How it works
One box, two jobs, and a valve that switches between them in about a second. Understanding that is most of what you need to pick the right size.
Get my fixed priceThe two parts that do the work
Everything else in a combi exists to serve these two components.
The detail
The main heat exchanger warms water and the pump sends it round the radiators, coming back cooler and going round again. A modern condensing boiler modulates — it turns itself down rather than cycling on and off — and the lower the temperature of the water coming back, the more efficient it is. That is why balancing your radiators and running a lower flow temperature genuinely saves gas.
Open a tap and a flow sensor notices within a second. The diverter valve swings across, the radiators stop getting heat, and the burner goes to full output to feed the plate heat exchanger. Cold mains goes in one side and hot water comes out the other, continuously, for as long as you keep the tap open.
There is no storage. That is the whole trick and it is also the whole limitation.
| Flow rate | What that feels like | Suits |
|---|---|---|
| 9 to 10 l/min | A thin but usable shower | One bathroom, one or two people |
| 11 to 12 l/min | A comfortable shower, bath in 11 minutes | The typical three-bed house |
| 13 to 14 l/min | Strong shower, bath in about 9 minutes | Four-bed, or anyone who baths often |
| 15 l/min and up | As good as a combi gets | The most you can get without a cylinder |
Quoted at a 35°C temperature rise, which is the industry standard. Real-world winter performance is lower — see the section below.
A boiler is rated on how much it can raise the temperature of the water, not on how hot the water ends up. If the mains comes in at 15°C in summer, getting to 50°C is a 35°C rise. In February the mains might be 5°C, so the same 50°C is a 45°C rise — nearly a third more work.
The boiler cannot make more heat than it has, so it slows the water down instead. A 12 l/min combi quietly becomes a 9 l/min one, which is exactly when you notice.
We size for the harder case, not the easier one. Our quote tool never sizes a cylinder-to-combi conversion below 28kW, because somebody used to a stored cylinder is used to a fast bath fill and a 24kW combi will feel like a downgrade in January. That rule costs us margin on some jobs and it prevents a complaint call on all of them.
Flow rate in the real world
Questions
On a combi it is mostly about hot water, not heating. A 30kW combi does not put 30kW into your radiators — it modulates down to maybe 6kW for heating and saves the full output for making hot water on demand.
So when you compare a 24kW and a 35kW for the same house, you are comparing shower performance, not warmth.
Nine to ten litres a minute is a thin but usable shower. Twelve is comfortable. Fourteen or fifteen fills a bath in about eight minutes and feels genuinely powerful.
The figure on the brochure is the maximum at a 35°C rise. In February, with colder incoming mains, you will get less.
Because the boiler is doing more work for the same result. Incoming mains water is about 15°C in August and can be 5°C in February. To deliver water at the same temperature the boiler has to add twice as much heat, so it slows the flow to keep up.
It is not a fault. It is physics, and it affects every combi ever made.
That is the diverter valve doing its job. A combi can only do one thing at a time, so the instant a hot tap opens it sends all of its output to the hot water exchanger.
For a five-minute shower nobody notices. If somebody baths for forty minutes on a January evening, you will.
Not properly. Two outlets share the same flow, so both get roughly half. If two bathrooms genuinely get used at the same time in your house, you want a system boiler and a cylinder.
Our quote tool will not offer you a combi if you tell it you have three or more bathrooms and showers.
Bathtubs, separate showers and radiator count decide the output. Fifteen questions and you will see the right one.
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