A method of operating a thermal energy exchanger for exchanging thermal energy between a first fluid flowing through the thermal energy exchanger in a first duct and a second fluid flowing through the thermal energy exchanger in a second duct , the method comprising the steps of: recording in a control system a plurality of measurement data sets, each of the measurement data sets including for a different point in time values of flow (φ 4/5 ) of the first and second fluids and a value of a dehumidification rate (DHu) of the second fluid; the control system computing for each of the measurement data sets normalized values of flow (φ norm,4/5 ) as functions of the values of flow (φ 4/5 ) of the measurement data set and associating the normalized values of flow with the measurement data set; the control system computing for each of the measurement data sets at least one normalization variable (DHu max,ideal ) as a function of the dehumidification rate (DHu) of the measurement data set and associating the at least one normalization variable (DHu max,ideal ) with the measurement data set; the control system computing for each of the measurement data sets a value of heat exchanger effectiveness (HXeff) by relating the value of the dehumidification rate (DHu) of the measurement data set to the at least one normalization variable (DHu max,ideal ) associated with the measurement data set and associating the heat exchanger effectiveness (HXeff) with the measurement data set; the control system computing for each of the measurement data sets normalized data points (φ norm,4/5' HXeff) defined by the values of normalized flow (φ norm,4/5 ) associated with the measurement data set and by the value of heat exchanger effectiveness (HXeff) associated with the measurement data set; and the control system determining for the thermal energy exchanger a characteristic transfer function which fits the normalized data points (φ norm4/5 ,HXeff). recording in a control system a plurality of measurement data sets, each of the measurement data sets including for a different point in time values of flow (φ 4/5 ) of the first and second fluids and a value of a dehumidification rate (DHu) of the second fluid; the control system computing for each of the measurement data sets normalized values of flow (φ norm,4/5 ) as functions of the values of flow (φ 4/5 ) of the measurement data set and associating the normalized values of flow with the measurement data set; the control system computing for each of the measurement data sets at least one normalization variable (DHu max,ideal ) as a function of the dehumidification rate (DHu) of the measurement data set and associating the at least one normalization variable (DHu max,ideal ) with the measurement data set; the control system computing for each of the measurement data sets a value of heat exchanger effectiveness (HXeff) by relating the value of the dehumidification rate (DHu) of the measurement data set to the at least one normalization variable (DHu max,ideal ) associated with the measurement data set and associating the heat exchanger effectiveness (HXeff) with the measurement data set; the control system computing for each of the measurement data sets normalized data points (φ norm,4/5' HXeff) defined by the values of normalized flow (φ norm,4/5 ) associated with the measurement data set and by the value of heat exchanger effectiveness (HXeff) associated with the measurement data set; and the control system determining for the thermal energy exchanger a characteristic transfer function which fits the normalized data points (φ norm4/5 ,HXeff). <EOS>