Cold fresh water from the tap is best to brew coffee. Water should be fresh; if it has been sitting too long (or has been heated and then cooled), it will be missing the dissolved air that is an important component of the water's taste. Distilled water and mineral water are not suitable as too much mineral content will hasten damaging lime scale buildup inside the coffee maker, while an absence of minerals will give your coffee and espresso a flat taste while also harming the boiler inside the machine.