Wiring Communicating Controls

In some ways, communicating controls are easier to wire than conventional 24 volt control systems. There are fewer wires, and generally speaking, everything connects letter to letter. Still, it seems technicians keep figuring out ways to incorrectly wire four wires.

Wire Type

Early communicating systems used shielded cables that looked like computer cables – because that is what they were. You did not connect bare wires to anything, you plugged in the connector to the socket on the control board. Besides being somewhat expensive, these proved to be less robust than was needed for equipment installed outside. Most communicating control systems for traditional split systems and packaged units work fine with traditional thermostat wire: 18 gauge. In fact, the connections are designed expecting that type of wire.  Do not use anything smaller than 18 gauge. The control wire is generally not shielded. The control wire should not be run parallel to power wire to avoid interference. If you have to cross a power wire, it is best to do it at right angles. As with any installation, the control wire and power wire should not be run in the same conduit. It is fine to tape the wire to the line-set.

Mini-Splits

Nearly all mini-splits and multi-splits use communicating controls. Their wiring breaks all the rules discussed above. Typically, they use 14-4 cables. These cables contain four #14 gauge, stranded wires. Two wires in the cable are for powering the indoor unit and two provide communication. Sometimes this cable is shielded. If you do use shielded cable it is important to only ground one end, not both. I prefer grounding the end at the outdoor unit because you are closest to the power supply and the equipment ground. The connectors on mini-split and multi-split units are made to use stranded wire. Although there are only four connections, it is important that the same wire used to connect to each outdoor terminal connects to a similarly labeled indoor terminal. Sounds simple, but it is amazingly easy to cross up even just four wires. After connecting the outdoor wires, take a picture with your phone so you can verify that you are connecting them to the correct terminals inside.

Multi-Splits

Most multi-split units have terminal connections for each head at the outdoor unit. A 14-4 wire runs from the outdoor unit to each head. A crucial detail is to insure that the wires for each head correspond to the correct set of refrigerant lines. Although this seems simple, it is very easy to screw up. One way to avoid getting wires and lines crossed is to tape the 14-4 cable for each head to the line-set for each head.

Read and Follow Instructions

When it comes right down to it, most installation issues could be avoided by actually reading and following the instructions. Most manufacturers provide specific instructions for wiring their equipment, even if it is as simple as connecting four labeled terminals to four other similarly labeled terminals. There is not really an industry standard for the terminal labels. However, that is not important if you follow the manufacturer’s instructions. Most of the time, whatever label is used outside it also used inside.