  We describe a new tool for the analysis of the UV to the sub-millimeter
(sub-mm) spectral energy distribution (SED) of spiral galaxies. We use a
consistent treatment of grain heating and emission, solve the radiation
transfer problem for a finite disk and bulge, and self-consistently calculate
the stochastic heating of grains placed in the resulting radiation field.
  We use this tool to analyse the well-studied nearby edge-on spiral galaxy
NGC891. First we investigate whether the old stellar population in NGC891,
along with a reasonable assumption about the young stellar population, can
account for the heating of the dust and the observed far-infrared (FIR) and
sub-mm emission. The dust distribution is taken from the model of Xilouris et
al. (1999), who used only optical and near-IR observations to determine it. We
have found that such a simple model cannot reproduce the SED of NGC891,
especially in the sub-mm range. It underestimates by a factor of 2-4 the
observed sub-mm flux.
  A number of possible explanations exist for the missing sub-mm flux. We
investigate a few of them and demonstrate that one can reproduce the observed
SED in the FIR and the sub-mm quite well, as well as the observed radial
profile at 850 microns. For the models calculated we give the relative
proportion of the dust radiation powered by the old and young stellar
populations as a function of FIR/sub-mm wavelength. In all models we find that
the dust is predominantly heated by the young stellar population.
