BMS Overview

A typical building will have HVAC plant consisting of air handling units to supply occupants with conditioned fresh air, chillers for cooling, boilers for space heating and domestic hot water and then various pumps and fans to provide this flow of water and liquid around the building.

The illustration below shows the typical layout of a BMS network. This highlights the different layers and sub-systems that form a modern BMS. The purple shaded area represents the controllers and field devices within a single building. These will be networked (i.e. wired together to permit communication between controllers) and connected to the Supervisor to allow the end user to control the site via a single PC as shown in the grey shaded area. Depending on how the system is setup it may also be possible for the BMS to send information remotely. For example, system alarms can be sent via SMS, energy reports can be sent via e-mail or fax and there may even be a web-based Supervisor to permit full graphical monitoring of the site from any web-enabled PC. The blue shaded area shows how remote sites can be connected to the main site either via the internet or through the site’s telephone line. The green areas indicate third-party devices such as A/C units for example, which can either communicate with the BMS directly, via an interface device or across the internet.

 

Management Level

The supervisors reside on this level and provide the main connection point between the field devices with the external connection points such as remote sites and communications to personnel and web-based applications via the internet, SMS and e-mail services. The supervisor will usually require a number of upgrades during the life of the building in order to keep graphics up to date and to make use of any bolt-on type packages such as alarm lists, energy management software and general improvement to the aesthetics of the system. Typical life-cycle of management level: 3-5years

Automation level

The BMS controllers provide the ‘brains’ of the system. A typical controller will usually have a range of configurable I/O (inputs and outputs). These are used to hardwire points in the field back to the controller. Software must then be engineered by a specialist to make use of the inputs in order to control the various outputs. The points used on a BMS will fall into one of four categories:

• Digital Input: A binary input to monitor a single status, e.g. a fault from a device
• Digital Output: A binary output use to send an ‘enable’ signal to a device, e.g. to start a fan.
• Analogue Input: An input which has variable value, e.g. a temperature sensor
• Analogue Output: An output from the controller whose value will vary depending on what the control is required. An AO can be used to modulate the position of a heating valve for example.

The control software varies between systems but in any case it will make use of logic blocks/line code to compare, measure and record all of the values available from the field to determine the necessary outputs and enable signals to the mechanical plant.

Controllers are constantly improving in terms of their communications methods, connectivity and processing capacity. Within the past 10 years, communications have become more standardised and software has become easier to engineer, modify and commission. The continuing developments of IT systems and web-based applications are helping controllers to become very flexible in terms of their communications methods. As a result most new controllers can be integrated with other building services (such as lighting and security) where in the past this simply was not possible. A range of controllers are typically supported by their manufacturer for no more than 12 years before they are considered ‘obsolete’ and in need of upgrade. It is very common to find old obsolete systems still in place in many buildings. This is typically due to the client not having sufficient budgets in place to upgrade, or instead funding projects which are deemed to be of a higher priority. At present the typical life cycle of controllers is approximately 7-12 years.

Field level

The field level consists of the many BMS related devices that reside around the plantrooms and occupied areas of the building. Sensors, actuators, fan coil unit controllers and the like fall into this category. These devices can often remain in place even when the management level (controllers) is upgraded. However, it is important to ensure that all of these devices are well maintained and calibrated in line with the manufacturer’s recommendations due to the important role they play in the control system. For example, a single faulty sensor could cause the entire heating system to switch on unnecessarily. With proper maintenance, field devices have an operating life of 10-20 years.