  We present a study of electrical and thermal transport in Weyl semimetal
WTe$_2$ down to 0.3 K. The Wiedemann-Franz law holds below 2 K and a downward
deviation starts above. The deviation is more pronounced in cleaner samples, as
expected in the hydrodynamic picture of electronic transport, where a fraction
of electron-electron collisions conserve momentum. Phonons are the dominant
heat carriers and their mean-free-path do not display a Knudsen minimum. This
is presumably a consequence of weak anharmonicity, as indicated by the
temperature dependence of the specific heat. Frequent momentum exchange between
phonons and electrons leads to quantum oscillations of the phononic thermal
conductivity. Bloch-Gr\"uneisen picture of electron-phonon scattering breaks
down at low temperature when Umklapp ph-ph collisions cease to be a sink for
electronic flow of momentum. Comparison with semi-metallic Sb shows that normal
ph-ph collisions are amplified by anharmonicity. In both semimetals, at
cryogenic temperature, e-ph collisions degrade the phononic flow of energy but
not the electronic flow of momentum.
