  Motivated by the recent discovery of a large anomalous Nernst effect in
Co$_2$MnGa, Fe$_3X$ ($X$=Al, Ga) and Co$_3$Sn$_2$S$_2$, we performed a
first-principles study to clarify the origin of the enhancement of the
transverse thermoelectric conductivity ($\alpha_{ij}$) in these ferromagnets.
The intrinsic contribution to $\alpha_{ij}$ can be understood in terms of the
Berry curvature ($\Omega$) around the Fermi level, and $\Omega$ is singularly
large along nodal lines (which are gapless in the absence of the spin-orbit
coupling) in the Brillouin zone. We find that not only the Weyl points but also
stationary points in the energy dispersion of the nodal lines play a crucial
role. The stationary points make sharp peaks in the density of states projected
onto the nodal line, clearly identifying the characteristic Fermi energies at
which $\alpha_{ij}$ is most dramatically enhanced. We also find that
$\alpha_{ij}/T$ breaks the Mott relation and show a peculiar temperature
dependence at these energies. The present results suggest that the stationary
points will give us a useful guiding principle to design magnets showing a
large anomalous Nernst effect.
