Many methods are used for controlling temperatures in heat exchanger systems. Direct manipulation of the flow rate of either the hot or the cold stream is most often used when that stream is a utility (cooling water, steam, hot oil, or refrigerant). When the flow rates of both streams are set by process requirements, heat-exchanger bypassing is widely used. A portion of one of the streams (either hot or cold) is sent through the heat exchanger, and the remainder is bypassed around the exchanger. The temperature of the mixed steam is controlled by valves in each path. This system provides very tight temperature control, since the dynamics of blending a hot stream and a cold stream are very fast. This paper explores the design and control issues when heat-exchanger bypassing is used. The design optimization variables include the fraction of bypassing, the area of the heat exchanger, and the design pressure drops over the control valves. Dynamic rangeability requires heat-transfer rates to be adjustable over a wide range. As expected, results demonstrate that a larger area and more bypassing improve the ratio of maximum-to-design heat transfer rates, which is important for dynamic controllability. An unexpected, counterintuitive result is that control valve design pressure drops have little effect on rangeability in heat-exchanger bypass systems in which variable-speed pumps are used to maintain total flow rates.