The effects of various transition-metal dopants on the electrical and thermal transport properties of Fe1-xMxSi alloys (M = Co, Ir, Os) are reported. The maximum thermoelectric figure of merit ZT max is improved from 0.007 at 60 K for pure FeSi to ZT = 0.08 at 100 K for 4% Ir doping. A comparison of the thermal conductivity data among Os-, Ir-, and Co-doped alloys indicates strong electron-phonon coupling in this compound. Because of this interaction, the common approximation of dividing the total thermal conductivity into independent electronic and lattice components (κtotal = κelectronic + κ lattice) fails for these alloys. The effects of grain size on thermoelectric properties of Fe0.96Ir0.04Si alloys are also reported. The thermal conductivity can be lowered by ∼50% with little or no effect on the electrical resistivity or Seebeck coefficient. This results in ZTmax = 0.125 at 100 K, still approximately a factor of 5 too low for solid-state refrigeration applications. © 2011 American Physical Society.