What Is a Combi Boiler?
A combination (combi) boiler is a single unit that provides both central heating and domestic hot water. It heats water directly from the mains on demand — without a separate hot water storage cylinder or cold water tank in the loft. Combi boilers account for around 70% of new boiler installations in the UK.
How Combi Boilers Work
When you turn on the heating, the boiler fires up and heats water that circulates through your radiators. When you open a hot water tap, a diverter valve switches the boiler to heat the water supply directly from the mains. Because water is heated on demand, there is no stored hot water to run out of — provided the boiler can keep up with demand.
Advantages of Combi Boilers
Space efficiency: No cylinder or loft tank means more usable space — ideal for smaller properties.
Instant hot water: No waiting for a tank to heat up.
Mains pressure hot water: Strong, consistent flow to showers and taps.
Lower installation cost: Simpler system with fewer components.
Energy efficiency: Only heats water when needed — no standing losses from a stored cylinder.
Limitations of Combi Boilers
Simultaneous demand: A combi can struggle if multiple taps or showers run at the same time. Flow rate is shared across all outlets.
Flow rate limits: Dependent on mains water pressure. Low mains pressure means poor hot water performance.
Size limitations: Not ideal for large families in bigger homes with high simultaneous hot water demand.
What Size Combi Do I Need?
Output is measured in kilowatts (kW). A 24–28 kW unit suits smaller homes; 30–35 kW for medium homes; 38–42 kW for larger properties with higher demand. Always get a proper heat loss survey rather than relying on rule-of-thumb figures.
Halo Heating provides free, no-obligation surveys to identify the right combi boiler for your home. Call 01483 676108.
Need Professional Help?
Our Gas Safe registered engineers cover Surrey and surrounding areas. Call for a free quote or to book an engineer.
Call 01483 676108