A regenerative heat exchanger is an important component of a thermal system in power units. It is crucial to know the performance of the regenerative heat exchanger in off-design conditions during its design and operation. Advanced regenerative heat exchanger simulators have been developed for many years to describe the performance in off-design conditions. The simulators involve the use of equations for mass, momentum, and energy balances and criteria relations for heat transfer coefficients; the geometrical data of the heat exchanger are also required. Due to high complexity, the calculations are performed iteratively. For this paper, a different approach was taken: The heat exchanger was considered as a "black box." Based on the data obtained from the simulator, the effect of input variables on the output ones was investigated, so as to propose a relation describing the regenerative heat exchanger performance. To assess this performance, heat transfer effectiveness was proposed, and its two variants were considered. Since two heat transfer effectiveness definitions were assumed, two approximate relations concerning the regenerative heat exchanger were determined. The relations were verified against data obtained from a simulator of a high-pressure regenerative heat exchanger in a medium-power steam condensing unit. A satisfactory accuracy of the proposed relations was obtained.