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Every technician remembers the first time they saw a ductless mini-split unit and the panic they felt inside. Not only do these units look unusual, but they also behave somewhat differently than traditional split systems. Some troubleshooting tips that may stem from installation issues still cover basic design fundamentals. Understanding the following design aspects of mini-splits can help the technician determine what’s going on inside the system and educate the next generation about the future of all-electric HVAC. For these tips, I sat down with Rachel Samala, the owner of a mechanical HVAC company in San Diego who has nearly a decade of experience installing and servicing mini-splits.
1. The motors can go up and down
When opening up a mini-split air conditioner, the technician may be surprised to see that there isn’t much to see – just a control board and sensors everywhere. That’s because the compressor and both fans are inverter-powered motors, meaning they use DC voltage to change speed. Samala explains the basics of how it works like this: “AC voltage goes to a rectifier, which converts the voltage to DC voltage. That DC voltage then essentially goes through a switch, the IGBT (Insulated Gate Bipolar Transistor). The IGBT then feeds DC voltage to the compressor motor in a way that mimics AC voltage, creating the inverter effect. This allows the compressor to start smoothly and ramp up and down slowly.”
2. Mini-Split vs. Multi-Split vs. VRF.
By definition, all mini-splits are VRF (variable refrigerant flow), “meaning they have inverter compressors to vary the refrigerant flow,” Samala explained. When a mini-split system has more than one indoor unit and one outdoor unit, they are called multi-splits. Samala explained that technicians tend to differentiate advanced VRFs from minis and multi-splits by the advanced systems’ ability to vary the refrigerant flow between evaporators to heat some units and cool others. This is done through the use of numerous sensors and branch control boxes. One way to check if the system is just a multi-split is to look at the number of pipes connected to the outdoor unit. If the outdoor unit has several pairs of pipes, it is a multi-split, but if it has only two (high pressure and low pressure) or three pipes (for heat recovery), then it has at least one junction box inside that is connected to the indoor units.
Since multi-splits lack the ability to vary the refrigerant flow between evaporators, all units must be commanded to either heat or cool.
3. These systems are “critical loads”.
Most manufacturers classify their mini-splits as “critical load,” meaning that the only approved method of loading these units is loading by weight, which is different from methods such as superheat, subcooling, or clear sight glass. The units come factory charged, and the manufacturers specify how much refrigerant to add starting at a certain line length, which in Mitsubishi’s case can be anywhere from 25 to 70 feet, so it is always recommended to check the installation manual for this, as well as the minimum line length required.
Click on the diagram to enlarge
Figure 1: Loading diagram for a typical critically loaded Mitsubishi outdoor unit. (Courtesy of Lianna Schwalenberg)
4. The expansion valve is outside.
In these systems, the expansion valve is located in the outdoor unit. There are many logical reasons for this decision – noise and limited space, for example. Because the evaporator is so small, mounting the valve directly on the coil runs the risk of flooding the evaporator more quickly. Because the meter is located outside, both lines are at low pressure when cooling and high pressure when heating. When charging a unit in the winter, it is helpful to pump the refrigerant into the high pressure side using a recovery machine.
5. Overheating is usually minimal.
Sometimes mini-splits have only one service port, usually on the suction side, and it can be tempting to measure the superheat. While this is possible, don’t be surprised that these units tend to run at very low superheat, usually 0-5°. These systems are designed to run at low superheat to maximize the efficiency of the small evaporator size, allowing more saturated refrigerant to fill the coil and take the heat load. The outdoor unit usually has at least one accumulator mounted on the suction side to protect the compressor from flooding.
6. Condensation management prevents water damage.
Both refrigerant lines should be insulated with foam to prevent condensation. If the insulation is not wrapped tightly and touches the lines, moisture will collect in the insulation and drip wherever possible. Wrapping is especially important for sets of lines that run behind the unit and through locations where the lines are colder than the dew point. To prevent water buildup, the condensate pump, if installed, is usually wired to temporarily cut power to the indoor unit when a float switch trips to give the pump time to operate and empty its reservoir.
7. Defrost control in heating mode.
Unlike a single speed heat pump, an inverter mini split can stay warm longer and doesn’t go into defrost mode as often. This is accomplished by modulating the compressor and condenser fan and using multiple temperature sensors to make better decisions. Frost occurs when humidity in the air condenses on the lines and fins when the condenser coil temperature is below 90°F. During defrost mode, the outside fan is turned off and the reversing valve is switched to allow hot refrigerant gas to enter the condenser coil until a certain amount of time has passed and certain sensors are satisfied. Mini splits typically have an electric floor pan heater to ensure that melted water drips out of the unit.
8. Most indoor units are controlled by wired or remote controls.
Mini-split indoor units do not use the RGYW connectors that technicians are most familiar with. When asked why this is, Samala said, “Using standard thermostat wires and thermostats on inverters makes an inverter system pointless because inverters use DC power, which requires special wiring and controls.” If the system is controlled by a wired controller, a shielded 18/2 cable is usually required between the indoor head and the controller. With wireless control, a remote sends an infrared signal to a receiver on the indoor unit, which converts the light data into electrical signals. Because of this, technicians can usually swap remotes without having to program data.
9. Flares are the most common source of leaks.
The most common causes of leaks are the flare connections, either because installers did not tighten them to specifications and check for leaks, or because the factory flare on the line harness was botched. Many experienced installers, like Samala, recommend cutting out the factory flare on the line harness and making a new one, as well as using thread sealant to prevent leaks.
10. Leak stop and dye are not recommended.
The principles of leak detection apply to mini splits as they do to any other system. If there is refrigerant in the system, start with an electronic leak detector and add nitrogen if necessary. Pressurize the system and use soap bubbles as the next method. Change Schrader and hose seals and apply thread sealant to the gauges if necessary. Most leaks in mini splits can be found and repaired without using dye or leak stop. There are many incorrect methods of adding dye to a system and some compressor manufacturers have not studied the effects of dye and leak stop to properly approve or disapprove their use.
