Conventional central heating systems in German buildings are hot-water heating systems. These systems consist of a boiler and radiators or heating coils in the floor. A heating characteristic maps the current outdoor temperature to the set-point of the boiler outlet-temperature. Conventional PI-controllers with fixed parameters adjust the boiler outlet-temperature to the set-point. Since the process of heating water in the boiler is nonlinearly dependent on the water-mass-flow, knowledge of the flow-rate could improve the quality of control. This paper presents two different observer-based schemes for the estimation of the water flow rate through a central heating boiler. One is a disturbance observer and the other is a parameter observer.