Building automation (smart buildings)

Building automation is a programmed, computerized, "intelligent" network of electronic devices that monitor and control the mechanical and lighting systems in a building. The intent is to create an intelligent building and reduce energy and maintenance costs.

Parts of the system

Controller

The controller is normally one or more programmable logic controllers, often with custom programming. PLCs come in a wide range of sizes and capabilities to control devices that are common in buildings. Usually the primary and secondary buses are chosen based on what the PLCs provide.

Most PLCs provide general purpose feedback loops, as well as digital circuits.

Occupancy sensors

Occupancy is usually based on time of day schedules. Override is possible through different means. Some buildings can sense occupancy in their internal spaces by an override switch or sensor.

Lighting

Lighting can be turned on and off with a building automation system based on time of day, or the occupancy sensors and timers. One typical example is to turn the lights in a space on for a half hour since the last motion was sensed. A photocell placed outside a building can sense darkness, and the time of day, and modulate lights in outer offices and the parking lot.

Air handlers

Most air handlers mix return and outside air so less temperature change is needed. This can save money by using less chilled or heated water (not all AHUs use chilled/hot water circuits). Some external air is needed to keep the building's air healthy.

Analog or digital temperature sensors may be placed in the space or room, the return and supply air ducts, and sometimes the external air. Actuators are placed on the hot and chilled water valves, the outside air and return air dampers. The supply fan (and return if applicable) is started and stopped based on either time of day, temperatures, building pressures or a combination.

Constant Volume Air-Handling Units

The less efficient type of air-handler is a "Constant Volume Air Handling Unit," or CAV. The fans in CAVs do not have variable-speed controls. Instead, CAVs open and close dampers and water-supply valves to maintain temperatures in the building's spaces. They heat or cool the spaces by opening or closing chilled or hot water valves that feed their internal heat exchangers. Generally one CAV serves several spaces, but large buildings may have many CAVs.

Variable Volume Air-Handling Units

A more efficient unit is a "Variable Air Volume (VAV) Air-Handling Unit," or VAV. VAVs supply pressurized air to VAV boxes, usually one box per room or area. A VAV air handler can change the pressure to the VAV boxes by changing the speed of a fan or blower with a variable frequency drive or (less efficiently) by moving inlet guide vanes to a fixed-speed fan. The amount of air is determined by the needs of the spaces served by the VAV boxes.

Each VAV box supply air to a small space, like an office. Each box has a damper that is opened or closed based on how much heating or cooling is required in its space. The more boxes are open, the more air is required, and a greater amount of air is supplied by the VAV air-handling unit.

Some VAV boxes also have hot water valves and an internal heat exchanger. The valves for hot and cold water are opened or closed based on the heat demand for the spaces it is supplying.

Central plant

A central plant is needed to supply the air-handling units with water. It may supply a chilled water system, hot water system and a condenser water system, as well as transformers and emergency power. If well managed, these can often help each other. For example, some plants generate electric power at period with peak demand, using a gas turbine, and then use the turbine's hot exhaust to heat water or power an absorptive chiller.

Chilled water system

Chilled water is often used to cool a building's air and equipment. The chilled water system will have chiller(s) and pumps. Analog temperature sensors measure the chilled water supply and return lines. The chiller(s) are sequenced on and off to chill the chilled water supply.

Condenser water system

Cooling tower(s) and pumps are sequenced on and off to supply cool condenser water to the chillers. The condenser water system may have cooling tower(s) and pumps. Analog temperature sensors measure the condenser water supply and return lines.

Hot water system

The hot water system supplies heat to the building's air-handling units or VAV boxes. The hot water system will have a boiler(s) and pumps. Analog temperature sensors are placed in the hot water supply and return lines. The boiler(s) and pumps are sequenced on and off to maintain supply.

Alarms and security

Many building automation systems have alarm capabilities. If an alarm is detected, it can be programmed to notify someone. Notification can be through a computer, pager or audible alarm.

• Common temperature alarms are Space, Supply Air, Chilled Water Supply and Hot Water Supply.
• Differential pressure switches can be placed on the filter to determine if it is dirty.
• Status alarms are common. If a mechanical device like a pump is requested to start, and the status input indicates it is off. This can indicate a mechanical failure.
• Some valve actuators have end switches to indicate if the valve has opened or not.
• Carbon monoxide and carbon dioxide sensors can be used to alarm if levels are to high.
• Refrigerant sensors can be used to indicate a possible refrigerant leak.

Security systems can be interlocked to a building automation system. If occupancy sensors are present, they can also be used as burglar alarms.

Fire and smoke alarm systems can be hard-wired to override building automation. For example: if the smoke alarm is activated, all the outside air dampers close to prevent air coming into the building. Life safety applications are normally hard-wired to a mechanical device to override building automation control.

Topology

Most building automation networks consist of a primary and secondary bus which contain programmable logic controllers, input/outputs and a user interface (also known as a human interface device).

The primary and secondary bus can be optical fiber, ethernet, ARCNET, RS-232, RS-485 or a wireless network.

Most controllers are proprietary. Each company has its own controllers for specific applications. Some are designed with limited controls: for example, a simple Packaged Roof Top Unit. Others are designed to be flexible. Most have proprietary software that will work with BACnet, LonTalk and ASHRAE standards.

Inputs and outputs are either analog or digital (some companies say binary).

Analog inputs are used to read a variable measurement. Examples are temperature, humidity and pressure sensor which could be thermistor, 4-20 mA, 0-10 Volt or Platinum RTD (resistance temperature detector), or wireless sensors.

A digital input indicates if a device is turned on or not. Some examples of a digital input would be a 24VDC relay or air flow switch.

Analog outputs control the speed or position of a device, such as a variable frequency drive, a I-P (current to pneumatics) transducer, or an actuator. An example is a hot water valve opening up 25% to maintain a setpoint.

Digital outputs are used to open and close relays and switches. An example would be to turn on the parking lot lights when a photocell indicates it is dark outside.

Protocols and Industry Standards

• ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers) is an international organization for people involved in heating, ventilation, air conditioning, or refrigeration (HVAC&R).
• BACnet is a network communications protocol for building automation and control systems. *LonTalk is a protocol created by Echelon Corporation for networking devices.
• KNX standard, a system for Home and Building Controls (promoted by "Konnex Association")is truly open (no royalties for Konnex members) and platform independent; guarantees multi-vendor and cross-discipline interoperability, ensured via certification and symbolized by the KNX trademark; supports many configuration methods (PC tools, device configurators and plug+play) and media (TP, PL, RF, Ethernet)
• Energy Star is program created by the United States government to promote energy efficient consumer products.