Combi vs System vs Conventional Boiler — UK
Comparing the three main boiler types for UK homes — pros, cons, property size suitability, hot water demand and installation costs.
Side-by-Side Comparison
| Property | Combi | System | Conventional |
|---|---|---|---|
| Hot water delivery | On demand (no storage) | Stored in unvented cylinder | Stored in vented cylinder |
| Tanks in loft | None needed | None needed | Cold water tank + F&E tank |
| Hot water cylinder | Not needed | Unvented (pressurised) | Vented (gravity-fed) |
| Flow rate (hot water) | 10-14 l/min | 18-22 l/min (mains pressure) | 8-12 l/min (gravity) |
| Simultaneous outlets | 1 (flow drops at 2+) | 2-3 (stored water) | 1-2 (low pressure) |
| Typical output | 24-35kW | 12-25kW | 12-25kW |
| Boiler cost (supply) | £800-1,500 | £900-1,500 | £700-1,200 |
| Installed cost (replacement) | £2,000-3,500 | £3,500-5,500 | £2,500-4,000 |
| Space needed | Minimal (wall-mounted) | Boiler + cylinder cupboard | Boiler + cylinder + loft tanks |
| Best for | 1-2 bed, 1 bathroom | 3-5 bed, 2+ bathrooms | Replacing existing conventional |
Pros and Cons
Combi Boiler
Pros
- + Compact — no cylinder or tanks
- + Hot water on demand (never runs out)
- + Cheaper to install
- + Frees up loft and airing cupboard space
- + Simple system with fewer components
Cons
- - Poor flow at multiple outlets
- - Slow to fill a bath
- - Not compatible with solar thermal
- - Dependent on mains pressure
- - No stored hot water if boiler fails
System Boiler
Pros
- + Mains pressure hot water everywhere
- + Supplies multiple outlets simultaneously
- + Compatible with solar thermal
- + No loft tanks needed
- + Best for larger homes
Cons
- - Needs space for hot water cylinder
- - More expensive to install
- - Hot water can run out (cylinder size)
- - Heat loss from stored water
- - Wait time for cylinder to reheat
UK Cost Comparison (2026 Prices)
| Scenario | Typical Cost | Includes |
|---|---|---|
| Combi swap (like-for-like) | £2,000-3,000 | Boiler, flue, controls, install |
| System swap (like-for-like) | £3,000-4,500 | Boiler, cylinder, controls |
| Conventional to combi conversion | £3,000-4,500 | Remove tanks/cylinder, new pipework |
| Combi to system conversion | £4,000-6,000 | New cylinder, pipework modifications |
When to Choose Each
Choose Combi When
- 1-2 bathrooms
- Small to medium property
- Space is limited
- Low hot water demand
Choose System When
- 2+ bathrooms
- High hot water demand
- Solar thermal planned
- Want mains pressure showers
Choose Conventional When
- Replacing existing conventional
- Gravity-fed system works well
- Low mains pressure area
- Budget is very tight
Related Calculators
- Boiler Size Calculator — calculate the right boiler output for your property
- Hot Water Cylinder Calculator — size unvented cylinders for your demand
How We Calculate This
Frequently Asked Questions
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Last updated: April 2026
All calculations are estimates. Verify with your supplier.