  We use recent X-ray observations of the intracluster medium (ICM) of the
galaxy group NGC 5813 to confront theoretical studies of ICM thermal evolution
with the newly derived ICM properties. We argue that the ICM of the cooling
flow galaxy group NGC 5813 is more likely to be heated by mixing of post-shock
jets' gas residing in hot bubbles with the ICM, than by shocks or
turbulent-heating. Shocks thermalize only a small fraction of their energy in
the inner regions of the cooling flow; in order to adequately heat the inner
part of the ICM, they would overheat the outer regions by a large factor,
leading to its ejection from the group. Heating by mixing, that was found to be
much more efficient than turbulent-heating and shocks-heating, hence, rescues
the outer ICM of NGC 5813 from its predestined fate according to cooling flow
feedback scenarios that are based on heating by shocks.
