The regularities of formation of temperature fields in a plane three-layer system with a not-through cross connection (connector) are considered with boundary conditions of the second kind on one exterior surface of the system. The character of distribution of the temperature fields in the zone of influence of the steel connector is investigated experimentally. The results of the experiment are compared to the numerical solution of the problem with boundary conditions of the second and third kind on the interior surface of the wall. Their satisfactory agreement is shown.