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Which is better, a combi or system boiler?

A combi boiler is best for London flats and homes up to 3 bedrooms — it is compact, cost-effective and provides instant hot water with no cylinder. A system boiler paired with an unvented cylinder is better for larger homes with 3+ bathrooms where multiple simultaneous showers are needed.

Can I switch from a combi to a system boiler?

Yes. Switching from a combi to a system boiler requires installing a hot water cylinder (usually in an airing cupboard) and typically takes 1–2 days. It is worthwhile if your hot water demand has outgrown what a combi can deliver.

Which boiler type is cheaper to run in London?

Combi boilers have lower running costs as there is no cylinder standby heat loss. System boilers cost slightly more to run due to cylinder heat loss, but deliver far better hot water performance for larger households with multiple bathrooms.

Can a combi boiler run two showers at the same time?

A combi boiler can supply two showers in sequence, but running both simultaneously will usually result in reduced flow at each outlet. If two or more bathrooms need hot water at the same time, a system boiler with an unvented cylinder (Megaflo) is the correct specification.

How much does a system boiler with a cylinder cost in London?

A system boiler installation with an unvented cylinder in London costs £4,800–£6,000 including the boiler, cylinder, power flush, commissioning and Gas Safe certification. A combi replacement costs £3,200–£4,200 by comparison.

Do I need a Megaflo with a system boiler?

A system boiler requires a hot water cylinder — Megaflo is the most popular unvented cylinder brand in London, but OSO and Heatrae Sadia are equally suitable. The cylinder stores hot water at mains pressure, delivering full flow to multiple outlets simultaneously.

Which boiler is best for a London flat?

A combi boiler is the best choice for most London flats — it is compact, requires no cylinder, and provides instant hot water. Vaillant ecoTEC and ATAG One combi boilers are our most common recommendations for London apartments.

Which boiler is best for a large London townhouse?

A system boiler with an unvented cylinder is the best choice for large London townhouses with 3+ bedrooms and multiple bathrooms. It delivers mains-pressure hot water to several outlets simultaneously, which a combi cannot do.

Boiler Comparison

Combi vs System Boiler — Which Is Right for Your London Home?

The most common question our London engineers are asked. The answer depends entirely on your property and hot water demand.

Choosing between a combi and a system boiler is the single most important decision in any boiler replacement project. Get it wrong and you'll either waste money on a system larger than you need, or end up with inadequate hot water pressure in a property that demands more. Our Gas Safe engineers install both daily across W1, SW1, NW3 and beyond.

Choose a combi boiler if…

Your home has 1–2 bathrooms, you want instant hot water with no cylinder, and space is at a premium. Combi boilers are the standard choice for London flats and smaller terraced houses. Installation cost: £3,200–£4,200. Combi boiler installation →

Choose a system boiler if…

Your home has 3+ bedrooms or 2+ bathrooms, you need mains-pressure hot water at multiple outlets simultaneously, or you have underfloor heating zones. System boilers require a cylinder (Megaflo). Installation cost: £4,800–£6,000. System boiler installation →

Side-by-Side Comparison

Feature	Combi Boiler	System Boiler
Hot Water Storage	None — on-demand	Yes — cylinder required
Flow Rate	Limited — 1 outlet at a time	High — multiple outlets
Water Pressure	Mains pressure	Mains pressure (unvented)
Space Required	Minimal — boiler only	Boiler + cylinder (airing cupboard)
Hot Water Delivery	Instant, but may wait	Instant from cylinder
Simultaneous Showers	Poor — pressure drops	Excellent
Solar/Heat Pump Compatible	No	Yes (dual coil cylinder)
Running Costs	Lower (no standby loss)	Slightly higher (cylinder heat loss)
Best Property Size	Flats, 1–3 bed homes	Large homes, 4+ bedrooms
Typical Install London	£2,000–£3,500	£3,500–£6,500 (boiler + cylinder)

Combi Boiler

Best for: flats, 1–3 bed London homes

Compact — no cylinder or water tank required

Lower installation cost for smaller properties

Instant hot water — no wait for cylinder to reheat

Ideal for London flats and smaller homes

Lower running costs — no standby cylinder heat loss

Limited flow rate — pressure drops with multiple showers

Not suitable for large homes with high hot water demand

No solar or heat pump compatibility

Combi Boiler Installation

System Boiler

Best for: large homes, high hot water demand

Unlimited simultaneous hot water — multiple showers at once

Excellent for large homes and multi-bathroom properties

Compatible with solar thermal and heat pump pre-heat

Consistent water pressure throughout the property

Better suited for London townhouses and large flats

Requires space for an unvented cylinder

Higher upfront installation cost

Cylinder loses some heat when not in use

System Boiler Installation

RARE Engineer Verdict

Our Recommendation

Choose a combi boiler for London flats and homes up to 3 bedrooms with 1–2 bathrooms. It's the most space-efficient and cost-effective solution, and the majority of London properties we work on in Chelsea, Pimlico and Marylebone fit this profile perfectly.

Choose a system boiler with Megaflo for larger townhouses and properties with 3+ bathrooms. If you've ever had a lukewarm shower because someone else is running a tap, a system boiler with an unvented cylinder will transform your home. This is our standard recommendation for larger properties in Hampstead, Knightsbridge and Belgravia.

Get a Quote Boiler Installation

Related installation & cost guides

Combi Boiler Installation

For flats & smaller homes

System Boiler Installation

For larger homes & plant rooms

Hot Water Cylinder Installation

Megaflo, OSO & unvented cylinders

Megaflo Installation & Repairs

G3-qualified Megaflo specialists

Boiler Installation Costs

Full London pricing guide

Boiler Replacement Guide

Repair vs replace explained

Heat Loss Calculator

Size your boiler correctly

Get a Boiler Quote

Fixed price in 3 steps

More Comparisons

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