The author considered the problem statement on the pipes freezing in emergency regimes of building engineering systems and external pipe nets using liquid water as working fluid under boundary conditions of the 3rd type. This problem is a high-priority task now because of actualization of building standards in Russian Federation and because of the increasing requirements to safety and security of heat supply. That’s why it is very important to find a simple but accurate enough dependence for the freezing time in pipe nets. The system of differential and algebraic equations of external heat exchange and internal heat transfer with account for heat ingress from hydraulic friction at water flow and Stephan’s condition on the freezing front is presented. The analytical solution of the given system is obtained as a quadrature for the dependence of the current coordinate of the freezing front. The results of numerical calculation of the corresponding integral are shown and their comparison with the former author’s researches concerning the solution of the considered problem at the boundary conditions of the 1st type is conducted. It is shown that the account of intensity of external heat exchange causes retarding of freezing because of adding thermal resistance on the external surface of the pipe. The former author’s conclusion on the existence of the ultimate water velocity, when freezing doesn’t take place, is verified. The area of use of the presented dependence is found. The obtained model contains is easy to use in engineering practice, especially during preliminary calculations. The presentation is illustrated with numerical and graphical examples.