What Is a Combi Boiler? How It Works, Pros, Cons and Who It Suits

A combi boiler, short for combination boiler, heats your central heating and your hot water from one compact unit. When you turn on a hot tap, cold water from the mains runs through the boiler, gets heated on the spot and comes straight out. There is no hot water cylinder in an airing cupboard and no cold water tank in the loft. That makes combis the most common boiler in London flats and smaller homes, but it also sets a limit on how much hot water the boiler can deliver at once.

If you are weighing a combi against the other main option, our guide to combi vs system boilers goes through the head-to-head comparison for different London property types.

How a combi boiler works

A combi has two circuits inside the same case.

• The heating circuit. A sealed loop of water runs from the boiler to the radiators and back. It is pressurised, which is why a combi has a pressure gauge that should normally read around 1 to 1.5 bar when the heating is cold.
• The hot water circuit. When you open a hot tap, a flow sensor tells the boiler to switch a diverting valve over. The burner heats a plate heat exchanger, and mains water passing through it is warmed before it reaches the tap.

Because the boiler can only do one job at a time, the heating pauses while you run a hot tap. You rarely notice this, as radiators stay warm for the few minutes a shower takes.

Nearly every gas combi fitted today is also a condensing boiler. Condensing describes how efficiently the boiler burns gas, not how it supplies hot water, so a combi and a condensing boiler are not alternatives. Our article on condensing vs combi boilers clears up that common mix-up.

Advantages of a combi boiler

• Space. No cylinder and no loft tank. In a flat, that can mean a whole cupboard back.
• Hot water on demand. You never run out in the way a cylinder does, and you never wait for a tank to reheat.
• Mains pressure hot water. With a decent mains supply, showers and taps run at mains pressure without a pump.
• Less to go wrong in the loft. No tank to leak, freeze or collect debris, and no ball valve to stick.
• Lower heat losses. There is no stored hot water cooling down in a cylinder between uses.
• Simpler installation in most small homes, because there is less pipework to run.

Disadvantages of a combi boiler

• Limited flow. A combi heats water as it passes through. Run a bath and a shower together and both suffer. Filling a bath also takes longer than from a cylinder.
• Dependent on the mains. If your mains flow is weak, your hot water will be weak too. Read our guide to low water pressure if your taps already struggle.
• No backup. With a cylinder and an immersion heater, a boiler breakdown still leaves you some hot water. When a combi fails, heating and hot water go together.
• Sensitive to hard water. Most of London has hard water, and limescale builds up in the plate heat exchanger. A scale reducer on the cold feed is worth fitting.
• Harder to pair with solar or a heat pump later, since these usually rely on a hot water cylinder.

Who a combi boiler suits

Home	Is a combi a good fit?
Flat or maisonette, one bathroom	Usually yes. Space saving matters most, and one shower at a time is normal.
Two or three bed terrace, one bathroom	Usually yes, provided the mains flow is good.
House with a bathroom and a separate shower room	Borderline. Fine if the showers rarely run together; otherwise look at a system boiler.
Larger house, two or more bathrooms, busy mornings	Often not. A system boiler with a cylinder copes much better.
Weak or shared mains supply	Get the flow measured first. A combi may disappoint.

Boiler output is quoted in kilowatts, and on a combi the hot water output is what matters most. A bigger output heats more water per minute, but it cannot create flow your mains does not provide. An engineer should check both before recommending a size.

How to tell if you already have a combi

1. Look for a hot water cylinder. No cylinder anywhere in the home usually means a combi.
2. Check the loft. No cold water storage tank and no small feed-and-expansion tank points the same way.
3. Look at the boiler. A pressure gauge on the front and the word "combi" or a letter "C" in the model name (for example "30C") are common signs.
4. Run a hot tap. If the boiler fires up as soon as you open a hot tap, even with the heating off, it is almost certainly a combi.

Switching to a combi from an older system

Many London homes still have a regular boiler with tanks in the loft and a cylinder. Converting to a combi means removing the tanks and cylinder, rerouting some pipework and connecting the hot water side straight to the mains. Older radiators and pipework also have to cope with a pressurised system, so the engineer will check for weak joints and corroded valves first. Cleaning the system before the new boiler goes on protects it from old sludge.

Replacing a boiler is notifiable gas work and must be done by a Gas Safe registered engineer. For budgeting, see our boiler installation cost guide, and for timing, how long a boiler installation takes. If your current boiler is simply getting old, how long boilers last will help you decide whether to replace it now or later.

If you would rather have an engineer look first, our labour rate is £95 an hour, and the £60 call-out fee is waived on jobs that run over 2 hours.

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