A container assembly (10) for a pump is described, provided with at least one pumping group (12, 14, 16, 18) and with at least one system (20) for transmitting power to such pumping group (12, 14, 16, 18). The container assembly (10) comprises at least one elastic element (26) sealingly housed inside such container assembly (10) at a predefined internal wall (28) thereof. Inside the elastic element (26), at least one cavity (32) is obtained which defines a corresponding air chamber configured for damping the variations of volume and the expansion of the fluid contained inside the pump following a possible change of state of the fluid itself when subjected to temperatures lower than its freezing point.