How the two systems actually differ

A combi boiler heats your radiators and your hot water from the same unit, with no cylinder or tank anywhere. When you turn on a hot tap, the boiler fires and heats cold water straight from the mains as it passes through a plate heat exchanger. Nothing is stored, so there is no standing heat loss and no loft tank to worry about.

A system boiler also heats your radiators, but it sends hot water to a separate cylinder, usually an unvented one, which stores it until you need it. The boiler is sealed, so it takes its cold water directly from the mains, but it works on a store-and-reheat principle rather than instant delivery. Both are condensing boilers and both can hit similar efficiency figures on paper, which is why the choice comes down to your home and your habits rather than the badge on the casing.

Space, cylinders and where everything lives

The practical difference most homeowners feel first is space. A combi frees up the airing cupboard and lets you remove the cold water tank and header tank from the loft, which matters in flats, terraces and anywhere you want an extra wardrobe. It also suits homes where pipework runs are tight, because there is no cylinder to site.

A system boiler needs somewhere for that cylinder. Unvented cylinders are typically 1.2 to 1.6 metres tall and around 550mm in diameter, and they need space for the expansion vessel, controls and, ideally, a service gap around them. In a two-bedroom terrace this can be awkward. In a larger house with a utility room, garage or a decent airing cupboard, it is rarely a problem. Remember that unvented cylinders must meet building regulations, need a temperature and pressure relief valve piped to a safe discharge point, and should be installed by someone holding the relevant G3 qualification.

Hot water demand and mains pressure

This is where the decision is usually won or lost. A combi's hot water output is limited by your mains flow rate and by the boiler's own capacity. A typical domestic combi might deliver around 10 to 13 litres per minute at a 35°C rise, which is comfortable for one shower at a time but struggles if someone opens a second tap or runs a bath.

An unvented cylinder, by contrast, can deliver multiple outlets at once at mains pressure, provided your incoming flow rate is decent. For a cylinder to perform properly, aim for a dynamic flow rate of at least 20 litres per minute and a pressure of roughly 1.5 to 3 bar. If you have two bathrooms, teenagers, a power shower or a large freestanding bath, a system boiler with a 180 to 250 litre cylinder will usually serve you far better. If you live alone with one bathroom, a combi is often the simpler and cheaper answer.

Worth noting too: a combi is limited by that flow rate, so if your mains supply is weak, hot water can be noticeably slower in winter when the incoming water is colder.

Running costs and efficiency in real homes

Both types are condensing and both carry ErP efficiency ratings in the low nineties. The differences show up in how you use hot water. A cylinder loses some heat through its insulation and pipework, typically 25 to 50 watts continuously, though a modern cylinder with a good insulation level and a well-lagged pipe run keeps this modest.

A combi wastes a little water and gas every time you run a tap, because it has to clear the dead leg of cold water in the pipe first and then heat up. Short, frequent draw-offs are its weak spot; long showers and baths are its strength. Cylinders, on the other hand, are efficient for lots of small draws because the water is already hot.

Another point in favour of the system boiler is future-proofing. A cylinder can be paired with solar thermal panels, and it works well with a heat pump if you plan to move away from gas later. A combi cannot easily use a thermal store or solar hot water in the same way.

Which setup suits which home

• One bathroom, one or two occupants, limited space: a combi is usually the sensible pick.
• Two or more bathrooms, or a household that showers in sequence: a system boiler with a suitably sized unvented cylinder is generally better.
• Low mains pressure or poor flow rate: check this before choosing, as it can rule out a combi entirely.
• Planning solar thermal or a future heat pump: lean towards a cylinder-based system.

Questions to settle before you book the install

Ask for a proper heat loss calculation rather than a rule-of-thumb sizing based on radiator count. The boiler's heating output should match your home's demand, while its hot water output should match your bathroom and cylinder needs. That means combis are often specified at 30 to 40kW for hot water performance, even though heating demand might be far lower.

Also ask about pipework. Upgrading to a combi often means removing tanks and rerouting feeds, and showers fed from a cylinder may need a different valve or pump arrangement. If you currently have a gravity-fed system, the switch to mains pressure is a genuine upgrade, but it needs doing properly.

Finally, weigh up the whole picture: purchase price, installation disruption, cylinder replacement in 10 to 15 years, and how you actually use hot water day to day. Get two or three quotes, ask each installer to justify their recommended cylinder size or combi output, and check they are registered for unvented work if a cylinder is involved. Choose the system that fits your home and habits, and either option will serve you well for years.