What is a combi boiler?
A combi boiler is a single appliance that does two jobs. It heats water that circulates round your radiators, and it heats fresh water for your taps and shower on demand. Because hot water is made only when you need it, there is nothing to store. That is the big difference from a system or regular boiler, which heats a tank of water in advance and keeps it in a cylinder, usually in an airing cupboard.
Almost every combi fitted in the UK today runs on mains gas. Some run on LPG in homes without mains gas, oil combis exist for off-grid homes, and there are electric combis too, which work differently and are covered in our guide to the electric combi boiler. Since 2005, new gas boilers fitted in England and Wales have had to be condensing boilers under the Building Regulations, so a condensing combi boiler is simply what a modern gas combi is.
How does a combi boiler work?
Inside the casing there are two circuits that share one burner.
Heating mode
When the room thermostat or timer calls for heat, the burner lights and heats water in the main heat exchanger. A pump pushes that water round the radiators and back to the boiler in a sealed loop. This is the same water going round again and again, kept at a pressure of around 1 to 1.5 bar when cold, which is why a combi has a pressure gauge.
Hot water mode
When you open a hot tap, a flow sensor detects the water moving and the boiler switches over. A diverter valve sends the hot water from the main heat exchanger through a second, smaller plate heat exchanger instead of the radiators. Cold mains water passes through the other side of that plate and comes out hot. Hot water takes priority, so the radiators pause while a tap runs, which most people never notice because showers are short and radiators hold their heat.
Two parts do most of the work in this switch: the flow sensor and the diverter valve. When either fails, the typical result is heating with no hot water, which our guide to no hot water but heating works covers, or the reverse, covered in heating not working but hot water is.
What the kW rating means for your hot water
The number in a combi's model name is mostly about hot water. Most homes need well under 20 kW to keep the radiators hot, but heating water instantly for a shower needs much more power. The more kW, the more litres a minute the boiler can heat.
A useful rule comes straight from physics. Raising one litre of water by 35°C, roughly from cold mains to shower temperature, takes about 146 kJ. One kilowatt supplies 60 kJ a minute, so each kW of hot water output heats around 0.4 litres a minute by 35°C. Manufacturers' own figures match this closely:
| Model (hot water output) | Hot water flow rate | Source |
|---|---|---|
| Worcester Bosch Greenstar 4000 25kW | 9 litres a minute at a 40°C rise | Worcester Bosch product data |
| Vaillant ecoTEC plus 826 | 10.6 litres a minute at a 35°C rise | Vaillant technical data |
| Vaillant ecoTEC plus 832 | 13.0 litres a minute at a 35°C rise | Vaillant technical data |
| Vaillant ecoTEC plus 840 | 16.3 litres a minute at a 35°C rise | Vaillant technical data |
Two things can stop you getting the rated flow. The first is your incoming mains: a combi can only heat the water that arrives, so a slow mains supply means a slow shower however big the boiler. The second is winter. Mains water is colder in January, so the boiler has to raise it further and the flow at the same temperature drops.
What size combi boiler do I need?
Size a combi by hot water demand, not by the number of radiators. As a rough guide used by many installers:
- 24 to 28 kW: flats and smaller houses with one bathroom and a single shower.
- 28 to 35 kW: houses with one bathroom and a separate shower or en suite, used one at a time.
- 35 to 40 kW: larger homes with higher hot water demand, as long as the mains supply can keep up.
An installer should measure your mains flow rate and pressure before recommending a size. A bigger combi on a weak mains supply gains you nothing.
Is a 30kW combi boiler enough?
For most three bedroom homes with one main bathroom, a combi around 30 kW is a common choice. Working from the rule above, around 30 kW of hot water output gives roughly 12 litres a minute at a 35°C rise, which runs one good shower comfortably. It will not run two powerful showers at full flow at the same time.
Can a combi boiler run two showers at once?
It can supply two outlets at once, but both will be weaker. The boiler's flow is shared, so a combi that gives 13 litres a minute at a 35°C rise gives each of two showers about half that. If two showers in regular use at the same time matters to you, a system boiler with an unvented cylinder, which stores hot water at mains pressure, is usually the better fit. Our system boiler guide explains how that works.
Does a combi boiler need a hot water cylinder?
No. A combi heats water as you use it, so there is no cylinder and no cold water tank in the loft. That frees up the airing cupboard and removes the tank from the loft, which is one of the main reasons people switch to one. The trade-off is that there is no stored hot water to fall back on. If the boiler breaks down, you lose heating and hot water together, whereas a cylinder with an immersion heater gives you a backup for hot water.
Advantages and disadvantages of a combi boiler
| Advantages | Disadvantages |
|---|---|
| No cylinder or loft tanks, so it saves space | Flow is shared between taps running at the same time |
| Hot water on demand, so no waiting for a tank to heat | Performance depends on your mains flow and pressure |
| Mains pressure showers without a pump | No backup hot water if the boiler fails |
| No energy lost keeping stored water hot | Harder to pair with solar water heating, which normally needs a cylinder |
| Usually the cheapest type to install | A few seconds' wait for hot water at distant taps |
Controls and efficiency rules for a new combi
In England, the Boiler Plus rules introduced in April 2018 mean a new gas combi must be fitted with time and temperature controls and at least one extra energy-saving measure. The options are flue gas heat recovery, weather compensation, load compensation, or a smart thermostat with automation and optimisation. Ask your installer which one they are including.
To get the best from a condensing combi, keep the heating flow temperature low enough for the boiler to condense. Many combis leave the factory set high, and turning the radiator flow temperature down, while keeping rooms comfortable, can cut gas use. The boiler's user manual shows how to change it.
How long does a combi boiler last and what does it need?
A gas combi typically lasts 10 to 15 years, and often longer if it is serviced every year and the heating water is kept clean with inhibitor and a magnetic filter. Manufacturers back this with guarantees. Worcester Bosch, for example, offers 7 years on the Greenstar 4000 as standard, rising to 8 years with its Greenstar system filter and 10 years with the filter and an extra payment.
Keep the guarantee valid with an annual service by a Gas Safe registered engineer. Our guide to boiler service costs explains what that involves. When yours reaches the end of its life, our guides to new boiler costs and repairing or replacing a boiler will help with the decision.
Only a Gas Safe registered engineer can legally install, move or repair a gas combi boiler, including anything behind the casing. If you smell gas, call the National Gas Emergency Service on 0800 111 999.