Most people are offered a combi by default, because for a lot of houses it is the right answer. For some it is the wrong one, and the thing that decides it is usually measured at the kitchen tap.
The difference between a combi and a system boiler is not really about the boiler. It is about whether your hot water is stored before you need it or made at the moment you ask for it, and which of those suits a house depends on things that have nothing to do with the brand on the front.
Key Takeaways
- A combi makes hot water on demand, so it can only deliver what your mains supply brings in
- A system boiler stores hot water in a cylinder, which copes far better with two taps at once
- Measuring your mains flow rate at the kitchen tap is the single most useful test
- One bathroom and a strong main usually points to a combi, two bathrooms usually does not
- A cylinder takes up space, but it leaves the door open to solar or a heat pump later
What the Three Types Actually Are
It is worth being clear on the terms, because they get used loosely.
A combi is a single unit that does both jobs. It heats the radiators, and when you open a hot tap it diverts to heating water straight from the mains as it passes through.
There is no cylinder and no tank in the loft, which is why it suits smaller houses and flats.
A system boiler heats a separate hot water cylinder, usually an unvented one fed directly from the mains. The pump and expansion vessel are built into the boiler, so the only other component is the cylinder itself.
Hot water is heated in advance and stored at pressure, ready to be drawn off.
A regular or heat only boiler does the same job, but relies on a cold water tank in the loft. It needs a separate feed and expansion tank, with the pump sitting outside the boiler. These are mostly found in older properties. When one is replaced it is often converted to a system arrangement.
Start With Your Mains Flow Rate
A combi has no reserve. Every litre of hot water it gives you has to arrive through the incoming main at that moment, which means your water supply sets the ceiling on performance no matter how powerful the boiler is.
Water companies are only obliged to deliver a fairly modest minimum flow at the boundary. That is well below what a combi wants in order to feel good in use. Plenty of homes get considerably more than the minimum, and some do not. There is no way to know which you are without measuring.
The test is simple, and worth doing yourself before you speak to anyone. Take a container of a known size, a two litre jug or a bucket you have measured. Run the kitchen cold tap fully open with nothing else drawing water, and time how long it takes to fill.
A house comfortably into double figures will run a combi well. One that struggles to reach it will have a combi that disappoints in the shower regardless of what is fitted, and that is a cylinder conversation instead.
How Many Bathrooms, and When?
The second question is about overlap rather than count.
A combi splits its flow between outlets, so two showers running at once means each gets roughly half. On a cold morning in January, when the incoming water is at its coldest and the boiler has to work hardest to raise it, that is noticeable.
One bathroom, or two that are never genuinely used at the same time, is combi territory.
A cylinder behaves differently, because the water is already hot. Two showers draw from the same store, so the pressure and temperature hold up. The limit is the size of the cylinder rather than the rate the main can supply. The trade off is that once you have emptied it, you wait for it to reheat.
So the honest question is not how many bathrooms you have. It is how often two hot outlets genuinely run together, and whether anyone in the house would notice if they could not.
Space for a Cylinder
A combi frees up a cupboard, and for a lot of people that is a real benefit rather than a technicality. Removing an old cylinder and the tanks above it can give back a useful amount of storage in a house that has none to spare.
That was the case on a job we did in Maidenhead for Alexander, where a leaking cylinder and a run of redundant pipework came out and a combi went in. For Bernard’s house in Crowthorne the answer went the other way, with a wall hung boiler working alongside an unvented cylinder, because the demand in that house needed stored hot water.
A cylinder needs somewhere sensible to live, and an unvented one has requirements beyond the space it occupies. It needs an expansion vessel and a safe discharge route for the pressure relief, and it has to be installed by someone holding the right qualification for unvented systems. It also needs checking annually, which is part of a service rather than an extra chore.
When a Combi Is the Right Answer
A combi makes sense in a house with one bathroom, a decent mains supply, and a use for the space a cylinder would take. It is simpler. There is less to go wrong, because there are fewer components. You are also not keeping a tank of water hot for the hours you are not using it.
It tends to be the cheaper installation too, because there is less equipment and usually less pipework.
When a Cylinder Is the Right Answer
A system boiler and cylinder earns its place in a few situations.
- Two or more bathrooms in real simultaneous use
- A weak mains supply that a combi cannot overcome
- A large bath that would take an age to fill at a combi’s flow rate
It is also the more future proof arrangement. Solar thermal, a solar diverter from PV panels and heat pumps all need somewhere to put heat that is not being used at that second, and a cylinder is that somewhere. If a heat pump is anywhere in your thinking for the next decade, fitting a combi now means taking the cylinder back out when the time comes.
What the Rules Require
Any new gas boiler has to be fitted by a Gas Safe registered engineer and notified to building control, which a registered installer handles as part of the job.
Since 2018, new installations in England also have to meet a minimum efficiency standard and include time and temperature controls. Combis in particular have to include one additional efficiency measure. In practice that usually means weather compensation, load compensation, flue gas heat recovery, or smart controls that learn how the house heats. These are not optional extras. A quote that makes no mention of controls is worth questioning.
Unvented cylinders carry their own requirements under the building regulations covering hot water safety, which is why that qualification matters.
Getting the Right One Specified
The most common way this goes wrong is a boiler being sized and chosen before anyone has measured anything.
A good survey looks at the mains flow rate, the number and position of the outlets, and the radiators the system has to serve. It also looks at how the household actually uses hot water. Only then does it recommend a type.
If you are weighing it up, we are happy to come and measure rather than guess. You can read more on our new boiler installation and unvented cylinder pages. Or get in touch for a straight answer about which suits your house. If your current boiler is what prompted the question, it may also be worth reading when you should replace a boiler.