Results of the fan comparison. (a) Acoustic behavior of two different fan systems installed in the demonstrator heat pump, diameter correspond to 500 mm. (b) Pressure characteristic curves as sum of two fans, based on measurements installed in the heat pump, curves below the heat pump characteristic curve extrapolated by quadratic function, the design airflow of the heat pump q_v,design is 5000 m³/h at a pressure difference of 20 Pa.