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 (12, 19, 22) 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 (12, 19, 22) 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 (12, 19, 22) 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 (12, 19, 22) 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 (12, 19, 22) 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 (12, 19, 22) 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 (12, 19, 22) 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 (12, 19, 22) 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 (12, 19, 22) 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 (12, 19, 22) 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 (12, 19, 22) 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 (12, 19, 22) determining for the thermal energy exchanger a characteristic transfer function which fits the normalized data points (φ norm4/5,HXeff).