An auxiliary coolant system, for a fuel cell stack system, includes a reversible coolant pump, a control valve, and plumbing lines. The pump enables the reversal of the coolant flow direction in the fuel cell stack system. The auxiliary coolant system is in parallel with the primary coolant system, and communicates with the primary coolant system via valves. During start-up, when coolant flow is from right-to-left through the fuel cell stack system, one valve of the primary coolant system will be partly open and control the amount of cold coolant make-up from the primary coolant system to the fuel cell stack while the valve of the auxiliary coolant system will be controlling the temperature differential between the inlet and outlet of the stack.