A combi heats water instantly and needs no cylinder, which makes it the right choice for most flats and smaller homes. A system boiler works with a hot water cylinder and can supply several taps at once, which makes it the better fit for a house with more than one bathroom. The deciding question is usually how many showers might run at the same time.
A combi does heating and hot water from one unit, with no cylinder and no tank in the loft. Open a tap and it fires up and heats water on demand.
The advantages are real. No cylinder means no airing cupboard given over to it, no tank in the loft, and no waiting for a cylinder to reheat. It also means no stored water sitting there losing heat.
The limitation is flow rate. A combi can only heat so much water per minute. Run two showers at once and both will suffer.
A system boiler heats a cylinder full of water and stores it. When you open a tap, it comes from the cylinder.
That means several outlets can run at once without the flow collapsing, because you are drawing from a store rather than heating on the fly. Two bathrooms both running is not a problem.
The trade-offs: it takes up space, and if you empty the cylinder you wait for it to reheat.
A cylinder plus a cold water tank in the loft. This is the traditional setup and you will find it in a lot of older Devon housing.
If your house is already plumbed this way, replacing like for like is often the cheapest and least disruptive option. Converting to a combi means ripping out the cylinder and the tank, and the pipework changes, which costs more.
One bathroom, one or two people: a combi, almost always.
Two or more bathrooms, or a family who all shower between seven and eight in the morning: a system boiler with a cylinder.
An older house already plumbed with a cylinder and a loft tank: think carefully before converting. The conversion cost often outweighs the benefit unless you were changing the pipework anyway.
And one more: if you are considering a heat pump at any point in the next decade, you will need a cylinder. Going combi now means putting one back in later.
Neither is better. A combi suits a one-bathroom home and saves space. A system boiler with a cylinder suits a house with two or more bathrooms where several outlets may run at once. The number of bathrooms is usually the deciding factor.
Not well. A combi heats water on demand at a limited flow rate, so running two showers at once means both will be weaker. If that is your household, you want a cylinder.
Only if there is a good reason beyond the boiler swap. Ripping out the cylinder and loft tank means significant pipework changes and cost. If the house is already plumbed for a cylinder, replacing like for like is usually cheaper and less disruptive.
Yes. Heat pumps heat water gently over a longer period, so they need somewhere to store it. If you are thinking about a heat pump in future, fitting a combi now means putting a cylinder back in later.
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