Before you compare brands or kW ratings, you need the right type of boiler. In most UK homes the choice is between a combi (combination), a system boiler with a hot water cylinder, or a regular (heat-only / conventional) boiler with a cylinder and cold-water storage tank.
Quick comparison
- Combi — Heating and instant hot water from one wall-mounted unit. No cylinder in most cases.
- System — Boiler heats radiators and a separate unvented (or vented) hot water cylinder. Many components are built into the boiler body.
- Regular — Boiler heats radiators and a cylinder; also needs a cold-water feed tank (usually in the loft) and often a feed-and-expansion tank.
How each type works
Combi boiler
A combi heats water on demand. When you open a hot tap, the boiler fires and passes mains cold water through a plate heat exchanger. For heating, a diverter valve sends flow through the primary heat exchanger to the radiators. There is no stored hot water — flow rate at the tap depends on the boiler’s hot-water output and your mains supply.
System boiler
A system boiler still heats a stored volume of hot water in a cylinder, but the expansion vessel, pump, and many hydraulic components are inside the boiler casing. The cylinder is usually unvented and mains-fed, giving good shower pressure without a loft tank.
Regular boiler
A regular boiler provides heat only. Hot water is stored in a cylinder, and a cold-water tank in the loft feeds the cylinder and system. This is the traditional layout in older UK properties and suits very high hot-water demand, but takes the most space.
Decision guide
Work through these questions with a heating engineer:
- How many bathrooms run at once? — One shower or bath at a time suits most combis. Two or more simultaneous outlets often need a cylinder (system or regular) or a high-output storage combi.
- Is space tight? — Combis win where there is no room for a cylinder or loft tanks.
- What is mains water pressure and flow? — Combis need adequate dynamic pressure and flow at peak demand. Poor mains supply can limit shower performance on any instantaneous boiler.
- Is there an existing cylinder setup? — Replacing like-for-like (system → system) can be simpler than converting to a combi, especially in larger homes.
- Do you need a hot-water backup? — A cylinder can retain some stored hot water or use an immersion heater if the boiler fails. A standard combi offers no backup.
When a combi is usually the best fit
- Flats and smaller houses with one bathroom (or one main bathroom plus a basin).
- Good mains water pressure and flow.
- Limited space — no cylinder cupboard or loft tank room.
- Moderate hot-water demand — one bath or powerful shower at a time.
When to consider system or regular instead
- Multiple bathrooms used together regularly.
- Large family with high simultaneous hot-water use.
- Very low mains flow — a stored system may deliver better usable flow at outlets.
- Property already designed around a cylinder and you want to avoid major pipework changes.
- Plans for solar thermal pre-heat or certain renewable interfaces that suit stored hot water.
Storage combi — a middle ground
Some combis include a small internal hot-water store (storage combi). They can improve flow when several outlets run briefly and reduce the “cold sandwich” effect at the start of a shower. They still need correct sizing and good mains supply — see our sizing guide.
Next steps
If a combi fits your home, read our choosing guide and sizing & efficiency articles. For installation quality, see installation best practice.
Boiler type selection should be confirmed by a competent heating engineer following a property survey and heat-loss calculation. Gas work must be carried out by a Gas Safe registered engineer.