While FDD can be used for other building systems, it focuses primarily on HVAC systems. HVAC systems are one of the more complex and energyconsuming systems in a building involving different processes and the interaction of different types of equipment. We measure the performance of an HVAC system in several different ways: indoor air quality, energy consumption, and thermal comfort.

FDD is based on research of faults in HVAC systems and the development of hierarchical relationships and rules between the different equipment and processes that make up the HVAC system. For example, a chilled water plant supplying chilled water to air handling units is a relationship; the chilled water plant is the single source and the air handling units are multiple loads. Another relationship is an air handling unit delivering supply air to terminal units; the air handling unit is the single source and the terminal units are the multiple loads. In an HVAC system this relationship between source and load can be via air or water. It is these relationships and the rules within the relationships that are at the core of FDD.

These hierarchical relationships are used to collect raw data, apply a set of rules and identify faults. For example, there is a set of rules for systems consisting of a chiller, a boiler, air handling units receiving hot and chilled water from the boiler and chiller, and terminal units receiving supply air from the air handling units. A different set of rules would be applied if there was staged heating and cooling directly at the air handling unit or for single-zone air handling units. There are also different rules for the same equipment based on the state of the equipment; for example a chiller will have a certain set of rules when it is off, another set of rules at start-up and still another during its steady-state.

An example from the US National Institute of Standards and Technology demonstrates how FDD can be applied. In this case it is FDD developed for residential split-system heat pumps. The US Department of Energy implemented a regulation in 2006 requiring a 30% increase in the minimum seasonal energy efficiency ratio (SEER) for central air conditioners. Equipment manufacturers made the improvements to their equipment and at the same time NIST's HVAC&R Equipment Performance Group developed a FDD approach for residential heat pumps to assess and assure the improvements in the equipment. NIST identified six common faults in the equipment: improper refrigerant charge, improper indoor airflow, incorrect outdoor airflow, flow restriction in the refrigerant liquid line, noncondensable gases in the refrigerant and reversing valve or compressor valve refrigerant leakage/ bypassing. They took a look at fault-free and faulty performance characteristics for both heating and cooling modes. The research resulted in a set of FDD rules that will be used in the commissioning and the detection of sensor failure for residential split-system heat pumps.

In a study on ongoing commissioning (an element of which is system diagnostics) Lawrence Berkeley Laboratories, showed an average energy savings of 10 percent and as much as 35 percent in some cases. Many of the FDD tools go beyond just identifying faults in building systems and can provide guidance on what the root cause of the fault may be. This is information that is valuable to the facility engineer and saves time. Some of these tools can also monetize the fault, where there is some indication of the severity of the problem or its wasted energy. The monetization of the faults tends to rearrange the priorities and urgency of work orders.

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