Copyright Goodheart-Willcox Co., Inc. 826 Air-Conditioning Systems 32.6.4 Ductless System Service and Troubleshooting When servicing and troubleshooting a ductless system, man- ufacturer information is critical to success. Manufacturers generally provide installation and service information on a website or phone app. Be familiar with these resources, and follow the manufacturer’s service procedures. The most common problems with ductless systems are the result of poor installation practices. Flare connections that were not made properly will begin leaking within 12 months. Use leak detection methods to identify poor connections. Wir- ing problems occur as well. Compare the wiring connections to what is specified in the installation guide. Improper mounting of the outdoor unit can create problems. A heat pump outdoor unit in a cold climate that is mounted directly to a concrete slab (without adequate clearance beneath) can be damaged by the refreezing of water created by the defrost cycle. If the water is pre- vented from flowing out from under the unit, the coil can be damaged. Ductless systems use complex computer controls that are continually monitoring the system conditions and adjusting EEVs accordingly. Due to this high degree of sys- tem monitoring, the control system is able to provide trou- ble codes to help troubleshoot problems. When working on ductless systems, you may need to contact the manufac- turer’s technical support group to provide specific trouble- shooting guidance. Thermistor failure is a common problem for duct- less units. A thermistor is an electronic device with resis- tance that changes as the temperature changes. In a PTC (positive temperature coefficient) thermistor, resistance increases as temperature increases. In a NTC (negative temperature coefficient) thermistor, resistance decreases and temperature increases. A thermistor problem is normally identified by a trouble code. When this occurs, the thermistor should be removed from the unit and tested. To test a thermistor, you first need to find a chart listing the resistance of the therm- istor at various temperatures. This information may be available in the manufacturer’s service materials or from other resources. Simply measure the resistance between the two terminals of the thermistor and compare the mea- sured value to the expected value. A common method of testing a thermistor is to submerge the thermistor in ice water for several minutes and then check its resistance. Compare this measured value to the expected value at 32°F. If the thermistor testing confirms that all thermis- tors are working properly, the problem is most likely the control board. The control is receiving the correct signal from the thermistor, but the signal is not being communi- cated correctly within the control board, resulting in the thermistor trouble code. EEV motor failure is another relatively common prob- lem for ductless systems. This failure can occur due to wear.