A combi boiler (combination boiler) provides both central heating and domestic hot water on demand, directly from the mains water supply. Unlike conventional boiler systems, a combi boiler doesn't require a separate hot water cylinder or a cold water storage tank in the loft, which makes them compact and ideal for properties where space is limited. Get instant hot water with a combi boiler. If you use a lot of hot water at once, the temperature and pressure may decrease; however, the hot water is readily and immediately replaced by more heated water.