What each one actually is
A combi heats water the moment you open a tap, straight from the mains, with no tank and no cylinder. It is compact, efficient and the default choice for most modern homes. A system boiler heats a cylinder of stored hot water instead, so the hot water you use was heated earlier and is waiting for you.
Older houses sometimes have a third arrangement, a heat-only boiler with tanks in the loft feeding a cylinder. Those systems can be converted to either a combi or a system boiler when the old boiler reaches the end of its life, and the right conversion depends on the same questions below.
The combi case, and its one real weakness
For the typical South Wales terrace or semi with one bathroom, a combi is hard to argue with. You free up the airing cupboard and the loft tanks, you stop paying to keep a cylinder of water hot, and you never run out of hot water because there is nothing to run out of.
The weakness is simultaneous use. A combi makes hot water at the rate the mains feeds it, so two showers running at once means both get less. If your mains flow is weak, even one good shower can feel underpowered. That is why a proper quote asks about your water pressure and flow, and it is one of the things checked before LMB confirms a combi is right for your house.
When a system boiler earns its place
Bigger households change the maths. If you have two or more bathrooms in regular use, a family that showers in a morning rush, or a bathroom habit built around deep baths, stored hot water means everyone gets full flow at the same time. The trade-offs are honest ones: the cylinder needs a cupboard, heated water you do not use is money spent, and the system costs more to install than a like-for-like combi swap.
One more honesty note: a modern unvented cylinder also leans on decent mains pressure to deliver its punchy showers. If your incoming main is genuinely poor, that needs looking at whichever boiler type you choose, and Lloyd will tell you straight rather than fit something that disappoints.
Whichever way your house points, the online quote tool asks the questions that matter and gives you a fixed fitted price, with installations from £1,900.
