A uniaxial flat front--"theory is presented for a gas loaded variable conductance heat pipe. The theory takes into account the inertial effects of the moving vapour, which can grow large under certain circumstances, but neglects the axial heat conduction of the wall and wick. A limited number of numerical examples indicates excellent agreement with the corresponding diffuse front theory. The differences with a similar flat front theory, which neglects the inertial effects of the moving vapour, can become substantial. In comparison with diffuse front based calculations, the computation times are very-short.