A function block for the fluid connection of a consumer, an untreated water pipe, an outflow and a water treatment system, comprising: a first connector, a second connector, and a third connector, to which the water treatment system can be connected; an untreated water connector, which can be connected to an untreated water pipe, wherein the first connector and the untreated water connector are fluidically connected; a wastewater connector and a consumer connector; characterised in that the function block also comprises the following: a pressure control unit, which is fluidically connected to the second connector and to the wastewater connector, wherein the pressure control unit is designed to maintain a predetermined pressure in the fluid connection to the second connector during the operation of the function block; a monitoring means, which is fluidically connected to the third connector and is fluidically connected to the consumer connector, wherein the monitoring means is designed to determine at least one property of a fluid flow between the third connector and the consumer connector; and a frame, to which at least one element from the following is fastened: the first connector, the second connector, the third connector, the untreated water connector, the wastewater connector, the consumer connector, the pressure control unit and the monitoring means. a first connector, a second connector, and a third connector, to which the water treatment system can be connected; an untreated water connector, which can be connected to an untreated water pipe, wherein the first connector and the untreated water connector are fluidically connected; a wastewater connector and a consumer connector; characterised in that the function block also comprises the following: a pressure control unit, which is fluidically connected to the second connector and to the wastewater connector, wherein the pressure control unit is designed to maintain a predetermined pressure in the fluid connection to the second connector during the operation of the function block; a monitoring means, which is fluidically connected to the third connector and is fluidically connected to the consumer connector, wherein the monitoring means is designed to determine at least one property of a fluid flow between the third connector and the consumer connector; and a frame, to which at least one element from the following is fastened: the first connector, the second connector, the third connector, the untreated water connector, the wastewater connector, the consumer connector, the pressure control unit and the monitoring means. a pressure control unit, which is fluidically connected to the second connector and to the wastewater connector, wherein the pressure control unit is designed to maintain a predetermined pressure in the fluid connection to the second connector during the operation of the function block; a monitoring means, which is fluidically connected to the third connector and is fluidically connected to the consumer connector, wherein the monitoring means is designed to determine at least one property of a fluid flow between the third connector and the consumer connector; and a frame, to which at least one element from the following is fastened: the first connector, the second connector, the third connector, the untreated water connector, the wastewater connector, the consumer connector, the pressure control unit and the monitoring means.