Birling Consulting Ltd
Birling Consulting Ltd

BMS & HVAC Control - The basics



These notes are intended to give a very brief overview of what BMS and HVAC controls are.


BMS & HVAC Control Configuration


Modern HVAC controls include discrete electronic controls, electronic controls with communications and BMS. HVAC plant manufacturers may also include control systems as part of their package, these range from relatively simple control functions, including safety related controls, through to comprehensive systems that can control additional plant.


Many HVAC controls include communications, either to communicate with a BMS head end/central facility, or via IT networks and the internet.


A BMS normally includes a PC based head end/central facility to provide the main interface for the building operation. The head end/central facility will include the BMS supplier’s software, normally running on a Windows based operating system. Larger systems can also include a separate server. Day to day operation is normally via graphical user interfaces with alarm management features, etc.


Modern BMS outstations often include graphical web interfaces that can be accessed without a head end/central facility software. These offer a low cost way of interfacing with the HVAC plant and building operation.


Local interface panels are also available that offer a wide range of functions, mimicking most of the standard BMS graphical functions.


BMS control of HVAC main plant is normally via an outstation, which is often installed in the main plant control panel.


HVAC terminal units (fan coils, VAV boxes, etc.) are normally controlled via ‘field’ controllers which have a limited range of functions, although outstations can also be used for terminal units. Field controllers are often supplied by the BMS manufacturer, but may be supplied by the terminal unit supplier, or other supplier.


BMS & HVAC Control Communications


BMS traditionally used proprietary communications networks and many still do for all, or part, of their communications between field controllers, outstations and central facilities.


All major BMS manufacturers and many HVAC plant suppliers have standardised on BACnet - A Data Communication Protocol for Building Automation and Control Networks. Developed under the auspices of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), BACnet is an American national standard, a European standard, a national standard in more than 30 countries, and an ISO global standard.


Other communication standards are available, which are generally associated with specific functions, BACnet can communicate with most other standards. 


BACnet does not resolve all communications and interoperability issues, but it goes a long way towards this goal. If the functionality of a packaged control system is limited it will not necessarily be improved just because it can communicate, although the communications might offer an alternative solution.

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