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 GBP 2,000 to GBP 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 GBP 800 to GBP 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 / May 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: GBP 2,000 to GBP 4,000 installed. Add GBP 800-GBP 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: GBP 700-GBP 1,500.

Vented (traditional copper)

Fed by a loft cold tank. Cheaper (GBP 350-GBP 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	kW	Unit RRP	Warranty
Worcester Bosch Greenstar 8000 Lifestyle	15, 18, 25, 30 kW	GBP 1,600 - GBP 2,300	12 years
Worcester Bosch Greenstar 4000	15, 18, 21, 25, 30 kW	GBP 1,100 - GBP 1,500	10 years
Vaillant ecoTEC Plus System	12, 15, 18, 25, 30 kW	GBP 1,250 - GBP 1,750	10 years
Ideal Logic Max System	15, 18, 24, 30 kW	GBP 900 - GBP 1,200	10 years
Baxi Megaflo System	15, 18, 24, 32 kW	GBP 1,000 - GBP 1,400	10 years
Viessmann Vitodens 100-W System	19, 26, 32 kW	GBP 1,350 - GBP 1,800	10 years

What is in a system boiler quote

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

Frequently asked

How much does a system boiler cost installed?

GBP 2,000-GBP 4,000 all-in for the boiler. Add GBP 800-GBP 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.