The paper investigates the transformation of thermal energy to mechanical work on pumping of liquid at steam formation in narrow vertical channels. Feature of boiling up of liquid in narrow channels is formation of the steam shells which are completely blocking the cross section of the channel and pushing out liquid from it. There is a fluctuation mode of vaporizing and condensation processes at a constant supply of thermal energy. The model of rod emission is used for the description of that processes. This paper carries out the pilot studies of pressure variation in the channel during the operation of the mechanical heat pump and experimentally defines the dependences of flow rate of liquid on its temperature, the input power and pressure. The physical model of pump operation is developed, its characteristics are constructed. The thermomechanical pump can find application for increase of technological process efficiency in those areas of equipment where pumping of liquid with rather small expense is necessary and there is a large number of waste thermal energy.