Expert System Diagnostics

Expert system diagnostics use computer technology to accurately determine performance and/or identify faults in HVAC systems. The purpose of a diagnosis is to select an effective treatment. The goal of a treatment is proper functioning of the HVAC system. To serve its purpose, the diagnostic system must bring about proper system function.The range of available diagnoses and identified treatments is dependent on the amount of artificial intelligence in the system, the amount of available information, and the accuracy of the available information. Microprocessors can perform high-level diagnostics and, to be consistently effective, need to be able to screen poor information from good information.

Information may be fed into the expert system through sensors, through human inputs, or a combination of the two.

Savings

The savings available from this technology are dependent on:

• the frequency of the faults it can identify,

• the faults’ effects on performance,

• the level of performance degradation it can identify,

• the accuracy of the treatment plan,

• the effective and timely communication of the treatment plan, and

• the effectiveness with which the treatment plan is carried out.

The savings can amount to 50% or better for the worst HVAC systems.

Another attribute of an effective system is improved occupant comfort. In some cases energy use will increase as occupant comfort preferences are addressed.

Products

One approach has been developed by Rossi and Braun (1997). Their product (ACRx) has been applied to air conditioners. It uses thermisters to determine the condenser air entering, suction line, and liquid line temperatures. This device uses pressure sensors to determine the high side and low side refrigerant pressures. A schematic of their basic array is shown in Figure 4.3.5.

Figure 4.3.5. ACRx Diagnostic Tool Basic Sensor Array

Additional temperature sensors are also available for this product.

Rule sets are used to diagnose 5 possible faults: refrigerant leakage, liquid-line restriction, compressor valve leakage, condenser fouling, and evaporator fouling.

The approach computes residuals based on a 1^st order system performance evaluation and classifies the residuals that exceed a certain threshold as faults.

The basic rules used to calculate residuals are shown in Table 4.3.1.

Table 4.3.1 Fault diagnostics rules (Rossi and Braun 1997)

Where: Tevap = Evaporator Saturation Temperature Tsh = System Superheat

Tcond = Condenser Saturation Temperature Tsc = Subcooling

Thg = Hot Gas Temperature

∆Tca = Condenser Approach

∆Tea = Evaporator Approach

As temperatures change from base performance calculations, the residuals compound until the threshold is met and the fault is flagged. This system uses general fault rules that were developed by Rossi and Braun.

Another expert system diagnostic product is the Enalysis™ eScan. This product uses sensors to determine the inside and outside airflows, temperatures and humidities. It also senses high and low side pressures, suction line and liquid line temperatures. This product uses radio frequency communication with the expert system in a laptop computer and provides an immediate on-site printed report to the building owner.

Figure 4.3.6 Enalysis eScan System

The Enalysis product provides diagnostic results for duct systems, furnace systems and air conditioning systems. This product senses more variables than the ACRx system and has the capability to provide more in-depth analyses.

Another expert system is the CheckMe!® system developed by the authors of this report. That system has been applied to air conditioners, heat pumps and

duct systems. The basic products diagnose refrigerant charge, airflow, and duct system faults.

With the basic AC CheckMe!® thermocouples are used to determine the condenser air entering, suction line, liquid line, evaporator entering and

evaporator leaving temperatures. A wet bulb thermocouple is used to determine the evaporator entering wet bulb temperature and standard refrigerant gauges are used to determine the high side and low side refrigerant pressures. The preferred airflow measurements use the TrueFlo™ Flow Grid shown in Figure 4.3.7. These data (along with

make, model number, etc.) are phoned into a call center where the data are recorded and analyzed by the expert system. The immediate diagnosis and repair plan are communicated to the technician during that phone call. The analysis is based on manufacturers’

specifications supplemented by error checking and statistical algorithms.

Figure 4.3.7. TrueFlo™ Flow Measurement When the expert system or the call center operator detects problems or technician confusion the technician is immediately transferred to one of the on-call technical experts for support and assistance. Once the technician has performed the repairs

the system is retested and its final performance verified. A third party report mailed to the building owner within a week of the technician visit.

Figure 4.3.8. CheckMe!® Performance Quality Assurance System

Copeland Corporation provides a system with onboard diagnostics to produce quick and accurate analysis of failures. Their system was tested on low

experience level technicians and found to increase the level of correct diagnosis and repair from 17% to 92%.

17%

92%

0%

20%

40%

60%

80%

100%

Percent Accurate

without Comfort Alert™

with Comfort Alert™

Diagnostic Accuracy

Figure 4.3.9. Success Rate for Copeland Comfort Alert™ System

Carrier-Aeroseal provides duct sealing that works by pressurizing the duct systems with sealant particles that lodge in the leaks and progressively seal them. The Aeroseal process is controlled by a computer that must be uploaded periodically to continue working. The uploaded data are analyzed for accuracy and technician performance. This system makes it possible for Carrier-Aeroseal to maintain quality performance by their franchisees.

Figure 4.3.9. Carrier/Aeroseal Duct Sealing References

Breuker, M., T. Rossi and J. Braun. 2000. “Smart Maintenance for Rooftop Units.”

ASHRAE Journal, November, vol. 42, no. 11, 41-47.

http://www.copeland-corp.com/cp_ac/cp_ac_1_1_5_comfortAlert.htm http://www.enalasys.com/products/escan.php

http://www.proctoreng.com/

Modera, Mark. Personal Communication Carrier/Aeroseal April 8, 2004

Neme, C., J. Proctor, and S. Nadel. 1999. Energy Savings Potential from Addressing Residential Air Conditioning and Heat Pump Installation Problems. American Council for an Energy Efficient Economy Report #A992, February 1999.

Rossi, T.M. and J.E. Braun. 1997. “A statistical, rule-based fault detection and diagnostic method for vapor compression air conditioners.” International Journal of Heating, Ventilation, and Air Conditioning and Refrigerating Research, 3(1):19-37.