A combi (or combination boiler) combines hot water production and space heating in one packaged appliance. It is commonly installed in properties where there is limited space for a separate hot water cylinder. Combi boilers have become highly prevalent in UK homes because hot water is generated instantaneously — whereas with a separate cylinder, pre-heated water may run out if the boiler cannot reheat the cylinder quickly enough during high demand.
A combi boiler is a central heating boiler and water heater combined into one unit. There is usually no separate hot water cylinder, no additional water storage tank, and no feed-and-expansion tank in the loft. Large combi and storage-combi models have emerged that can meet the requirements of larger homes, though technical sizing and specification are still required to confirm suitability.
Pros and cons of combi boilers
Advantages
- As long as the boiler is working, hot water is produced continuously for as long as it is needed.
- Space saving — no cylinder or cold-water storage tank required in most cases.
- Mains pressured, so booster pumps or cold-water storage tanks are not normally needed unless there is a problem with the mains supply.
- Lower installation cost and shorter installation time compared with system or regular setups.
Disadvantages
- Because hot water is generated instantaneously, if the boiler breaks down there is no backup such as an immersion heater in a cylinder.
- More components that can fail or become clogged — for example diverter valves, plate heat exchangers, and filters (though some advanced combi boilers use combined heat exchangers).
- Limited flow rates, so usually only one bath or shower at a time (storage combis with built-in tanks are an exception).
- Cannot be used alone with some renewable heat sources such as solar thermal or heat pumps without additional equipment.
Is a combi boiler right for you?
Combi boilers suit flats and smaller homes where space is tight and hot-water demand is moderate. Larger properties with multiple bathrooms may need a higher-output combi, a storage combi, or a system boiler with a cylinder. See our choosing guide and sizing guide for next steps.
Content adapted from guides originally published on Hub.MyBoiler.com and hub.myboiler.com/combi-boiler.