The multiple boiler system uses independent boilers for the water used to extract the coffee and for the steam. In this case, the boiler temperatures can be adjusted separately; the group is heated by "convention" and therefore, through direct contact with the boiler. The boiler (or boilers) heating the water for coffee extraction is at a constant pressure of 9 bar and therefore, it is usually made in stainless steel. Differently to the system using a single boiler for hot water and steam, the whole tank capacity is occupied by hot water. This means that temperature control is not possible by checking pressure but only by using a mechanical or digital thermometer (probe).