What a Combi Boiler Does
A combi boiler (short for combination boiler) heats your home and supplies hot water on demand from a single unit, without needing a separate hot water tank or cold water storage tank. When you turn on a hot tap, the boiler fires up, heats water as it flows through, and delivers it directly to you. When you turn the tap off, the boiler stops heating. The same boiler also connects to your radiators and heats them on a schedule you set.
The advantage is space: you save the cupboard or loft room that a traditional tank system would occupy. The disadvantage is that a combi cannot heat your radiators and supply hot water to multiple taps at the same time — if someone is running a bath while the heating is on, the radiators cool slightly, or the bath runs lukewarm.
Key Takeaways
- A combi boiler heats water on demand for taps and radiators from one unit, eliminating the need for a separate hot water cylinder or cold water tank.
- The boiler uses a heat exchanger to warm water as it flows through, so hot water arrives within seconds of opening a tap.
- A diverter valve inside the boiler switches water flow between the heating circuit (radiators) and the hot water circuit (taps) depending on demand.
- Combi boilers work best in smaller homes or flats where hot water demand is light; larger homes with multiple simultaneous users may experience reduced flow or temperature.
- The boiler requires mains water pressure to work, so it cannot function if your water supply is cut off or pressure drops significantly.
The Heat Exchanger: Where Water Gets Heated
At the heart of a combi boiler is the heat exchanger, a metal chamber where cold water passes through and gets heated by the boiler's flame or electric element. When you open a hot tap, mains water enters the boiler, flows through the heat exchanger, and emerges hot enough to use — usually within 5 to 10 seconds. The water does not sit in a tank waiting; it is heated as it travels.
The heat exchanger is designed to transfer heat efficiently from the burning gas (or electric heating element) to the water flowing past it. Modern condensing combi boilers use a second heat exchanger to capture heat from the exhaust gases that would otherwise escape up the flue, which is why they are more efficient than older models.
The Diverter Valve: Switching Between Heating and Hot Water
A combi boiler cannot heat your radiators and supply hot water to a tap at the same time because it has only one heat exchanger. Instead, a diverter valve inside the boiler automatically switches the flow of water between two circuits: the heating circuit (which goes to your radiators) and the hot water circuit (which goes to your taps).
When you open a hot tap, the drop in water pressure triggers the diverter valve to move, blocking water from the radiators and directing it through the heat exchanger to your tap instead. When you close the tap, pressure builds again, the valve switches back, and water flows to the radiators once more. This happens automatically and takes only a few seconds. If the diverter valve sticks or fails, you may get no hot water, no heating, or both circuits running at once (which wastes energy).
The Burner and Ignition System
When the boiler detects a demand for heat — either because a radiator thermostat calls for warmth or because you have opened a hot tap — the burner ignites. In a gas combi boiler, a spark or hot surface ignites the gas-air mixture. In an electric combi boiler, an electric element heats up. The flame or element then heats the water flowing through the heat exchanger.
A thermostat inside the boiler monitors water temperature and tells the burner when to fire and when to stop. If the water reaches the target temperature (usually 60°C for hot water, adjustable by you), the burner shuts down. This on-off cycling is why you hear a combi boiler click and hum — it is constantly adjusting to maintain the right temperature.
The Pressure Relief and Safety Systems
As water heats, it expands. A combi boiler has a pressure relief valve that opens if pressure inside the system gets too high, releasing water to a drain to prevent damage. You may see a small amount of water dripping from a pipe outside your home when the boiler is heating hard — this is normal and means the relief valve is working.
The boiler also has a reset button (usually red or black) that you press if the boiler shuts down due to a fault. Common reasons for shutdown include a drop in gas pressure, a blocked flue, or a fault detected by the boiler's internal sensors. If the boiler will not restart after pressing reset, or if it keeps shutting down, the boiler needs a service or repair.
Why Combi Boilers Need Mains Water Pressure
Unlike older boiler systems that use a cold water tank in the loft to create pressure, a combi boiler relies on mains water pressure from your water supplier to push water through the heat exchanger and out of your taps. If your water supply is cut off, or if pressure drops (which can happen during maintenance work on the water main), your combi boiler cannot supply hot water, even if it is working perfectly.
In areas with low mains pressure, a combi boiler may deliver hot water slowly or at a weak flow. Some homes install a pressure vessel (a small tank) to store pressurised water and improve flow, but this adds cost and complexity. If you live in a flat or a property fed by a shared supply, check your water pressure before choosing a combi boiler.
Condensing Combi Boilers: Higher Efficiency
Most new combi boilers sold are condensing models, which capture heat from the exhaust gases that leave the boiler. In a non-condensing boiler, these hot gases escape up the flue and are wasted. In a condensing boiler, a second heat exchanger cools the exhaust gases, extracting their heat and using it to warm the incoming water. This process is so efficient that water vapour in the exhaust condenses into liquid, which drains away as condensate.
Condensing boilers are 15 to 20 percent more efficient than non-condensing models, which means lower fuel bills and lower carbon emissions. The trade-off is that they cost more to buy and install, and the condensate drain must be routed to a suitable outlet (usually an internal drain or an external pipe). If the drain freezes or blocks, the boiler will shut down until it is cleared.
Frequently Asked Questions
Can a combi boiler supply hot water to two taps at once?
Yes, but the flow and temperature drop. The boiler heats water as it flows, so if you split the flow between two taps, each receives less hot water and cools faster. If you run a bath and someone opens a hot tap elsewhere, the bath water may cool noticeably. Larger homes with multiple users often choose a system boiler or heat-only boiler with a separate hot water cylinder instead.
What does it mean when my combi boiler is making a loud noise?
Combi boilers make clicking and humming sounds as the burner ignites and the diverter valve switches. Loud banging or kettling usually means limescale has built up inside the heat exchanger, restricting water flow and causing steam bubbles. This requires a service engineer to flush the system or replace the heat exchanger. Ignore it and the boiler may overheat and shut down.
Why is my combi boiler not producing hot water?
The most common causes are a stuck diverter valve, a blocked heat exchanger, low mains water pressure, or a fault that has triggered the boiler to shut down. Check that the boiler is switched on, that the reset button has not been pressed, and that your water supply is running. If these are fine, the boiler needs a service engineer.
Do combi boilers need a pressure vessel?
Only if your mains water pressure is below 1.5 bar (your water supplier can tell you). A pressure vessel stores pressurised water and improves hot water flow, but adds cost and takes up space. Most homes with normal mains pressure do not need one.
How often should a combi boiler be serviced?
Once a year, ideally before winter. An engineer will check the burner, clean the heat exchanger, test the safety devices, and look for leaks or corrosion. Regular servicing keeps the boiler efficient, reduces the risk of breakdown, and is often required by your boiler warranty.