  Energy transfer from electrons to phonons is an important consideration in
any Weyl or Dirac semimetal based application. In this work, we analytically
calculate the cooling power of acoustic phonons, i.e. the energy relaxation
rate of electrons which are interacting with acoustic phonons, for Weyl and
Dirac semimetals in a variety of different situations. For cold Weyl or Dirac
semimetals with the Fermi energy at the nodal points, we find the electronic
temperature, $T_e$, decays in time as a power law. In the heavily doped regime,
$T_e$ decays linearly in time far away from equilibrium. In a heavily doped
system with short-range disorder we predict the cooling power of acoustic
phonons is drastically increased because of an enhanced energy transfer between
electrons and phonons. When an external magnetic field is applied to an undoped
system, the cooling power is linear in magnetic field strength and $T_e$ has
square root decay in time, independent of magnetic field strength over a range
of values.
