Air handler units (AHUs) are commonly employed to condition and circulate air as part of a heating, ventilating, and air-conditioning (HVAC) system for buildings, and large buildings in particular. AHUs may provide heating, cooling, or both to change the supply air temperature, humidity level, or both depending on geographic location and current environmental factors. Such conditioning is provided by coil(s) (e.g., heat exchanger coils or cooling coils) located to interact with the AHU's air flow. AHUs may take the form of terminal units (also called fan coil units), packaged units or rooftop units, but will be referred to herein as AHUs for purpose of brevity.
Conditioning of the air flow typically employs the use of hot water or steam for heating and cool or chilled water, and sometimes refrigerant for cooling. Coils for transferring heat may take the form of metallic tubes combined with metallic fins to encourage heat transfer; whereas cooling coils may also employ eliminator plates and condensate pans to remove and drain condensation. In many applications, air flow control dampers (often called “economizer” dampers) also regulate the flow of outside air into the AHU to provide cooling requirements in place of the cooling coil when the outside air temperatures are sufficiently low. In large buildings or building complexes central boiler and chiller plants are generally employed to generate hot water/steam and cool water, respectively. The water is circulated to and through the building in dedicated conduit and valves controlled by the building automation system modulate the flow of the water or other fluid through the coils inside the AHUs.
In more modern HVAC systems, the AHUs may include smart (also referred to as self-ranging, self-balancing or self-regulating) valves that modulate the flow of the water through the coils inside the AHUs. These conventional smart valves typically communicate with water temperature sensors that monitor the temperature of the cold/hot water return and/or supply flows, and may also communicate with one or more air temperature sensors that monitor the temperature of the air flowing through the coil under its control.
When the water temperature sensors monitor both the return and supply flows, the smart valve may be configured to maintain a minimum temperature difference between the supply and return flows in addition to controlling the temperature of the air being discharged from the coil. In addition, the AHUs include fans or blowers that move the air flow over and/or through the coils to obtain a desired downstream (sometimes called “off coil”, “discharge” or “supply) air temperature and a desired airflow volume. The downstream air flow may be directed toward a desired space (e.g., a room or hallway) through air ducts and/or vents.
The control of the AHU, generally, and the control of at least some of the equipment comprising the AHU may be accomplished with an automation system such as, but not limited to a unit or building automation system (hereinafter generally referred to as a building automation system or BAS). Controlling the downstream air temperature, in particular, may be based on comfort requirements or desires, equipment capabilities, process requirements, efficiency goals, or some combination thereof.
Building automation systems typically have the capacity to provide alarms to a remote operator station or data server. However, this alarming capacity is limited. Generally, alarms can be set to annunciate when temperatures are out of expected range or when equipment fails to start or stop properly. These alarm functions typically have some interlock and time delay features so, for example, an out or range temperature value will annunciate only when the fan in the AHU is running and has been running for a predetermined period of time. However, alarm features currently available are very limited in their flexibility to detect faults, and have no diagnostic nor fault mitigation capabilities.