What Is a Combi Boiler and How Does It Work? A Plain-English Guide for Homeowners

What Is a Combi Boiler?

A combination boiler — universally called a combi — is a single unit that handles two functions: heating your home through radiators and providing hot water at the tap. The key distinction from older systems is that a combi heats water on demand, directly from the mains, rather than storing it in a separate hot water cylinder.

Around 70% of new boilers installed in UK homes are combis, and they have been the dominant type in London for the past two decades. For smaller London flats and terraced houses, they are typically the right choice.

How a Combi Boiler Works

Inside the boiler are two separate heat exchangers. When the central heating system calls for heat, the first heat exchanger (the primary circuit) heats water that circulates continuously around the radiators. This water stays within the sealed central heating system and is never consumed.

When you open a hot tap, the boiler detects the water flow and the diverter valve switches to prioritise the second heat exchanger (the domestic hot water circuit). Cold mains water passes through this heat exchanger, is heated instantly by the burner, and flows directly to your tap. There is no stored hot water — the temperature you get is determined by how fast the water flows through the heat exchanger and how much heat the burner applies.

When you close the tap, the diverter valve switches back to central heating mode.

Advantages

• No cylinder: Removing the hot water cylinder frees up an airing cupboard — a significant advantage in London flats and small terraces.
• Instant hot water: No waiting for a cylinder to heat up.
• Mains pressure hot water: Because the hot water comes directly from the mains, shower pressure depends on mains pressure rather than header tank height. In most London properties with adequate mains pressure, this is a significant improvement over gravity-fed systems.
• Efficiency: Modern condensing combis achieve efficiency ratings above 90%. There is no heat loss from a standing cylinder.
• Simpler installation: Fewer components mean lower installation cost in straightforward replacement scenarios.

Limitations

• Hot water flow rate: A combi can only heat water as fast as it can burn gas. Typical flow rates are 10–14 litres per minute. Running two showers simultaneously, or a shower while filling a bath, reduces pressure and temperature noticeably.
• Mains pressure dependency: If your London property has low mains pressure (common in some parts of outer London), a combi will not deliver satisfying shower performance. A system boiler with an unvented cylinder is often better in this situation.
• No backup hot water: If the boiler breaks down, you have no hot water at all. With a cylinder system, a dual-coil cylinder can be heated by an immersion heater as a backup.
• Not suitable for large homes: Properties with more than three bathrooms used simultaneously typically need a heat-only (regular) boiler with a large cylinder, or a system boiler.

Common Faults

No hot water but heating works: The diverter valve is stuck on the heating position. This is a mechanical fault in the valve itself and is one of the most common combi boiler repairs. A plumber or heating engineer can test and replace the diverter valve, typically costing £150–£300.

Heating works but pressure keeps dropping: The system has a leak (even a minor one) or the pressure relief valve is discharging. Locate the source — check all radiator valves and visible pipe joints. Do not keep re-pressurising without finding the cause.

Boiler fires but cuts out after a few minutes: Known as "short cycling." Causes include a blocked condensate pipe, low gas pressure, a faulty pump, or a heat exchanger beginning to fail. Requires professional diagnosis.

Noisy boiler (banging, kettling): Limescale build-up on the heat exchanger — very common in London's hard water areas — causes localised boiling and banging. A system flush with descaler can help in early stages. Severe scaling requires heat exchanger replacement or boiler replacement.

Condensate pipe frozen: In cold weather, the condensate pipe (the white plastic pipe that discharges outside) can freeze, causing the boiler to lock out with a fault code. Thawing with warm water usually gets it running again, but the discharge point should be repositioned internally if this recurs.