Combi Boiler vs System Boiler
A straight comparison of combi and system boilers, and how to tell which one your property actually needs.
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The choice between a combi and a system boiler comes down to one question: does more than one person in your household need hot water at the same time?
If the answer is no, a combi is usually the better option. If the answer is yes, a system boiler almost always gives a better result. Everything else is detail, and the detail is below.
The deciding factor: simultaneous demand
A combi heats water as it flows through the appliance, so its hot water output is limited by how fast it can raise the temperature of incoming mains water. Open a second outlet and that capacity is shared between them.
A system boiler heats water in advance and stores it in a pressurised cylinder. Drawing from two outlets simply empties the cylinder faster, and both outlets run at full pressure while it lasts.
This is why a combi in a house with two busy bathrooms is such a common regret, and why it is worth being honest at the survey stage about how your household actually uses hot water.
Where mains pressure changes the answer
A combi is only as good as the water coming into the property. If your incoming mains flow rate is poor, a combi will deliver a disappointing shower no matter which model you buy, because there is no stored water to draw on.
A system boiler is far more forgiving. The cylinder fills gradually and delivers at pressure when you need it, so a modest incoming supply matters much less.
We measure flow rate during the survey. It takes a few minutes and it removes the guesswork entirely.
What about space?
This is the combi’s real advantage. No cylinder and no loft tanks means an airing cupboard you can actually use and a loft free of plumbing.
If space is genuinely tight and hot water demand is low, that is a strong argument. If you have somewhere sensible to put a cylinder and more than one bathroom, the space saving is rarely worth the loss in hot water performance.
Cost differences
Like-for-like replacement of either type starts from £1,650 + VAT. The bigger cost difference is not the boiler, it is the conversion.
Converting from a system to a combi means removing a cylinder and tanks and altering pipework. Converting the other way means installing a cylinder and finding room for it. Either conversion adds significantly to the job, which is why staying with your existing system type is usually the cheapest route if it is serving you well.
Side-by-side comparison
| Combi boiler | System boiler | |
|---|---|---|
| Space required | No cylinder, no loft tanks. Frees up an airing cupboard and loft. | Needs a hot water cylinder, usually in an airing cupboard. No loft tanks required. |
| Hot water supply | Heated on demand. Never runs out, but flow is limited to what the boiler can heat. | Stored in a cylinder at mains pressure. Strong flow, but a finite volume before reheat. |
| Two outlets at once | Flow to each drops noticeably. Two showers together is a poor experience. | Handles simultaneous demand well. Two showers at full flow is normal. |
| Mains water pressure | Performance depends directly on incoming pressure and flow rate. | Cylinder buffers modest mains flow, so less sensitive. |
| Bath filling | Fills at the boiler's flow rate, which can be slow. | Fast fill from stored hot water. |
| Installation | Simplest where one is already fitted. Conversion means removing cylinder and tanks. | More components to install and site, so usually a longer job. |
| Backup if the boiler fails | None. No stored hot water at all. | An immersion heater can usually provide hot water. |
| Best suited to | One to three bedroom homes with a single bathroom and good mains pressure. | Larger homes, two or more bathrooms, or high simultaneous demand. |
Frequently asked questions
Which is better, a combi or a system boiler?
Neither is universally better. Combis suit smaller homes with one bathroom and good mains pressure. System boilers suit larger homes with two or more bathrooms or high simultaneous hot water demand.
Can a combi boiler run two showers at once?
Not well. Flow to each outlet drops noticeably. If two showers are used at the same time regularly, a system boiler is the better choice.
Do system boilers need a loft tank?
No. The expansion vessel and pump are built into the boiler. Only a traditional regular boiler needs loft tanks.
Is it expensive to switch from one to the other?
Converting between boiler types costs more than a like-for-like replacement because of the cylinder, tanks and pipework changes involved. We quote conversions individually after survey.