Purpose - Because of their widespread use in industry, induction through-heaters of various metal products must be of high effectiveness not only in "quasi" steady-state operation but in different transient modes as well. Nowadays, they are usually designed to provide the required characteristics in "quasi" steady-state operation mode mainly. The purpose of this paper is to examine numerical simulation of transient processes in induction through-heating lines generally and investigate dynamic temperature fields during the first start of the heaters particularly. Design/methodology/ approach - The research methodology is based on coupled numerical electromagnetic and thermal analyses using FEM approach. ANSYS simulations are supported with the developed tools for imitation of mass transfer effects in continuous induction heating lines. Findings - The results show that transient temperature fields in the heated strip or slab significantly differ from their "quasi" steady-state descriptions. Local temperature variations acquired in longitudinal as well as transverse flux induction heaters during the first start have been predicted. Practical implications - The received results can be used for design of induction through-heaters and improvement of their characteristics in dynamic operation modes. Originality/value - Investigation of dynamic characteristics of the heaters in dynamic modes can be only done by numerical modelling based on special algorithms providing a time loop additional to coupling between electromagnetic and thermal analyses. Such algorithms have been developed and used for investigation of two types of induction installations: through-heaters of cylindrical billets for forging and heating lines of strip or thin slab for rolling mills. © 2011 Emerald Group Publishing Limited. All rights reserved.