What is a combi boiler and how does it work?

What is a Combi Boiler?

A combination (combi) boiler is a single unit that provides both central heating and instant domestic hot water on demand — without a separate hot water cylinder or cold water storage tank.

How Does a Combi Boiler Work?

When you open a hot tap or shower, the combi boiler fires up and heats the water directly from the mains supply as it passes through a secondary heat exchanger. Hot water is delivered in seconds.

For central heating, the boiler heats water to around 70–80°C and circulates it through your radiators.

Advantages of a Combi Boiler

• Compact — no cylinder or loft tank needed
• Instant hot water — no waiting for a tank to heat up
• Lower cost — generally cheaper to install than a system boiler
• Energy efficient — no heat loss from a stored cylinder

Limitations

• Flow rate — limited to one outlet at a time without pressure drops
• Not suitable for properties with high simultaneous hot water demand (3+ bathrooms)
• No solar or heat pump compatibility for hot water

Is a Combi Right for Your London Home?

Combi boilers are ideal for London flats, 1–3 bedroom properties and homes with 1–2 bathrooms. For larger properties, a system boiler with a Megaflo cylinder is usually better.

Call RARE on 020 7458 4949 for an honest assessment.