  Observations of high-excitation molecular emission lines can greatly increase
our understanding of AGB winds, as they trace the innermost regions of the
circumstellar envelope. The PACS spectrometer on-board the Herschel Space
Telescope, provides for the first time the spectral resolution and sensitivity
necessary to trace these lines. We report on the first modelling efforts of a
PACS spectral scan for the OH/IR star V669 Cas. Central to our methodology is
the consistent treatment of both dust and gas by using a line radiative
transfer and a continuum radiative transfer code conjointly. Water emission
lines are found to be extremely sensitive to the dust-to-gas ratio, emphasizing
the need of consistent modelling for dust and gas.
