boilers
Combi Boiler vs System Boiler: Which Is Better for Your London Property?
What Is a Combi Boiler?
A combination (combi) boiler heats water on demand — it does not store hot water. Cold water from the mains passes through the boiler and is heated instantly. Combi boilers are the most common boiler type in London flats and smaller terraced houses.
Pros of Combi Boilers for London Properties
- Compact — no cylinder or header tank needed (valuable in London flats)
- Instant hot water — no waiting for a tank to heat up
- Efficient — only heats water when demanded
- Lower installation cost than system boiler
Cons of Combi Boilers for London Properties
- Flow rate limited — running two showers simultaneously is problematic in larger properties
- Reliant on mains pressure — poor mains pressure areas of London give disappointing flow
- Not suitable for HMOs with multiple occupants requiring simultaneous hot water
What Is a System Boiler?
A system boiler heats water and stores it in an insulated hot water cylinder. Multiple outlets can run simultaneously without loss of pressure. Common in larger London houses and HMOs.
Pros of System Boilers for London Properties
- Multiple simultaneous hot water outlets — essential for HMOs and large families
- Not affected by mains water pressure variations
- Better suited to properties with more than 2 bathrooms
Frequently asked questions
Which boiler is better for a London flat — combi or system?
For most London flats with 1–2 bathrooms, a combi boiler is the better choice. Space is at a premium in London flats, and a combi eliminates the need for a hot water cylinder. The majority of London flat boiler replacements use combi boilers for this reason.
Is a combi boiler suitable for an HMO in London?
Generally no — combi boilers struggle to serve multiple occupants requiring simultaneous hot water. A system boiler with a large hot water cylinder (180–300L) is typically more appropriate for London HMOs with 3+ occupants. Some large HMOs use commercial combination water heaters.
How much does a new combi boiler cost in London?
A new combi boiler installation in London (supply and fit) typically costs £2,000–£3,500 depending on the boiler brand, property access, and existing pipework condition. This includes removal of the old boiler, installation of the new boiler with new controls, and gas commissioning. Higher-specification brands (Viessmann, Worcester Bosch) sit at the top of this range.