Help for process design issues of water circulation system, expansion tank, and water supply pipe

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Our company has a mature hot water circulation process. The hot water tank, water pump, and plate heat exchanger are placed at an altitude of 5 meters on the second floor. The high-level hot water tank is arranged at an altitude of 17 meters on the third floor. The 42-degree hot water from the hot water tank on the second floor is pumped to the plate heat exchanger. After heating to 40 degrees, it is then sent to the water equipment at an altitude of 13 meters on the third floor. The temperature becomes 42 degrees, and then returned to the hot water tank on the second floor. A pipe is connected from the bottom of the hot water high tank at an altitude of 17 meters to a 42-degree return pipe at an altitude of 15 meters. The hot water pump has a flow rate of 80m3/h and a head of 50m. This process has been running for many years without any problems. Today I suddenly thought of a question, that is, the pressure of the 42-degree return pipe with an altitude of 15 meters should be about 4 kilograms. Will the water in this return pipe flow through the DN25 connecting pipe into the 17-meter high-level hot water tank and flow away through the overflow pipe? Can any expert give an answer?