Combi boiler A Combi boiler is the most common form of boiler system in the UK, and provides heat for your radiators and domestic hot water on demand. There are several different types of combi boiler to choose from, but they typically work by taking cold water straight from the mains supply and heating it as you need it – eliminating the need for a separate hot water cylinder or storage tank. They are much better economically as you don’t have to heat a tank of hot water all the time which is a lot of energy wasted. Combi boiler – pros and cons A gas combi boiler system contains almost all components within the boiler itself, which makes the boiler system more compact and neater looking – but as there are moving parts within the boiler unit, there is more that can potentially go wrong than with a conventional boiler. On the plus side, a combi gas boiler means there’s no need for a cold water or feed and expansion tank in the loft, for example, and you can immediately get hot water heated on demand. That does mean you’ll only be able to use hot water from a combi boiler system for one task at a time. For example, one person wouldn’t be able to have a shower while someone else does the washing up using hot running water. A combi gas boiler system is better suited to a household with a smaller number of people that tend not to need to use hot water simultaneously. If you’re interested in getting a combi boiler, see our boilers review for the latest models, including test scores and Best Buys. Our gas boiler brands satisfaction results also show how Which? members rate different combi boiler manufacturers.