Combi boilers
A combination (combi) boiler heats water directly from the mains for both central heating and hot water, on demand. There is no separate hot water cylinder or cold water tank in the loft.
Advantages:
- Compact โ fits in a kitchen cupboard
- Instant hot water โ no waiting for a cylinder to heat up
- No loft tank โ frees up loft space and removes frost risk
- Lower installation cost than system or regular boilers
Disadvantages:
- Hot water flow rate is limited โ multiple simultaneous outlets (shower + tap) can cause pressure issues
- Not suitable for low mains water pressure properties
- Not compatible with all bath fill rates in larger properties
Best for: Smaller to medium homes (1โ3 bedrooms), properties with good mains pressure, homes with one bathroom, Cardiff terraces, most Welsh suburban properties.
Popular models in Wales 2026:
- Worcester Bosch Greenstar 8000 (mid-high range, 7โ10 year warranty with accredited installer)
- Vaillant ecoTEC exclusive (mid-high range)
- Baxi 800 (budget-mid)
- Ideal Logic Max (budget-mid)
System boilers
A system boiler heats water that is stored in a separate hot water cylinder (usually an unvented pressurised cylinder). There is no cold water tank in the loft but an external expansion vessel is required (usually adjacent to the boiler).
Advantages:
- High hot water flow rates โ suitable for multiple simultaneous outlets
- Compatible with solar thermal hot water systems
- Good for 3+ bedroom homes with 2+ bathrooms
Disadvantages:
- Hot water cylinder takes up space (typically airing cupboard)
- Hot water is not truly instantaneous โ cylinder must have heated water available
Best for: Larger homes (3โ5+ bedrooms), properties with multiple bathrooms, homes with solar thermal potential.
Regular (heat-only) boilers
A regular boiler (also called a heat-only or conventional boiler) works with both a hot water cylinder and a cold water feed tank in the loft. This is the system that was standard in most Welsh homes built before 1990.
Advantages:
- Can deliver very high hot water volumes simultaneously
- Compatible with gravity-fed systems (useful in some older properties)
- Suitable where replacing an existing regular system without upgrading the whole system
Disadvantages:
- Requires both a hot water cylinder and a cold water loft tank
- Loft tank is a frost risk and takes up space
- Less efficient than system or combi options
Best for: Properties already on a regular system where replacing like-for-like is more cost-effective than converting; properties with gravity-fed shower systems; larger Victorian properties where combi flow rate would be insufficient.
Which is right for your home?
| Property type | Recommended boiler |
|---|---|
| 1โ2 bed flat or terrace | Combi |
| 3 bed semi, 1 bathroom | Combi |
| 3โ4 bed semi, 2 bathrooms | Combi (high output) or System |
| 4+ bed detached, 2+ bathrooms | System |
| Already has hot water cylinder | System |
| Low mains pressure | System |
| Rural property, gravity-fed | Regular |
Converting from regular to combi in an older Welsh property involves removing the loft tank, capping off old pipework, removing the cylinder, and connecting direct from the mains. This adds ยฃ400โ800 to a standard combi installation but is often worth it for the space saving and simplicity.
Low mains pressure is a common issue in some older Welsh towns and rural areas. A combi boiler on a low-pressure supply will deliver poor hot water flow rates. Ask your Gas Safe engineer to measure mains pressure before specifying a combi for an older property.