After exchangers have been in service, dirt and scale collect on their surfaces, usually at different rates, thus reducing the amount of heat that can be transferred, or in other words, reducing the exchanger's performance. As a practical matter it is not reasonable to clean an exchanger every time some scale buildup occurs. Rather, it is customary to minimize the problem by adding surface and attempting to anticipate the amount of buildup that will occur on both streams over a period of time while still maintaining performance. This is done by adding another resistance to each stream when calculating the overall heat transfer rate. These resistances are called fouling resistances or fouling factors. Their suggested values have been established using industry experience and operating conditions as a guide. The question remains, How much surface should be included? Some answers will be provided in this chapter via a look at plant conditions followed by examples.