  A series of full-Heusler alloys, $\rm Fe_2V_{1-x}W_xAl$, $0 \leq x \leq 0.2$,
was prepared, characterized and relevant physical properties to account for the
thermoelectric performance were studied in a wide temperature range.
Additionally, off-stoichiometric samples with similar compositions have been
included, and a 10~\% improvement of the thermoelectric figure of merit was
obtained. The V/W substitution causes i) a change of the main carrier type,
from holes to electrons as evidenced from Seebeck and Hall measurements and ii)
a substantial reduction of the lattice thermal conductivity due to a creation
of lattice disorder by means of a distinct different mass and metallic radius
upon the V/W substitution. Moreover $ZT$ values above 0.2 have been obtained. A
microscopic understanding of the experimental data observed is revealed from
ab-initio calculations of the electronic and phononic structure.
