The most common boiler question we get isn't about brands — it's combi or system. Here's how to actually decide.
The question that actually matters: how many bathrooms need hot water at once?
A combi boiler heats water on demand, directly from the mains, with no cylinder to store it — which works well for one bathroom but can mean reduced flow if two showers run simultaneously. A system boiler heats and stores hot water in a cylinder, giving a buffer for multiple bathrooms in regular simultaneous use. If your household only ever uses one bathroom at a time, a combi is usually the simpler, more space-efficient answer regardless of bedroom count.
Space is the second factor
A system boiler needs somewhere for a cylinder — an airing cupboard is typical. If you do not have that space, or you have already converted to a combi and freed it up, that is a real constraint worth weighing against the hot water benefit of a cylinder.
Mains pressure matters for a combi
A combi boiler's hot water performance depends on your incoming mains flow rate — low mains pressure limits any combi regardless of its rated output. This is checked as part of a proper survey before a combi is recommended.
There is no universal right answer
Both are reliable, modern, efficient boiler types when correctly sized and installed. The right choice is specific to your property's bathroom count, hot water habits and available space — which is exactly what a free survey is for.
Combi vs. system at a glance
| Combi boiler | System boiler | |
|---|---|---|
| Hot water | Heated on demand from the mains | Stored in a cylinder |
| Cylinder needed | No | Yes |
| Loft tanks | No | No |
| Best suited to | One bathroom in use at a time | Two or more bathrooms used together |
| Space | Boiler only | Boiler plus a cupboard for the cylinder |
| If the boiler breaks down | No hot water | Hot water from the immersion heater, if one is fitted |
A "combi system" simply means a heating system run by a combi boiler: no cylinder and no loft tank. If your home still has a tank in the loft feeding a cylinder, you have a regular (heat-only) boiler, the third option. The full three-way comparison is in combi vs. system vs. regular boilers.
Sizing either type
Once the type is settled, the output follows from your radiators and the heat loss of the house. See how many radiators a 28kW boiler can run and try the boiler size calculator. With a system boiler the cylinder is sized separately, which the hot water cylinder sizing guide covers.
Not sure which suits your home? A free survey checks your bathrooms, hot water use, mains flow and space, then gives you a fixed written quote. Book a free survey, or see our combi boiler replacement and system boiler replacement services.
Frequently asked questions
Can I just go by bedroom count?
Not reliably — bathroom count and how many people need hot water at the same time matters more than bedroom count for deciding between a combi and a system boiler.
Is a system boiler always better for a bigger house?
Not automatically — a large house with one bathroom and modest hot water demand can still suit a combi well. It genuinely depends on simultaneous hot water use, not size alone.
Can I convert between the two types later if I choose wrong?
Yes — conversion between boiler types is a common job, though it involves more work (and cost) than a like-for-like swap, since pipework, and a cylinder or its removal, are usually involved.
What is a combi system?
It is a heating system run by a combi boiler. The boiler heats the radiators and heats hot water on demand from the mains, so there is no cylinder and no loft tank. A system boiler, by contrast, works with a hot water cylinder.