Glossary · Boilers and boiler types
How does a combi boiler work?
A combi boiler is one gas appliance that supplies central heating and hot water without a hot water cylinder or cold water tank. It takes water from the mains, heats it with a gas burner and a heat exchanger when you ask for heat, and sends it to the radiators or the taps.
Key points
- Hot water is heated on demand from the mains, so there is no stored hot water and no cylinder.
- A motorised valve switches the heat between the radiators and the taps. Hot water takes priority, so heating pauses briefly when a tap runs.
- A plate heat exchanger passes the heat to the tap water, which stays separate from the water in the radiator circuit.
- Flow is limited. Vaillant and Glow-worm both say a combi suits one outlet at a time better than several.
- A combi needs reasonable mains pressure, and there is no stored hot water if the boiler stops working.
How a combi boiler works
A combination boiler, or combi, provides central heating and hot water from the same unit. Glow-worm describes it as working by heating water on demand, without an external hot water cylinder.[1]Glow-worm, What is a combi boiler and how does one work? Vaillant adds that the name comes from the idea of combining all the parts of a traditional central heating system into one compact unit.[2]Vaillant, What is a combi boiler?
Here is the sequence when you open a hot tap, based on the manufacturers’ descriptions:
- The boiler takes cold water directly from the mains.
- A sensor detects that you have turned on a hot tap or shower, or that the heating is calling for heat.
- The gas burner lights and its heat passes into the boiler’s main heat exchanger.
- For a hot tap, a motorised valve sends that heat to a second, plate heat exchanger, where it warms the mains water on its way to the tap.
- For the radiators, the valve sends the heated water round the heating circuit instead, to the temperature set on your controls.
- When the demand ends, the burner stops.
Steps 1 to 5 follow the Glow-worm and Vaillant descriptions of the process.[1]Glow-worm, What is a combi boiler and how does one work?[2]Vaillant, What is a combi boiler? Vaillant says the heat moves from the flame through the main heat exchanger to the primary circuit, and from there either directly to the radiators or to the taps through a plate heat exchanger.[2]Vaillant, What is a combi boiler? The tap water and the radiator water do not mix. The worked detail of the valve is on the diverter valve page and the exchangers on the heat exchanger page.
The parts inside
| Part | Job |
|---|---|
| Burner and main heat exchanger | Burns gas and passes the heat to the heating water |
| Plate heat exchanger | Passes heat from the heating water to the mains water for the taps |
| Diverter valve | Switches the heat between radiators and taps, with hot water first |
| Pump | Circulates the heating water round the radiators |
| Expansion vessel | Takes up the expansion of heating water as it warms |
| Pressure gauge | Shows the heating system pressure |
The Worcester Greenstar 24/28/30 combi manual lists these in one appliance: a plate heat exchanger, a three-speed heating pump with an automatic air vent, a pressure relief valve, a pressure gauge, an expansion vessel and a motorised three-way diverter valve that delivers hot water as a priority.[4]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions The pages on the central heating pump and expansion vessel cover two of them. All of these are inside the casing, so any repair is gas appliance work. Anyone employed to work on gas appliances in a home must be Gas Safe registered and competent in that area of gas work.[5]Health and Safety Executive, Domestic gas: frequently asked questions
What a combi does well and where it is limited
Vaillant says some modern combis keep a small amount of water ready inside the boiler at the temperature you want, so it reaches the tap faster.[2]Vaillant, What is a combi boiler? Ideal Heating says that because the water comes under mains pressure, there is often no need for a pump on the hot water side.[3]Ideal Heating, What is a combi boiler?
The limits come from the same design. Ideal Heating says the central heating will generally switch off for a moment when a tap is turned on, and that two hot taps at once give a noticeable drop in pressure.[3]Ideal Heating, What is a combi boiler? Glow-worm lists three points to weigh up: there is no backup hot water supply if the boiler fails, hot water suits one bathroom at a time, and the boiler needs reasonable mains pressure or flow to work properly.[1]Glow-worm, What is a combi boiler and how does one work? Vaillant says larger homes with several bathrooms may suit a system boiler better.[2]Vaillant, What is a combi boiler? You can compare them under system boiler and regular boiler. BASI’s combi boilers page covers sizing for a home.
For one specific model, the Worcester manual gives a minimum mains inlet pressure of 1.5 bar for maximum flow.[4]Worcester, Bosch Group, Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions That is a property of that model. Your boiler’s own manual has its own figure.
Example
Aisha lives in a three-bedroom semi in Morley with one bathroom. Her combi heats the radiators on a winter morning. When she runs the hot tap at the kitchen sink, the radiators pause briefly and the tap gives hot water within moments. When she turns it off, the radiators resume. A week later her teenage son and her husband run a shower and a tap at the same time and the shower runs cooler. That is the flow limit, not a fault.
What people get wrong
Many people think a combi has a hidden tank of hot water. It does not. It heats water as you use it, which is why it saves space and why it cannot serve several outlets at full flow.
The second misunderstanding is that a combi uses the same water in the taps as in the radiators. The two circuits are separate. The tap water comes from the mains, and the radiator water stays in a closed loop.
The bottom line
A combi boiler is one appliance doing two jobs. A burner and heat exchanger make the heat, a motorised valve points it at the radiators or the taps, and a plate heat exchanger warms the mains water for the taps on demand. It saves space and has limits on flow and backup. Any repair or adjustment inside the casing belongs to a Gas Safe registered engineer.
Common questions
Does a combi boiler store hot water?
No. It heats water from the mains as you use it, so there is no hot water cylinder. Vaillant notes that many modern combis hold a small amount in the boiler for a quicker start.
Why does my heating stop when I use a hot tap?
Hot water takes priority on a combi. Ideal Heating says the central heating generally switches off for a moment when you turn a tap on, and it resumes afterwards.
Can a combi boiler run two showers at once?
Not well. Glow-worm says only one shower can use hot water at any given time, and Ideal Heating says opening two hot taps gives a noticeable drop in pressure.
Can I repair a combi boiler myself?
No. Anyone paid to work on gas appliances in a home must be Gas Safe registered.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Glow-worm. What is a combi boiler and how does one work?. Checked 11 November 2026. ↩
- Vaillant. What is a combi boiler?. Checked 11 November 2026. ↩
- Ideal Heating. What is a combi boiler?. Checked 11 November 2026. ↩
- Worcester, Bosch Group. Greenstar 24/28/30 RSF combination boiler: Installation, commissioning and servicing instructions. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩