We are first strongly advising to use one open protocol (or several), but open protocols only. Please avoid any proprietary protocol that closes the solution and system. BEWARE! Sometimes protocols are opened but system is closed! For example devices and products are all based on open system but infrastructure and tooling is proprietary… Then system is closed.
To choose a protocol you definitely have to know it : here is a small history and position of the open standards :
LonWorks (EN/ISO 14908)
It’s a major protocol In HVAC control. It is broadly used by HVAC manufacturer proposing hardware devices including a LonWorks twisted pair interface (FTT10A) : Fan Coil Unit, VAV controler… LonWorks is broadly used at field level on large and medium sites where HVAC control is important but also where room control is important : typically for its flexibility to reconfigure life space and offices.
KNX (EN/ISO 14543 )
It’s a protocol often used in lighting, blinds and home and small building control. European big players in electrical installations basically have KNX devices. Moreover KNX is a standard coming from EIBus, Batibus et EHS. Then there is a broad existing product range. KNX is mainly used in home and hotel automation. It is also used in small and medium building automation but for field device only.
BACnet (EN/ISO 16484)
It’s a protocol born in the USA, created by ASHRAE organization to simplify and harmonize the management of several BMS coming from different providers. That protocol is mainly a protocol aimed to be at management level to supervise BMS networks. It includes natively interoperable objects to manage schedulers, trending and alarming. It is worldwide broadly used at SCADA level but few used in Europe at field level because it misses peer2peer data sharing and easy commissioning..