What's the difference between a combi and system boiler?

A combi boiler heats water directly from the mains when you open a tap, so there is no hot water cylinder or cold water tank in the loft. That makes it ideal for smaller homes, flats, and properties with limited space. The downside is that a combi struggles if two showers are running at the same time because it can only heat a certain flow rate. For a one or two-bathroom house, a combi is usually the best option. Our boiler cost guide covers pricing for both types.

A system boiler works with a hot water cylinder, usually an unvented pressurised cylinder in an airing cupboard. It heats a full tank of water and stores it so multiple outlets can draw hot water simultaneously without losing pressure. If you have three or more bathrooms, a family of four or more, or you want to add a high-flow rain shower, a system boiler with a 200-litre cylinder will perform far better than a combi. The trade-off is you need space for the cylinder and the water heats in advance rather than on demand.

Cost-wise, a combi boiler installation runs between £2,200 and £3,500 including the unit. A system boiler with a new unvented cylinder is typically £3,500 to £5,500 because of the extra pipework and cylinder. I always recommend clients think about their hot water demand first. If you are replacing an old conventional boiler with a tank in the loft, switching to a combi frees up space but switching to a system boiler with an unvented cylinder gives you better pressure throughout the house.