On the basis of the system of one-dimensional stationary differential equations of conservation laws а calculation of the injector working at water vapor is executed. The main parts of the injector are considered: an accelerating Laval nozzle, a mixing camera and a diffuser with a cylindrical entrance. Calculation allows determining the characteristic sizes of a design of the injector and change of the key physical parameters of water and steam by its length. The calculation results are compared to author’s own experimental data. The scheme with an injector for heating of network water is suggested and compared to existing ones. The injector usage instead of ordinary water heater with a pump leads to significant saving of fuel and electric energy.