Combi Boiler vs System Boiler Cost Calculator

How this calculator works

This calculator works in ranges rather than single figures, because real quotes, yields and tariffs genuinely vary that much. It starts from typical UK price, output and savings ranges drawn from published 2026 industry guides and official scheme documentation, then scales them by the quantities you enter and adjusts them for each choice you make below. The result is a low-to-high band that a realistic outcome is likely to fall within, not a promise of the figure you will achieve.

A combi installation costs to against to for a system boiler with cylinder in 2026, a gap of roughly to that buys the cylinder, its pipework and controls. Running costs are nearly identical, both being condensing gas boilers, with the system setup carrying only the cylinder’s standing heat loss, a modern insulated cylinder losing perhaps 1 to 2 kWh a day, worth to a year.

Home size

Home size scales the installation estimates: larger homes need higher output units, bigger cylinders where fitted and sometimes more pipework, while flats and small houses sit below the three bedroom baseline that published guide prices describe. It does not scale the running cost figures, which follow the demand number you enter instead.

Annual heat demand (kWh)

This is the heat your home needs over a year for space heating and hot water. A typical UK three bedroom house needs roughly 10,000 to 14,000 kWh; if you heat with gas today, your annual gas kWh from your bill multiplied by about 0.85 for boiler efficiency is a good estimate. Every running cost figure on this page scales directly with this number.

Boiler brand tier

Brand moves a quote by several hundred pounds either way: budget boilers do the same job with shorter warranties of around 2 to 5 years, mid-range brands balance price against 5 to 10 year cover, and the premium names, led by Worcester Bosch and Vaillant, cost 15 to 25 percent more for industry-leading reliability records and warranties up to 12 years. Over a 10 to 15 year boiler life, a long parts-and-labour warranty is worth real money, which is why premium brands hold their price.

Where you live

Heating labour is not priced uniformly across the UK, with published guides putting London and South East installation quotes roughly 15 to 25 percent above the national average. The hardware costs the same everywhere, so the difference sits in engineer day rates, access and parking.

What else affects the figures

The real decision is hot water behaviour, not money: a combi heats water only as it flows, so it never runs out but serves one outlet well at a time, while a cylinder stores litres ready for simultaneous showers, fast bath filling and homes where mornings are a queue. The practical rule is bathrooms: one bathroom, combi; two or more in regular simultaneous use, system boiler; borderline cases decided by mains flow rate, which caps what any combi can deliver. The cylinder also buys future flexibility, since heat pumps and solar diverters both want one, which is why some 2026 buyers choose system boilers as a bridge to a later heat pump. Conversions between the two add to in either direction, so the configuration you have is worth keeping unless its limits genuinely bite.

Certification, planning and grid connection

Every gas boiler installation, repair or removal is legally restricted to Gas Safe registered engineers, and working on gas without registration is a criminal offence, so checking the engineer’s Gas Safe ID card and registration number is the non-negotiable first step. The installation is notifiable under Building Regulations, which registered installers self-certify, and Boiler Plus rules require time and temperature controls plus an efficiency measure on new combi installations. Flue positions and condensate drainage follow set standards worth confirming in the quote, an annual service protects both safety and the manufacturer warranty, and a carbon monoxide alarm near the boiler is cheap, sensible and in rented properties legally required.