Purpose: The aim of the paper is optimization of a control strategy used in a seal fatigue tester. Design/methodology/approach: Analytical approach has been applied to get an understanding of a heat exchange process. It is required to optimize the control process algorithm. Findings: The initial control algorithm has been improved regarding numerous tests under different operating conditions. The final version of the control program allows to maintain the temperature according to the on-off strategy minimizing the temperature oscillations and potential overshoots. Research limitations/implications: It should be considered a new control strategy based on PID controller. Practical implications: The major component of the tester is the heat exchanger. It consists of a tube and embedded pipe circuit feeding with the external heating or cooling medium. The tube is a cylindrical housing where the seals are assembled at both sides. Two main pumps circulate the medium at a specified flow rate and pressure. The temperature is maintained to hold it almost constant during heating and cooling phases. Two thermocouples are placed close to the seals, and the one in the middle of the heat exchanger tube. The tester is equipped with a control system consisting of a PLC controller and LabView data acquisition application. It is possible to control the device remotely through LAN/WAN networks. The LabView application communicates with the controller via software with the use of PPI (Point-To-Point Interface) protocol. A new control algorithm has allowed to perform tests according to testing specification without significant overshoots. Originality/value: On-off control algorithm has been proposed for a combined heating-cooling installation rarely using in the industrial solutions.