Alt: Three types of UK boilers compared: combi, system and regular heat-only models

Combi boilers are the right choice for most UK homes. They account for over 80% of new installations, heat water on demand, and cost £1,800-£3,000 installed. System boilers suit larger homes with 2+ bathrooms and cost £2,500-£3,500 plus a cylinder. Regular (heat-only) boilers cost £2,000-£3,000 plus tank and cylinder, and are only worth fitting if you have an older gravity-fed system you want to keep. Here is how all three compare side by side.

At-a-glance comparison table

How does a combi boiler work?

A combi (combination) boiler heats water directly from the mains the moment you turn on a tap — no cylinder, no tank, no waiting. It provides both central heating and hot water from a single compact unit, typically wall-mounted in a kitchen cupboard or utility room.

Combi boilers are the dominant choice in the UK for good reason. They are compact, efficient, and cheaper to install because there is no cylinder or pipework to add. A 28-30kW combi suits a 1-3 bedroom home with one bathroom. A 32-35kW combi handles homes with up to two bathrooms.

Pros:

• No cylinder or tank needed — saves space
• Hot water on demand — no waiting for a tank to heat
• Lower installation cost
• Simpler pipework

Cons:

• Hot water flow drops if two showers run simultaneously
• Not ideal for homes with 3+ bathrooms
• Cannot integrate with solar thermal panels (no cylinder to store heated water)

Best combi models in 2026: Worcester Bosch Greenstar 4000, Vaillant ecoTEC Plus, Viessmann Vitodens 100-W. See our full guide to the best boilers for a 3-bedroom house for detailed model comparisons.

How does a system boiler work?

A system boiler heats water and stores it in a separate hot water cylinder, delivering strong flow to multiple bathrooms simultaneously. Unlike a regular boiler, the pump and expansion vessel are built into the unit, so it does not need a cold water tank in the loft.

System boilers are the go-to choice for larger families and homes with 2-4 bathrooms. The stored hot water means no drop in pressure when multiple taps or showers run at the same time.

Pros:

• Strong hot water flow to multiple outlets at once
• No loft tank needed
• Compatible with solar thermal panels
• Faster installation than a regular boiler

Cons:

• Needs space for a hot water cylinder (usually an airing cupboard)
• Hot water runs out once the cylinder empties — then you wait for it to reheat
• Higher installed cost than a combi

Best system models in 2026: Worcester Bosch Greenstar 4000 System, Vaillant ecoTEC Plus System, Viessmann Vitodens 100-W System.

How does a regular (heat-only) boiler work?

A regular boiler — also called a heat-only or conventional boiler — works with both a hot water cylinder and a cold water storage tank in the loft. The tank feeds cold water to the cylinder by gravity, and the boiler heats it. This is the oldest type of central heating system in the UK.

Regular boilers are rarely installed in new properties. They suit older homes with existing gravity-fed pipework where converting to a combi or system boiler would require extensive replumbing.

Pros:

• Compatible with older gravity-fed radiator systems
• Can supply hot water to multiple bathrooms
• Compatible with solar thermal panels
• Low water pressure from the mains is not an issue

Cons:

• Needs both a cylinder and a loft tank — takes up the most space
• Most complex and expensive pipework
• Least efficient of the three types
• Declining installer expertise and parts availability

When to choose a regular boiler: Only if you have an existing gravity-fed system and the cost of converting to a system or combi boiler is prohibitive. For most homeowners replacing a regular boiler, switching to a system boiler is the better long-term move.

Which boiler type should you choose?

For 80% of UK households, a combi boiler is the right answer. Use this decision guide:

• 1 bathroom, 1-3 bedrooms: Combi boiler (28-30kW)
• 2 bathrooms, 2-4 bedrooms: Combi boiler (32-35kW) or system boiler
• 3+ bathrooms, 4+ bedrooms: System boiler with 200-300 litre cylinder
• Older home with gravity-fed system, budget constraints: Regular boiler (like-for-like replacement)
• Planning solar thermal panels: System or regular boiler (both have a cylinder to store solar-heated water)

What about switching from one type to another?

Switching from a regular boiler to a combi is the most common upgrade in the UK. The cost of conversion adds £500-£1,000 on top of the standard installation price to cover removing the old cylinder, tank and associated pipework. Most installers complete the job in one day.

Switching from a combi to a system boiler is less common but may be necessary if you have added a second bathroom. Budget an extra £800-£1,500 for the cylinder and associated pipework.

For more on boiler costs and options, visit our main boilers page. If you are weighing up a boiler against a heat pump, read our new boiler vs heat pump comparison.

FAQ

Which boiler type is cheapest to install?

A combi boiler is the cheapest to install at £1,800-£3,000 because it needs no cylinder or tank. A system boiler costs £2,500-£3,500 plus £300-£500 for a cylinder. A regular boiler costs £2,000-£3,000 plus cylinder and tank.

Can a combi boiler handle two showers at once?

A standard 30kW combi boiler struggles to deliver full-pressure hot water to two showers simultaneously. If you regularly run two showers at the same time, a 35kW combi may cope, but a system boiler with a 200+ litre cylinder is the more reliable solution.

Is a system boiler more efficient than a combi?

Not necessarily. Modern combi boilers achieve 92-94% ErP efficiency, slightly higher than most system boilers at 90-93%. However, system boilers lose a small amount of heat from the stored cylinder, which combis avoid entirely.

Should I replace my regular boiler with a combi?

In most cases, yes. Switching from a regular to a combi boiler removes the need for a loft tank and airing cupboard cylinder, freeing up space and simplifying your heating system. The conversion typically adds £500-£1,000 to the installation cost.

What is the most popular boiler type in the UK?

Combi boilers dominate the UK market, accounting for over 80% of all new installations (Heatable, UK Boiler Statistics 2026). Their compact size, on-demand hot water, and lower installation cost make them the default choice for most homes.

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