The far IR spectral region is fundamental for the understanding of the earth's radiation balance, in particular because of a strong cooling to space due to the water vapor rotation band and continuum. Cirrus clouds play an important but poorly characterized role in this spectral region. Using a line-by-line code to model at high resolution the atmospheric transmittance, in association with a multiple scattering radiative transfer scheme, we simulate the impact of cirrus clouds on the atmospheric heating rates in the spectral region from 100 to 1000 cm-1. Compared with clear sky conditions, we find that cirrus clouds induce a heating from the ground to the cloud base at all wave numbers. Within the cloud, a heating effect occurs in the window region, whereas a strong cooling occurs in the far-IR. The particles' shape are modeled with either a size distribution of spheres or randomly oriented spheroids. We show that, for a range of different ice water path and particles, size, for a same average volume of ice per particle, spheroids have a bigger impact than spheres at all wave numbers on the cooling/heating rates as well as for the top of the atmosphere radiances.