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Combi for compact, system for multi-bath — which suits your home and what each install costs.
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Costs, timelines, pitfalls and the right questions to ask before any central heating starts on combi Boiler vs System Boiler.
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Combi undersized for big house
30kW combi can't deliver enough hot water to two showers simultaneously. System boiler needed.
Combi pressure issues
Mains water pressure under 1.5 bar gives weak combi output. Check mains pressure before specifying.
Cylinder space lost in conversion
System to combi conversion gives up the airing cupboard. Useful space.
Hard water scaling
Combi heat exchangers fail fast in hard-water areas. Specify water softener or descaler.
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
Combi boiler installationReplacing existing combi
£2.2k–£3.8k
System boiler installationReplacing existing system
£3k–£4.5k
Unvented hot water cylinderMegaflo, Telford, Heatrae
£800–£1.8k
Combi → system conversionRequires new cylinder install
£4k–£7.5k
System → combi conversionEasier — remove cylinder
£3.5k–£6.5k
Worcester Bosch Greenstar 30kW combiSupply only
£1.2k–£1.8k
!
Quote spread is typically ± 18% — always get 3 quotes.
At a glance
InfographicThe Central Heating briefing.
Infographic
Central Heating quick-view
The combi vs system boiler decision shapes your central heating efficiency for the next 15 years. Combi suits 90% of UK homes; system is better for 3+ bathroom or high-demand households. Expect to pay £2,200–£5,500 for the boiler installation alone in 2026.
What Does the Work Involve?
Combi boiler: heats water on demand — no hot water cylinder. Compact, wall-mounted. Mains-pressure hot water (no separate cold water tank in loft). System boiler: heats water that's stored in an unvented hot water cylinder. Requires cylinder space (airing cupboard or utility). Handles multiple bathrooms simultaneously.
Typical Costs
| Item | Low (£) | High (£) | Notes |
|---|---|---|---|
| Combi boiler installation | 2,200 | 3,800 | Replacing existing combi |
| System boiler installation | 3,000 | 4,500 | Replacing existing system |
| Unvented hot water cylinder | 800 | 1,800 | Megaflo, Telford, Heatrae |
| Combi → system conversion | 4,000 | 7,500 | Requires new cylinder install |
| System → combi conversion | 3,500 | 6,500 | Easier — remove cylinder |
| Worcester Bosch Greenstar 30kW combi | 1,200 | 1,800 | Supply only |
| Vaillant ecoTEC 30kW combi | 1,100 | 1,700 | Supply only |
How Long Does It Take?
Like-for-like combi swap: 1 day. System swap: 1–2 days. Combi to system conversion: 2–3 days. System to combi conversion: 1–2 days (remove cylinder, gain airing cupboard).
DIY or Professional?
Gas Safe registered only.
Choosing the Right Tradesperson
Manufacturer-accredited installer (Worcester Bosch Approved, Vaillant Advance, Baxi Premier). Get 10-year warranty in writing. Confirm which boiler size — undersized leads to insufficient hot water, oversized cycles inefficiently.
UK Regulations
Gas Safe Registration mandatory. Building Regs Part L (efficiency), Part J (gas combustion safety), Part P (electrical). Boiler Plus efficiency upgrade required.
Common Problems
- Combi undersized for big house: 30kW combi can't deliver enough hot water to two showers simultaneously. System boiler needed.
- Combi pressure issues: Mains water pressure under 1.5 bar gives weak combi output. Check mains pressure before specifying.
- Cylinder space lost in conversion: System to combi conversion gives up the airing cupboard. Useful space.
- Hard water scaling: Combi heat exchangers fail fast in hard-water areas. Specify water softener or descaler.
Frequently Asked Questions
Which is more efficient?
Combi: 92–94% ErP at burner — high. System: 90–92% at burner but loses some via cylinder standby heat. Combi wins for typical 2-bed home; system wins for high-demand homes (heating stored water once vs heating in instances multiple times).
Which has higher running cost?
Combi: cheaper to run for low-demand homes. System: cheaper for households with heavy hot water use because it's heated in bulk.
Combi for a 4-bedroom house?
Possible with a 30kW+ combi if only 1 bathroom in heavy use. Two simultaneous showers — system boiler much better.
Best boiler brand 2026?
Worcester Bosch Greenstar 8000 Style: market leader. Vaillant ecoTEC Plus: equivalent quality. Baxi 800 series: good value. Ideal Logic: budget option.
How long does a boiler last?
15+ years for quality brands. Some Worcester Bosch and Vaillant boilers reach 20+ years. Cheap brands fail at 8–12.
Can I keep my system boiler when changing radiators?
Yes — boiler is independent of radiators. Just verify boiler output matches new total heat demand.
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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