Combi vs System Boiler:
Which Do You Need?

This is the most common question we field at the survey stage of a boiler installation. The answer depends on four things: the size of your property, the number of bathrooms, your mains water pressure, and your current hot water storage setup. Here is a practical breakdown from engineers who fit both types across South East London every week.

What Is a Combi Boiler?

A combination (combi) boiler heats both central heating water and domestic hot water from a single unit — directly from the mains, on demand. There is no hot water cylinder and no cold water storage tank in the loft. When you open a hot tap, the boiler fires and heats the water as it flows through.

Combi boilers are compact, efficient, and eliminate the standing heat loss of a hot water cylinder. The trade-off is flow rate: a combi can only heat so much water per minute (measured in litres per minute, or L/min), and if two showers run simultaneously in a larger property, pressure and temperature can drop noticeably.

What Is a System Boiler?

A system boiler heats water and stores it in a separate insulated hot water cylinder — typically 150 to 300 litres, depending on demand. It connects directly to the mains (unlike older open-vented systems which required a cold water tank) and pressurises the heating circuit internally.

The main advantage is simultaneous draw: multiple bathrooms can run hot water at the same time without pressure loss, because hot water is drawn from the stored cylinder rather than heated on the fly. The trade-off is the space required for the cylinder — typically an airing cupboard of at least 60×60cm footprint and 1.6m height.

Head-to-Head Comparison

Factor	Combi Boiler	System Boiler + Cylinder
Space required	Boiler unit only — no cylinder, no loft tank	Boiler + hot water cylinder (airing cupboard)
Simultaneous hot water	Limited — flow rate degrades with multiple draws	Strong — multiple bathrooms draw from stored volume
Mains pressure requirement	Higher — typically min. 1.5 bar, 12 L/min	Lower — stored cylinder buffers low-pressure mains
Waiting for hot water	Instant — no cylinder to heat up	Near-instant if cylinder is well-insulated and correctly sized
Running cost	Lower standing losses — no cylinder to maintain	Modern unvented cylinders have very low standing losses
Installation cost	Lower — no cylinder installation required	Higher — cylinder and associated pipework add cost
Best suited to	1–2 bathroom properties, adequate mains pressure	2+ bathroom properties, low mains pressure, high demand

Which Is Right for South East London Homes?

The Victorian and Edwardian terraces common across Greenwich, Charlton, Lewisham, and Blackheath typically have one or two bathrooms and mains pressure that varies significantly by street. The 1930s semi-detached stock in Eltham, Plumstead, and parts of Charlton tends to have more consistent mains pressure and often has an existing cylinder setup that makes a system boiler replacement more straightforward.

Victorian Terraces (SE3, SE7, SE10, SE13)

A modern 35kW or 40kW combi boiler suits most Victorian terraces with one bathroom, provided mains pressure tests above 1.5 bar at the stopcock. If a property has had its loft conversion or a second bathroom added, we almost always recommend a system boiler with a 200-litre unvented cylinder instead.

1930s Semis (SE9, SE18)

Many 1930s semis in Eltham and Plumstead have existing cylinder setups. Where the cylinder is in good condition and the property has two bathrooms, we generally recommend retaining the system boiler configuration and replacing the boiler only. This avoids unnecessary pipework reconfiguration and makes the installation faster and cheaper.

What About Heat Pumps?

Air source heat pumps are increasingly common in newer South East London properties and larger period homes with loft extensions. They operate on a fundamentally different principle — they do not burn gas — and are beyond the scope of a like-for-like boiler replacement in most cases. If you are interested in heat pump options, we can advise on suitability during a site visit, but the vast majority of domestic properties in SE3–SE18 are currently better served by a modern, high-efficiency gas boiler.