  The alloys with 8/18/24 valence electron count (VEC) are promising candidates
for efficient energy conversion and refrigeration applications at low as well
as high temperatures. The full potential linearized augmented plane wave method
as implemented in WIEN2k code was used to investigate electronic structure and
TE transport properties with the PBE$-$GGA and TB$-$mBJ exchange potentials and
Boltzmann transport theory. The calculated single crystal elastic constants,
phonon dispersion and phonon density of states confirm that these systems are
mechanically and dynamically stable. The TE transport properties is calculated
by including the lattice part of thermal conductivity ($\kappa_{L}$) obtained
from two methods one from the calculated elastic properties calculation
($\kappa^{elastic}_{L}$) and the other from phonon dispersion curve
($\kappa^{phonon}_{L}$). The strong phonon$-$phonon scattering by large mass
difference/strain fluctuation of isovalent/aliovalent substitution at Ti/Sn
sites of Fe$_{2}$TiSn reduces the lattice thermal conductivity which results in
high \textit{ZT} value of 0.81 at 900\,K for
Fe$_{2}$Sc$_{0.25}$Ti$_{0.5}$Ta$_{0.25}$Al$_{0.5}$Bi$_{0.5}$. The comparative
analysis of TE transport properties using the band structures calculated with
the PBE$-$GGA and TB$-$mBJ functional shows that the \textit{ZT} value obtained
from TB$-$mBJ scheme is found to be significantly higher than that based on
PBE$-$GGA. The calculated relatively low lattice thermal conductivity and high
\textit{ZT} values suggest that isovalent/aliovalent substituted Fe$_{2}$TiSn
are promising candidates for medium to high temperature waste heat recovery.
