  We investigate the origin and nature of the profiles of water and ammonia
observ ed toward the L1157-B1 clump as part of the HIFI CHESS survey
(Ceccarelli et al. 2010) using a new code coupling a gas-grain chemical model
with a parametric shock model. Fir st results from the unbiased survey (Lefloch
et al. 2010, Codella et al. 2010) reveal different molecular components at
different excitation conditions coexisting in the B1 bow shock structure, with
NH$_3$, H$_2$CO and CH$_3$OH emitting only at relatively low outflow velocities
whereas H$_2$O shows bright emission at high velocities. Our model suggests
that these differences are purely chemical and can be explained by the presence
of a C-type shock whose maximum temperature must be close to 4000 K along the
B1 clump.
