Abstract: An expansion tank for accommodating hot expanded fluid in a heat transfer fluid system has a tank body defining an internal volume. A fluid inlet is provided on the tank body for connection to the heat transfer fluid system. A membrane, such as a diaphragm or a bladder, divides the internal volume into first and second sections. The first section includes a buffer zone between the fluid inlet and the membrane. The buffer zone contains a predetermined volume of tempering fluid for cooling down the incoming hot expanded fluid at its entry in the expansion tank. The buffer zone has an air valve for venting air from the buffer zone while the buffer zone is being filled up with the tempering fluid. The second section on the opposed side of the membrane is maintained under pressure with a gas.