A system (20) for a PCR reaction includes an array of reaction vessels mounted on a thermal mount (21). The thermal mount (21) is provided with a liquid path therein coupled to a cooling liquid input port (22), a heating liquid input port (23) and a liquid output port (24). A pump (38) is used to pump liquid from cooling liquid source (29) either along a cooling liquid path (28) to the cooling liquid input port (22), or via a heating liquid source (31), where the liquid is heated, and along a heating liquid path (30) to the heating liquid input port (23). A temperature sensor (34) measures the temperature of the thermal mount (21) and a processor (27) controls the pump, valves (26) at the input and output ports and valves (41-44) at either side of the pump (38), to control whether heating or cooling liquid is input to the thermal mount, and at what flow rate, in order to obtain the correct temperature of the thermal block (21).