A combi boiler (combination boiler) heats both your water and your home without a separate tank. 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. Combi boilers are very popular in flats and houses with less hot water requirements since they deliver hot water instantly to taps and showers. 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.