What Is a Combi Boiler and How Does It Work?

Updated July 2026 · Emergency Plumber Chelsea

A combi boiler does two jobs with one burner. A traditional heating system splits the work: a boiler heats water, a cylinder stores the hot water for your taps, and tanks in the loft feed the system. A combi puts everything in one box on the wall, usually in a kitchen cupboard. There is no stored hot water. Instead, the boiler heats water only when you need it, which is why combis are sometimes called instantaneous or on-demand boilers. Almost all combis sold today are condensing boilers, which recover extra heat from the exhaust gases, and our guide to what a condensing boiler is explains that part.

Heating mode works like any sealed system. The heating side of a combi is a closed loop of water that runs from the boiler, round every radiator and back again. A pump pushes it round, and the burner heats it through the main heat exchanger. Because the loop is sealed, it has a pressure gauge on the front of the boiler, which should read around 1 to 1.5 bar when cold, and an expansion vessel inside the casing that absorbs the extra volume as the water heats up. Our guide on what pressure your boiler should be explains how to read and top up that gauge. Our guide on how central heating works explains the pump, pipes, radiators and controls in more detail.

Hot water mode is where a combi is different. When you open a hot tap, a flow sensor inside the boiler detects the cold mains water moving and tells the boiler to switch over. The burner fires at full rate, and a component called the diverter valve redirects the hot heating water away from the radiators and through a small plate heat exchanger. On the other side of that plate heat exchanger, fresh mains water passes through and picks up the heat, so it comes out of your tap hot a few seconds later. The two waters never mix. When you close the tap, the flow sensor detects it and the diverter valve swings back to heating.

Hot water always takes priority. While a hot tap is running, the radiators stop getting heat. For a quick hand wash you will not notice, but during a long shower on a cold morning the radiators may cool slightly. This is normal combi behaviour, not a fault. It is also why a combi that gives heating but no hot water, or hot water but no heating, often points to the diverter valve, and our guides to hot water but no heating and no hot water cover those symptoms.

The hot water flow depends on your mains supply. A combi can only heat water as fast as it arrives, so its hot water performance is limited by two things: the boiler's output and the flow rate of your cold mains. Combis are usually sold by output in kilowatts, and a larger output heats water faster. But if the mains flow into the property is weak, even a powerful combi will give a disappointing shower. In some older Chelsea buildings, a narrow or ageing supply pipe, or a long shared rising main in a mansion block, limits the flow more than the boiler does. Our guide on what size boiler you need explains how an installer checks this before recommending a model.

Combis suit most flats and smaller houses. They save space because there is no cylinder cupboard and no tanks in the loft, they give hot water whenever you want it without waiting for a cylinder to heat, and they only heat the water you actually use. Hot water comes at mains pressure, so showers are often stronger than on an old gravity-fed system. For a one-bathroom flat in a Chelsea mansion block or a converted period house, a combi is usually the obvious choice. If one tap sits a long way from the boiler and hot water takes a while to arrive there, a point of use water heater at that tap can solve it.

Combis struggle when several hot taps run at once. Because the boiler is heating water on the fly, two showers running together share the same flow, and both get weaker. In a larger house with two or more bathrooms in regular use, or a household that tends to shower at the same time every morning, a system boiler with a cylinder usually works better. Our guide comparing a combi and a system boiler sets out the choice in more detail. A combi also gives no backup: if it breaks down, you lose heating and hot water together, whereas a cylinder with an immersion heater can still give hot water.

Knowing the parts helps you describe a fault. When a combi goes wrong, the symptom often points to one part. A pressure gauge that keeps dropping points to a leak on the sealed heating side, or a dripping pressure relief valve. A pressure gauge that shoots up whenever the heating runs points to the expansion vessel. Hot water that runs hot and cold, or takes a long time to arrive, points to scale in the plate heat exchanger or a sticking flow sensor, both common in London's hard water. A boiler that will not fire at all for either heating or hot water points to the gas supply, ignition or controls, and our guide to a boiler ignition lockout covers that. Telling an engineer which of these you are seeing speeds up the diagnosis.

Hard water shortens the life of some combi parts. The plate heat exchanger heats mains water every time you run a hot tap, and in a hard-water area like Chelsea, limescale builds up on it over the years. The first sign is often hot water that fluctuates in temperature or loses heat when you turn the tap down. Many installers fit a scale reducer on the mains inlet to a new combi, and an annual service is the chance to spot problems early. Our guide on how often to service a boiler explains what a service covers.

A few simple habits keep a combi running well through the winter. Check the pressure gauge when you first put the heating on in autumn and top it up if it has dropped below about 1 bar. Keep the condensate pipe clear, and in a frost, check the plastic pipe outside if the boiler locks out. Do not block the flue terminal outside or pack the cupboard around the boiler with storage. Test your carbon monoxide alarm when you turn the heating on for the season, and book the annual service before the cold weather arrives.

If your combi is not heating the radiators, not giving hot water, or losing pressure, Emergency Plumber Chelsea sends Gas Safe registered engineers for boiler repairs and boiler installation across Chelsea, Knightsbridge, South Kensington, Sloane Square and Fulham. We explain what has failed and agree the work before we start. There is no call-out fee on jobs over 3 hours. Call 020 3002 6667.

Quick tips

• A combi heats water on demand from the mains; there is no cylinder and nothing in the loft.
• Radiators pausing while a hot tap runs is normal: hot water always takes priority.
• Weak mains flow limits a combi's hot water more than the boiler's size does.
• Check the pressure gauge reads 1 to 1.5 bar cold at the start of each heating season.
• Two showers at once share the same flow; for busy households a cylinder may suit better.

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