By integrating the second hot water circuit, the system could be adapted to the specific exhaust gas components of the exhaust air from a tunnel kiln, whereby critical plant conditions, such as exhaust gas condensation, and maintenance/cleaning costs could be reduced or the plant service life increased. The internal hot water circuit (primary) was raised in its temperature level by approx. 35°C, the supplementary hot water circuit (secondary) is coupled with the primary circuit for heat transfer via a plate heat exchanger system. The secondary circuit is operated at the original temperature level (110/90°C) and can thus be reconnected to the existing hot water plant network without further adaptation and feed the recovered heat energy into the network.