A vehicle comprising: a coolant header tank defining a storage volume for storing a fluid coolant and having a minimum fluid level within the storage volume, the coolant header tank comprising a port connected to the portion of the storage volume that contains fluid when filled to the minimum fluid level; and a temperature regulating system configured to carry the fluid coolant, the temperature regulating circuit comprising a branch connected between an air collecting region of the temperature regulating system and the port of the coolant header tank to permit air to flow from the temperature regulating system to the coolant header tank, the temperature regulating system being configured so that the air collecting region is higher than a horizontal plane that is defined by the minimum fluid level.