Guide
Combi, system or regular: which boiler suits your house
The answer is decided by your mains water flow rate and how many bathrooms run at once, not by the size of the house or the price of the boiler.
The three types, briefly
A combi heats water on demand straight from the mains, so there is no cylinder and no loft tank. A system boiler heats a pressurised cylinder, which stores hot water at mains pressure. A regular or conventional boiler heats a vented cylinder fed by gravity from tanks in the loft, which is the arrangement in most houses built before the 1990s.
Combis are the default for flats and smaller houses because they take up no space and cost least to install. That does not make them right for every property.
The number that decides it
Mains flow rate, measured in litres per minute at the kitchen tap with everything else shut off. A combi delivers hot water at whatever rate the incoming main can supply, so a house with 10 litres per minute will never give two decent showers from a combi, whatever boiler is fitted.
Below about 12 litres per minute, a combi will disappoint. Between 12 and 18 it suits a single bathroom household. Above 18, and with a 22mm incoming main, a combi can serve two bathrooms if they are not used simultaneously. The test takes five minutes with a weir gauge and any installer should do it before recommending anything.
When a cylinder is the right answer
Two or more bathrooms in simultaneous use, a household that showers at the same time each morning, a large bath, or a property where mains flow is poor. In all of those, a system boiler with an unvented cylinder gives mains pressure hot water to every outlet at once, because the cylinder is a buffer the mains fills up slowly overnight.
A cylinder also future-proofs the house. Every heat pump installation needs one, so a property that already has a well-sized unvented cylinder is a much shorter step away from a low temperature system than one with a combi.
What it costs to change type
Staying with the same type is always cheapest. A combi swap is £1,900 to £3,600. Converting a regular system to a combi is £3,200 to £5,500, because tanks and cylinder come out and a new mains cold supply goes in. Going the other way, from a combi to a system boiler and unvented cylinder, is £3,800 to £6,500 and needs somewhere to put the cylinder.
A conversion is worth doing when the current arrangement genuinely does not suit how the house is used. It is not worth doing to save space in an airing cupboard.
Common questions
How do I measure my mains flow rate?
Run the cold kitchen tap full into a measuring jug for exactly ten seconds with everything else off, then multiply by six. Under 12 litres per minute and a combi will struggle.
Can I have two showers running off a combi?
Only with a high flow rate, a 22mm incoming main and a high output boiler, and even then the flow at each shower drops. Two bathrooms in simultaneous use is a cylinder job.
Do I need a cylinder for a heat pump?
Yes. Heat pumps heat water at a low temperature, so hot water has to be stored, and the cylinder needs a larger coil than a boiler cylinder.