ASHRAE Journal - February 2012 - 78

EMERGING TECHNOLOGIES

HVAC Fault Detection
A
By Matthew Wiggins, Ph.D.; and James Brodrick, Ph.D., Member ASHRAE

significant amount of energy is wasted by malfunctioning or illmaintained building systems. Faults relating to HVAC systems rep-

resent between 1% and 2.5% of total commercial building consumption. Owners/operators are often unaware when units are malfunctioning and wasting energy. The average commercial AC unit has an evaporator airflow 15% to 25% less than optimum commissioned values,1 34% of residential air

conditioners are undercharged and 28% are overcharged,* 35% of commercial rooftop units’ dampers fail within several years of installation,1 and 50% to 67% of air conditioners (residential and commercial) are either improperly charged or have airflow issues.2,3
Despite the performance seen when a system is ﬁrst commissioned, efﬁciency over time decreases. Though scheduled maintenance is purported to maintain a system at its peak performance level, the typical maintenance person cannot do the in-depth tests required to deliver this performance without lengthy service calls. Such calls are often prohibitively expensive. Automated fault detection and diagnostics (FDD) systems attempt to address these issues by identifying faults when they occur and, if they are of sufﬁcient severity, communicating the fault to the owner or maintenance personnel. This can eliminate scheduled maintenance costs, reduce diagnostic labor, reduce wasted energy, reduce peak electricity demand, and minimize downtime. Residential and commercial HVAC systems are excellent systems for application of FDD. Units that tend to be under-maintained (such as many smalland medium-sized commercial/industrial building’s rooftop or other packaged air conditioners) have the potential for the shortest payback periods.

Technology
Generally FDD works by measuring a subset of temperatures, pressures, and humidity levels in several stages of the HVAC system. Automated comparison of what these values should be for a given system with what they actually are can provide strong indicators of fault conditions. Targeted faults include: • Low refrigerant charge; • Refrigerant overcharge;

• Noncondensable gas in the refrigerant; • Liquid-line restriction; • Compressor valve leaks; and • Condenser and evaporator fouling. Several types of HVAC FDD devices exist. First, embedded FDD in each device’s controller can be convenient and accurate, with the algorithms speciﬁcally tailored to the model by the manufacturer. This requires additional sensor and communications hardware beyond what is traditionally ﬁelded, and therefore, adds to the system’s upfront cost. Embedded FDD offers continuous, dedicated monitoring allowing it to record and report faults as they occur, allowing repairs to be performed in a timely manner. The National Institute of Standards and Technology (NIST) has developed a set of algorithms for detecting faults in air-handling units (AHUs) and variableair-volume (VAV) boxes that has been integrated into some control units.4 These higher-end units give alarms for a number of conditions such as fouled coils, incorrect damper positions, and leaking valves. Next, software as a service (SaaS) FDD systems offer continuous monitoring and feedback on system faults by sending sensor information to a data server that continuously monitors connected systems and reports faults. The FDD algorithms on the server can be continuously updated independent of the data coming from the sensors. In addition, the server’s increased computational power provides the potential for more complex and accurate diagnostic capabilities. Generally, SaaS FDD systems require a monthly fee, but can provide more personalized service. Finally, handheld or in-ﬁeld FDD units can be carried by maintenance personnel to

*These values are based on data collected from 4,000 residential cooling systems in California. Previous investigations revealed an undercharge of 15%.2

ASHRAE Journal

ashrae.org

February 2012

ASHRAE Journal - February 2012

Table of Contents for the Digital Edition of ASHRAE Journal - February 2012