The invention concerns a process for detecting scale deposit in the liquid supply means of a water pump driven machine comprising at least a water tank, a pump and a heating means, where water is pumped from the water tank and fed to the heating means, and where the pump is energized by a controller by providing the pump with an energizing signal to provide an intended water flow rate F, wherein the actual water flow rate f is measured and the discrepancy Δ between the actual water flow rate f and the intended water flow rate F is directly and/or indirectly compared to an operating instruction related to scale deposit.