A coolant reservoir and associated multi-functional unit for a thermal sub-system of a fuel cell system. The coolant reservoir includes a flame arrester positioned in a gas region of the reservoir. The multi-functional unit includes a first pressure relief valve that automatically opens if the pressure within the gas region goes above a first predetermined pressure. The multi-functional unit may also include a second pressure release valve that quickly releases pressure in the coolant reservoir if the pressure within the gas region goes above a second higher predetermined pressure. The multi-functional unit also includes an air-line and a check valve for allowing ambient air to enter the gas region if the pressure within the gas region falls below ambient pressure. The multi-functional unit also includes a cooling fluid fill line having plumbing that causes the coolant reservoir to vent prior to a cooling fluid cap being removed.