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Ductless Mini-Split Installation Tips

5/4/2018

Here are some tips to help make the installation of a ductless mini-split go smoothly.

Wiring: Many ductless systems require three 14-gauge stranded conductors with a dedicated ground between the indoor and outdoor units, or a 14/4 stranded cable. The ground is very critical and a dedicated ground wire must be used on every fan coil or head. Do not rely on the conduit to ground the fan coil! The third wire, marked 3 or S, is the communication wire and should never be spliced or broken for any reason, even by a condensate safety switch. If a splice is unavoidable it should be soldered and shrink wrapped, as splices or breaks in the line will cause communication faults. These wires are polarity sensitive between the indoor and outdoor unit. Take care to not cross L1 or L2 with the third wire (3 or S) that will most likely burn out the diode bridge on the outdoor unit circuit board. If you crossed the wire, there won’t be a blinking light on the main board. Don’t worry, it’s a common mistake!

Wall Penetrations: Line set wall penetrations are often overlooked as a cause for poor temperature control. Unlike a traditional A/C system, ductless mini-splits maintain temperature by sensing from the return air (RA) temperature sensor, not the sensor on the remote control. Cold or warm outdoor air leaking through this wall penetration can affect the RA sensor, causing space overheating or overcooling, so be sure to seal all wall penetrations.

Fan Coil Clearances: Fan coil clearances to obstructions are often ignored during installation, causing a number of problems including improper air distribution, short cycling, and improper temperature control. Wall fan coils absolutely require a minimum clearance below. Even a book shelf placed improperly under a wall fan coil will cause poor air distribution and short cycling. Ceiling cassettes require a minimum of five feet away from a wall on all four sides. This will cause recirculation of the supply air into the return, resulting in short cycling and poor temperature control. Be sure to check the recommended clearances.

Cover Installation: Make sure covers and ceiling trim grills are installed properly so that all gaskets are compressed and there is no recirculation from the supply into the return. This will cause short cycling and poor temperature control. Never attempt to operate any mini-split with the covers removed.

Proper Maintenance:  Ductless units generally have much lower air temperature than a standard split air conditioning system. These lower temperatures can cause condensation to form in the air passages, which will collect dirt. Make sure to annually properly clean and maintain the fan coils or they will get quite dirty and even start to grow mold. Several cleaning products are available that are specifically designed for mini-splits. The addition of PanPad or tab to prevent sliming should also be used.

Leaking Flare Fittings: Mini-splits utilize flares as a means of connection for the line sets. Only use a R410 flaring tool and a flare gauge to ensure that a proper flare is made. Always use a torque wrench when tightening the flare nuts to ensure they are not overtightened, and then pressure test to 550lbs with dry nitrogen for 24 hours if possible.