How much does a system boiler cost installed in the UK in 2026?

A like-for-like system boiler replacement in the UK in 2026 costs £2,000 to £4,000 all-in including supply, labour, removal, and Gas Safe certification. Cylinder replacement (often needed if the existing cylinder is over 15 years old) adds £800 to £1,500. Converting from a regular boiler to a system boiler saves on loft tank removal but is otherwise a similar cost.

What size hot water cylinder do I need with a system boiler?

Rough sizing: 120 litres for 2-3 occupants, 150-180 litres for 3-4 occupants, 210-250 litres for 4-5 occupants, 300+ litres for larger homes. Megaflo and similar unvented cylinders give mains-pressure flow but need a temperature relief discharge route; vented cylinders are cheaper but need a loft feed tank.

UK 2026 / June 2026

System boiler replacement cost

Q: When does a system boiler make more sense than a combi?

When the home has 3+ bedrooms with 2 or more bathrooms, and hot water is regularly used at multiple taps simultaneously. A combi cannot serve two showers at full flow; a system boiler with a properly-sized hot water cylinder can because the cylinder buffers the demand. The break-even is usually around 4 occupants or 2 bathrooms.

UK 2026 system boiler replacement: £2,000 to £4,000 installed. Add £800-£1,500 if the hot water cylinder also needs replacing.

When system beats combi

A combi boiler heats water on demand directly from the mains. A system boiler heats a stored cylinder which then feeds taps. The break-even point is roughly:

3+ bedrooms and 2+ bathrooms
4+ occupants regularly using hot water at the same time
Low mains pressure (combi flow rate is mains-pressure-limited)
Existing hot water cylinder you want to keep (en suite + main bathroom)
Plans for solar thermal (cylinders integrate; combis don't)

Cylinder considerations

The hot water cylinder is the load-bearing partner to the system boiler. Two main types:

Unvented (Megaflo, OSO, Heatrae Sadia)

Mains pressure direct to taps, strong flow. Needs a discharge pipe routed outside (Part G of the Building Regs). Annual inspection by a G3-qualified engineer. Cylinder cost: £700-£1,500.

Vented (traditional copper)

Fed by a loft cold tank. Cheaper (£350-£600) but gravity-fed flow can be weak upstairs. Often the existing setup in older UK homes.

Rough cylinder sizing: 120 L for 2-3 occupants, 150-180 L for 3-4, 210-250 L for 4-5, 300 L+ for larger homes.

Top system boiler models

Brand + model	Warranty (manufacturer terms)
Ideal Vogue Max System	Up to 12 years
Ideal Logic Max System	10 years
Baxi 800 System 2	10 years with the supplied Adey filter
Baxi 600 System 2	7 years
Vaillant ecoTEC plus system	5 years, up to 10 with the Advance kit
Viessmann Vitodens 100-W system	7 years on registration, 10 or 12 for a fee

What is in a system boiler quote

System boiler unit (£900-£2,300)
Hot water cylinder replacement if needed (£700-£1,500 unvented, £350-£600 vented)
Installation labour 1-2 days (£700-£1,500)
Flue + fittings (£100-£300)
Magnetic system filter (£80-£150)
Powerflush on systems over 10 years (£300-£600)
Building Control notification (included, £100-£200)
G3 unvented cylinder inspection certificate if applicable

Frequently asked

How much does a system boiler cost installed?

£2,000-£4,000 all-in for the boiler. Add £800-£1,500 for cylinder replacement if the existing cylinder is over 15 years old or undersized.

When does a system boiler make more sense than a combi?

3+ bedrooms, 2+ bathrooms, regular simultaneous hot water demand, or low mains pressure.

What size cylinder do I need?

120 L for 2-3 occupants, 150-180 L for 3-4, 210-250 L for 4-5, 300 L+ for larger homes.

Can I keep my old cylinder?

If under 15 years old, well-insulated (factory foam jacket, not loose insulation), and correctly sized, yes. Otherwise replacement at the same time is the right call to avoid a future second job.

Smaller home, one bathroom? Combi cost usually wins. Older home with a loft tank? Regular boiler cost.