Buying Guides
System Boilers: The Guide for Bigger Homes
Reviewed by our Gas Safe registered engineers · Last updated 22 July 2026
A system boiler heats a stored cylinder of hot water, making it the go-to for homes with more than one bathroom. Here’s how it differs from a combi, who it suits, and what one costs.
Quick answer: A system boiler works with a hot-water cylinder (usually in an airing cupboard) to store hot water, so several taps and showers can run at once without the flow dropping. Most key components are built into the boiler, so unlike an old ‘regular’ system it needs no tanks in the loft. It suits medium-to-large homes and anywhere with two or more bathrooms. The trade-offs are the cylinder taking up space, and hot water being limited to what the cylinder holds until it reheats.
At a Glance
What to expect from a system boiler
How a system boiler behaves day to day, so you can weigh it against a combi.
| Aspect | What to expect |
|---|---|
| Hot water | Stored in a cylinder — several outlets at once, until it runs low |
| Space needed | Needs an airing-cupboard cylinder, but no loft tanks |
| Best suited to | Medium-to-large homes, or two-plus bathrooms |
| Water flow | Strong and steady to several taps at once |
| Install cost | More than a combi (cylinder plus pipework) |
| Recovery | Once the cylinder empties, it takes time to reheat |
Pros & Cons
The upsides and trade-offs
The upsides
Great hot-water flow to several outlets at once, so multiple showers aren’t a problem. Works well with solar or a hot-water schedule, and needs no loft tanks. The natural choice for family homes.
The trade-offs
The cylinder takes up cupboard space, hot water is limited to what’s stored until it reheats, and installation costs more than a combi. Not the best fit for a compact one-bathroom flat.
Engineer Insight
It comes down to the cylinder
The deciding factor is almost always simultaneous hot-water demand. If two showers regularly run at the same time, or you’ve a big bath to fill, a system boiler and a properly sized cylinder keeps everyone happy in a way a combi can’t. We size the cylinder to the household — too small and you run out, too big and you waste energy keeping water hot you don’t use. It’s the sizing, not the badge, that makes a system boiler work.
Costs
What a system boiler costs
Typical North London guide prices — you’ll always get an upfront quote before any work starts.
FAQs
System boilers — your questions answered
What is a system boiler?
A system boiler heats a stored cylinder of hot water, so several taps and showers can run at once. Most components are inside the boiler, so it needs no loft tanks — unlike an older ‘regular’ boiler.
What’s the difference between a system and a combi boiler?
A combi heats water on demand with no cylinder; a system boiler stores hot water in a cylinder. Combis suit smaller homes; system boilers suit larger homes and multiple bathrooms used at once.
Do I need a system boiler?
If you have two or more bathrooms in regular simultaneous use, or a large household, usually yes. For a flat or small home with one bathroom, a combi is normally simpler and cheaper.
How much does a system boiler cost?
A like-for-like system boiler in London is typically £2,200–£3,500 fitted, more if a new cylinder is needed. Converting from a combi costs more because of the extra cylinder and pipework.
Can a system boiler run out of hot water?
Yes — hot water is limited to what the cylinder holds, and once it empties it takes time to reheat. Sizing the cylinder to your household avoids this in normal use.
Related
Related guides
The compact option for smaller homes.
Read guideBuying GuidesRegular Boiler GuideFor homes with a traditional tank setup.
Read guideBuying GuidesBest Boiler BrandsWhich make to choose.
Read guideBuying GuidesNew Boiler Cost LondonWhat a replacement really costs.
Read guideHot WaterNo Hot WaterTroubleshooting a cylinder or combi.
Read guideCentral HeatingHow Central Heating WorksHow the boiler fits the system.
Read guideBrowse all Buying Guides, or head back to the Help & Advice Centre.
Bigger home? Let’s size it right.
Book a Gas Safe engineer to size a system boiler and cylinder to your household, or call for honest advice. North London, upfront pricing.
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