Choosing Between Combi and System Boilers

Start With How You Actually Use Hot Water

The real answer to the combi versus system question usually arrives the moment you stop thinking about the boiler and start thinking about your morning. A combi heats water on demand — turn on a tap and it fires up, with no cylinder and no tank in the loft. A system boiler heats your radiators and, separately, tops up a hot water cylinder that stores water ready for whenever you need it.

So count your simultaneous demands. If two people regularly shower at the same time, or someone runs a bath while the dishwasher is filling, a combi will feel the strain. Most domestic combis deliver somewhere between 10 and 16 litres per minute, and a decent shower alone can pull 8 to 12. A system boiler paired with a 180–250 litre cylinder will happily feed two bathrooms at once without the temperature wobble.

Think about the whole day, not just the peak. A household of one or two, out at work until six, rarely asks much of a stored cylinder. A family of five with a bath-time routine and a teenager who showers for twenty minutes is a different story entirely.

Space Matters More Than You Think

A combi is the space-saver. It hangs on a wall — often in a kitchen cupboard — and frees up both the airing cupboard and the loft. It still needs a flue route to an outside wall and a condensate pipe that can drain away safely, which occasionally catches people out in flats or where the boiler sits against an internal wall.

A system boiler needs the boiler itself plus a cylinder, typically around 1.2 to 1.8 metres tall and 550mm across, along with an expansion vessel and controls. That usually means the airing cupboard or the loft. If you are planning a loft conversion in the next few years, putting a cylinder up there now is a decision you will regret.

Bear in mind that a hot water cylinder is not just a tank — it needs to be somewhere you can get to it for servicing, and somewhere a leak would not destroy a ceiling.

The Money: Upfront and Ongoing

Expect a combi installation to land somewhere between £1,800 and £3,000 fitted, depending on output, brand and how much pipework needs altering. A system boiler plus an unvented cylinder generally comes in at £2,500 to £4,000 or more, and if your mains supply needs upgrading to feed an unvented cylinder, that figure climbs again.

Running costs are less clear-cut than the sales patter suggests. A combi avoids the standing losses of a stored cylinder, but it fires up every time you rinse a cup. A modern well-insulated cylinder loses surprisingly little — often under 2 kWh a day — and a system boiler can be paired with solar thermal, an off-peak immersion tariff or, later, a heat pump.

One practical advantage of a cylinder: if the boiler breaks down in January, the immersion heater keeps hot water flowing. With a combi, no boiler means no hot water at all.

Pressure, Flow and Your Mains Supply

This is the bit homeowners most often overlook. An unvented cylinder needs decent mains pressure and flow — think 1.5 bar and 20 litres per minute as a comfortable benchmark. In older terraces, shared supplies or low-pressure areas, that is not always available, and an accumulator or a different approach may be needed.

A combi is limited by the same mains supply, so a weak incoming flow means a weak shower whatever boiler you choose. Old gravity-fed systems with a tank in the loft are increasingly being retired, but if your home still has one, the pipework and controls will usually need attention either way. Whatever you decide, ask your installer to measure pressure and flow before quoting — it takes ten minutes and saves a great deal of disappointment.

When a Combi Wins

• One bathroom, one or two occupants, straightforward routines.
• A flat, small terrace or anywhere wall space is tight.
• You want the loft and airing cupboard back.
• You want the lower upfront cost and a simpler installation.
• Hot water use is light and spread through the day rather than concentrated.

When a System Boiler Earns Its Keep

• Two or more bathrooms, or a household of four or more.
• You want to run a bath and a shower at the same time without a drop in temperature.
• Solar thermal, an off-peak tariff or a future heat pump is on your radar.
• You value the insurance of an immersion backup.
• You already have a cylinder and the pipework suits keeping one.

Before you commit, ask your installer a few direct questions: what flow rate can my mains supply actually deliver, what size cylinder or boiler output do I need for my household, and what happens to my hot water if the boiler fails? A Gas Safe registered engineer will answer all three without flinching, and a good one will steer you towards the option that fits your home rather than the one that fits their van.