“Excess reliability” is system capacity exceeding that which is needed to meet the
demand and to maintain system operation in case of an unexpected shutdown of the
largest contributor to a utility balance. Therefore, excess reliability is not only unneeded
but can also prove very costly. For example, an extra boiler might be run to provide
additional reliability (often called N +1). The actual load on the system may be low
enough, however, that two of a facility’s boilers must be run at minimum capacity,
instead of shutting down one boiler and running the remaining boiler at higher capacities.
The difference in efficiency between the lower end of a boiler’s operating envelope and
the middle of that envelope can be as much as 20–25%. Excess reliability, intended to
prevent infrequent, high-cost issues, can lead to long-term operation at reduced
efficiency, which can increase annual steam production costs by $1,000,000 or more
at a large facility.