Smart HVAC equipment that recognizes when it is failing or has failed, or when environmental conditions have drifted outside its optimum capability range, could save substantial amounts of energy if the equipment sent useful information to the building owner or contractor. The necessary capabilities are referred to as automated fault detection and diagnostics (FDD). Implemented on commercial rooftop units (RTUs), these capabilities could save 10 percent or more.

Even the most efficient HVAC equipment will waste energy if it is operated incorrectly. In addition, the performance of HVAC equipment often degrades over time, so periodic maintenance is required to maintain equipment efficiency. Some of the problems that can affect the energy efficiency of RTUs include:

• Insufficient evaporator airflow

• Condenser coil fouling

• Incorrect refrigerant charge

• Compressor valve leakage

• Liquid line restrictions

• Economizer damper failure

• Sensor failure/degradation

Fault detection and diagnostics for rooftop units refers to technologies that monitor components, sense problems as “faults,” and can optimize operation and/or notify personnel, ensuring timely identification and correction of operating and service issues.

In recent years, advances in electronics have improved sensor capability and reduced the costs of sensors and controllers. Automated FDDs have been introduced for applications as diverse as nuclear power plants, aircraft, chemical process plants, and automobiles.

Manufacturers now produce “smarter” equipment that can self-diagnose faults, and some that can detect and adjust operations based on real-time conditions and performance data. This is partly due to growing awareness by building operators for the need to monitor building performance, which has resulted in the growth of building operation and maintenance services, and demand for extensive data reporting. This, coupled with the increasing use of information technology in buildings and building equipment, has helped to create basic infrastructure to enable widespread adoption of automated FDDs in RTUs.

Besides the energy efficiency benefits arising from ensuring the proper operation of RTUs, automated FDD on RTUs can also help:

• Provide greater comfort to occupants by providing information for building management and control systems (BAS, EMS);

• Minimize interruptions to building operations due to system failures;

• Reduce time (and costs) needed for maintenance and troubleshooting; and

• Avoid damage to RTU components (and replacement costs) by predicting component failure (prognostics) and implementing preventive maintenance.

Automated FDD with prognostics analyzes diagnostic data from sensors and generates trend lines to predict possible failures. It can also help promote other emerging technologies in HVAC: A common market barrier to emerging technologies is that customers are concerned about the availability of the technology, such as whether spare parts would be quickly available if there is a need to replace them. Prognostics can help address this by allowing the HVAC supplier ample time to ship the necessary parts and replace them even before failures occur.


Automated FDD can help reduce utility and maintenance costs. It allows building operators to take early action to correct faults in the RTUs, helping to reduce the amount of time the RTUs are operating in an inefficient manner due to these faults.

Diagnostic data can help building operators better understand energy consumption patterns and may highlight ways in which to reduce energy consumption. The data allow building operators to take a more active approach towards building management, shifting towards a more “reliability-centered” approach from the current “reactive” one. It can also reduce the cost for the building to undergo continuous commissioning, since the data needed for commissioning is already collected.

A meta-analysis of building commissioning reviewed the cost-effectiveness of commissioning in improving energy efficiency. The analysis found that most of the reported building performance problems were due to the HVAC systems. A report by the International Energy Agency reported that typically 20-30 percent energy savings can be achieved in commercial buildings by correcting faulty operation in the HVAC system. It is likely that the potential energy savings from intelligent HVAC systems will exceed this estimate, since intelligent HVAC systems provide opportunities for energy savings that go beyond just the correction of faults (such as dynamic matching of capacity to load when installed with variable output components).

In a paper prepared for the U.S. Department of Energy (DOE), TIAX estimated that FDDs could reduce national primary energy consumption by 3-18 percent for RTUs, assuming that they only address three key faults: insufficient evaporator airflow, condenser coil fouling, and incorrect refrigerant charge.

A California Public Interest Energy Research (PIER) project on advanced automated HVAC fault detection and diagnostics commercialization estimates that at least 10 percent of the energy used in California commercial buildings is wasted due to excessive run time and problems in the HVAC equipment and controls - problems that could be addressed by diagnostics/prognostics.

Based on a review of field studies on commercial rooftop units in the Pacific Northwest and California, it was found that:

• An average of 46 percent of the units tested had refrigerant charge that deviated by more than 5 percent from the specifications. Correcting the refrigerant charge is estimated to result in 5-11 percent savings in the cooling energy;

• An average of 64 percent of the units tested had economizers that failed or required readjustment. Repairing a failed economizer is estimated to result in 15-40 percent savings in the cooling energy, depending on the climate zone and other factors; and

• An average of 42 percent of the units tested had airflow that was out of range. The correction of airflow is estimated to result in about 10 percent savings in the cooling energy.

Our study assumes 10 percent savings, a conservative estimate.


The commercial rooftop unit market is fiercely competitive, and typically dominated by commoditized products sold as least purchase price solutions. Utility incentives, such as those coordinated by the Consortium for Energy Efficiency (CEE), are considered essential for moving the market to higher performance. It will be important for CEE to consider the results of field research and begin to require automated FDD measures for program eligibility.


The fastest way to increase market penetration of FDD could be through building energy code provisions in progressive states like California. However, it is not clear whether a state can adopt an efficiency-related equipment requirement without violating the federal preemption requirements of the National Appliance Energy Conservation Act (NAECA) and Energy Policy Act (EPACT).

In the absence of state action, voluntary programs such as utility and public benefit incentives are likely to work well for increasing penetration and understanding of automated FDD’s savings potential.

Publication date:08/23/2010