It is not uncommon for heat transfer engineers to be asked why a system is not performing, even when the equipment was not sized by their organization. Minimum data may be all that's available. This chapter will provide the engineer with ideas about what to look for when units do not perform.
A related question is, How does one know when a unit is not performing? In refineries, chemical plants, and power plants, underperformance is known by measuring output. In air-conditioning systems it is known when rooms become hot and uncomfortable and users ask why. In electronic systems, problems become known when equipment fails and immediate attention is required. In efforts to get a plant back onstream quickly, it should be understood that the best solution is often overlooked in favor of the expedient one.