|---|
| Hot water source | On-demand | Cylinder-stored |
| Cylinder needed | No | Yes |
| Loft tank | No | No (modern) |
| Multiple tap use | Limited flow | Full flow |
| Initial cost | £1,800-4,500 | £2,200-5,500 |
| Space needed | Compact (boiler only) | Boiler + cylinder cupboard |
| Heat loss | None | Slight from cylinder |
| Backup if boiler fails | No hot water | Cylinder water available |
When to Choose Combi
- 1-2 bathrooms.
- Small to medium home.
- Limited cupboard space.
- Want to maximise floor space.
- Family unlikely to run multiple hot taps simultaneously.
When to Choose System
- 3+ bathrooms.
- Large family with high simultaneous demand.
- Want hot water available during power cut (combined with cylinder thermal store).
- Existing pipework optimised for cylinder.
- Want best flow rate on demand.
Typical Install Costs
| Project | Low £ | High £ |
|---|---|---|
| Combi swap (like-for-like) | 1,800 | 3,500 |
| System swap (like-for-like) | 2,200 | 4,000 |
| System → combi conversion | 2,800 | 5,000 |
| Combi → system conversion | 3,800 | 6,500 |
UK Regulations
- Both: Gas Safe + Building Control.
- System needs G3 (unvented cylinder qualification).
Common Problems
- Combi in 3-bath home — flow rate inadequate during simultaneous use.
- System with tiny cylinder — runs out in family use.
- Conversion costs — moving cylinder location adds significantly.
Frequently Asked Questions
Quick decision rule?
1-2 bathrooms: combi. 3+ bathrooms: system. Borderline: depends on simultaneous demand and lifestyle.
Combi for a 3-bath home?
Possible but compromised. Large combi (40 kW+) handles one or two simultaneous taps. Not for all three baths at once.
Can I convert later?
Yes but expensive — £3,000-£6,000 for either direction. Get it right first time if possible.
What about flow rate?
Combi flow depends on boiler kW + mains pressure. System unlimited by mains (cylinder stored under pressure).
Heat-only conventional?
Third option, used in older systems. Now uncommon for new installs.
Power cut behaviour?
Combi: no hot water during cut. System: stored cylinder water available until cooled.
This guide was written with AI assistance and is intended for general information only. Prices are estimates based on UK averages and may vary by region. Always get at least three quotes and consult a qualified professional before starting any work.
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