Comparison Guide · Updated 14 September 2026
Combi Boiler vs System Boiler: Best Choice for a London Terraced House
A combi boiler heats water on demand with no tank, making it the simpler and usually cheaper choice for a one-bathroom terraced house. A system boiler uses a hot water cylinder, so it costs more to install and needs airing cupboard space, but it supplies multiple bathrooms with hot water at once without the flow dropping. For most London terraces the deciding factor is straightforward: how many bathrooms are you running at the same time.
At a Glance
Combi Boiler
Best for: one bathroom, no cylinder space
VS
System Boiler
Best for: multiple bathrooms, simultaneous use
Combi figures from our own Central Heating & Gas Work page. System figures: Checkatrade, MoneySuperMarket 2026.
Side by Side
Combi vs System Boiler: The Full Comparison
Installation cost and time for a straightforward combi swap are our own quoted figures. System boiler figures are triangulated from named UK industry sources, since it is a less common install for us than a like-for-like combi swap.
| Criteria | Combi Boiler | System Boiler |
|---|---|---|
| Hot water cylinder | None, heats on demand | Required, typically 120–210L for a London terrace |
| Cold water storage tank | None | None (unlike a regular/conventional boiler) |
| Simultaneous hot water use | Struggles with more than one outlet at once | Supplies multiple bathrooms at full flow simultaneously |
| Typical installation cost | £1,500 – £4,500 (like-for-like swap) | £2,500 – £4,500 (including new cylinder) |
| Installation time | 1 day for a straightforward swap | 2 to 4 days |
| Space needed | Just the boiler itself, wall-mounted | Boiler plus an airing cupboard for the cylinder |
| Best suited to | Flats and smaller terraces, one bathroom | Family terraces with two or more bathrooms |
| Annual service cost | £70 – £150 | £70 – £150 |
| Gas Safe engineer required | Yes | Yes |
Running costs are broadly similar between the two for the same fuel and boiler efficiency rating. What actually drives your bill is how much hot water your household uses, not which of the two boiler types you choose.
Which One Is Right For You
When Each Option Actually Makes Sense
For a London terraced house, the number of bathrooms you run at the same time is usually the deciding factor, not the size of the property alone.
A combi boiler makes more sense when…
- Your terrace has one bathroom, or bathrooms are rarely used at the same time.
- You do not have spare airing cupboard space for a cylinder.
- You are replacing an existing combi like-for-like, keeping installation quick and cheap.
- You want the simplest system with the fewest components to maintain.
- Mains water pressure at your property is good and consistent.
A system boiler makes more sense when…
- You have two or more bathrooms and want full flow from each at the same time.
- You have, or can create, airing cupboard space for a cylinder.
- Mains pressure is weak or inconsistent, since the cylinder buffers against this.
- You are converting from an older regular boiler and already have cylinder space from the existing tank.
- You want the option to add solar thermal or a heat pump alongside the system later.
Common Questions
Combi vs System Boiler: Frequently Asked Questions
A combi boiler heats water on demand with no separate tank, suiting flats and smaller homes. A system boiler uses a hot water cylinder but no cold water tank, suiting homes with higher hot water demand. A regular boiler, common in older properties, uses both a cylinder and a tank.
Usually, yes, for a like-for-like swap. A straightforward combi replacement typically costs £1,500 to £4,500. A system boiler installation costs more, roughly £2,500 to £4,500, because it includes a new hot water cylinder and the extra labour and pipework that comes with it.
A combi boiler, in most cases. With only one bathroom, you rarely need hot water at two outlets simultaneously, so a combi’s simpler setup and lower installation cost make it the more practical choice. A system boiler only earns its extra cost when there is real simultaneous demand to justify it.
Yes. A system boiler’s stored hot water cylinder is its main advantage: it can supply multiple bathrooms at full flow and temperature at the same time. A combi boiler heats water as it is used, so running two showers together typically drops the flow rate or temperature at one or both.
Beyond the boiler itself, you need airing cupboard space for the hot water cylinder. A typical London terrace takes a 120 to 210 litre cylinder depending on the number of bedrooms and bathrooms, standing roughly 1.2 to 1.5 metres tall. A combi boiler needs none of this, just the wall-mounted unit.
Yes. Any gas boiler installation, repair or service, combi or system, must be carried out by a Gas Safe registered engineer by law. This applies equally to both boiler types and to the annual service and gas safety check that follows.
A straightforward combi-to-combi swap usually takes one day. A system boiler installation, including fitting a new hot water cylinder and connecting the pipework, typically takes 2 to 4 days depending on the property and whether the cylinder location needs any preparation.
Running costs are broadly similar for the same fuel type and boiler efficiency rating. The bigger factor is how much hot water your household actually uses and how well-insulated your cylinder is, not which of the two boiler types you have. Choose based on hot water demand and space, not running cost alone.
Go Deeper
Related Reading
Still Deciding?
Talk It Through With Hussain Directly
The right boiler depends on details a guide cannot see: your mains pressure, your existing pipework, and how many bathrooms you actually run at once. Send us your address and current setup, and we will come back with a straight recommendation and a free, itemised quote within 2 hours.