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. A system boiler works with a hot water cylinder, usually an unvented pressurised cylinder in an airing cupboard, and heats a full tank of water in advance. The right choice depends on household size and hot water demand.
Our boiler cost guide covers pricing for both types.
Combi boilers
Combis are 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. Installation typically costs 2,200 to 3,500 pounds including the unit.
System boilers
A system boiler stores hot water 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 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. Installation with a new unvented cylinder typically costs 3,500 to 5,500 pounds because of the extra pipework and cylinder.
Switching from conventional to combi or system
If you are replacing an old conventional boiler with a tank in the loft, switching to a combi frees up loft and cupboard space but switching to a system boiler with an unvented cylinder gives you better pressure throughout the house. Think about hot water demand first — the boiler type follows from that.
What this means for a quote
A boiler installation quote should specify the boiler type and model, whether a new cylinder is included, any pipework alterations, and the flue route. If you use TailoredQuote, you can generate an itemised scope that covers the boiler, controls, pipework, and any associated building work so the customer can compare quotes on a like-for-like basis.
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