To save fuel as much as possible is one of the main problem for all Central Heating-and-Power Plants. In this goal, one way is to keep the temperature of combustion air at maximum level, in the case of dirty gaseous fuels, such as the coal, the most used air- preheater system is rotary Ljungstrom type preheater, like at the Central Heating-and-Power Plant of Bacau. Due to the many factors, the temperature of flue gas changes and, consequently, the number of rotation of the heat wheel must change. For this reason, is necessary to determine the maintaining time necessary for the hot side of the heat wheel to absorb maximum energy from the flue gas. This paper tries to establish a relation to determine the maintaining time in flue gas of metallic matrix of the heat wheel from rotary Ljungstrom type preheater in order to absorb maximum energy from the flue gas.