A theory of magnetohydrodynamic piston‐driven disturbances in a thermally‐conducting medium is presented, based on a phenomenological model which admits self‐similar solutions. The motion consists of a thermal precursor followed by an isothermal shock. The magnetic field plays a fundamental role; even a very modest transverse magnetic field keeps the peak densities down, blocks the heat flow, and widens the perturbed domain. This, together with a decrease (increase) of post shock density (temperature) compares favorably with some measurements in the solar wind.