Building automation systems

A building automation system (BAS) is the network of controllers, sensors and software that runs a building's mechanical and often electrical systems: it starts and stops equipment, holds temperatures and pressures, and raises alarms. When it is set up and maintained well, it saves energy and keeps occupants comfortable. When it drifts, it can quietly waste a great deal of both.

What a BAS does

• Scheduling: running equipment when the building is occupied and setting it back when it is not.
• Control: holding setpoints for temperature, humidity, duct pressure and water temperature by adjusting fans, valves, dampers and pumps.
• Coordination: making equipment work together, for example resetting chilled water temperature based on how many cooling valves are open.
• Monitoring and alarms: alerting operators to failures, high temperatures, filter loading or equipment running when it should not.
• Trending: recording values over time so operators can diagnose problems and verify performance.

Many systems also connect to lighting, metering, access control and other building systems, though fire alarm systems remain separate and must not depend on the BAS for their life safety functions.

How it is put together

Field devices

Sensors measure temperature, pressure, humidity, carbon dioxide, flow and status. Actuators move dampers and valves. Relays and variable frequency drives start and modulate motors.

Controllers

Controllers read the sensors, run the programmed sequences and command the actuators. Some are dedicated to a single VAV box or fan coil; others run whole air handlers or central plants.

Network and front end

Controllers communicate over a building network, commonly using open protocols such as BACnet, or vendor-specific ones. A front-end server and graphical interface let operators view equipment, change setpoints, review alarms and look at trends. Many are now reachable remotely, which brings cybersecurity into scope.

Sequences of operation

The sequence of operation is the written description of how each system should behave: when it starts, what it controls to and how it responds to conditions. It is the most important document for a BAS, and often the hardest to find. Without it, nobody can tell whether the system is doing what was intended. Ask for it at handover, and keep it updated when changes are made. Our guide to commissioning a new building explains how sequences are verified.

Common problems

• Points left in manual override, sometimes for years, so equipment ignores the schedule.
• Schedules that no longer match how the building is used.
• Failed or drifting sensors giving false readings, especially outdoor air, mixed air and duct static pressure sensors.
• Simultaneous heating and cooling caused by poorly tuned loops or overlapping setpoints.
• Economizers locked out or not functioning.
• Alarms so frequent that operators ignore them.
• Graphics that do not match the real equipment after renovations.
• No trend data, so problems cannot be diagnosed after the fact.
• Remote access with shared or default passwords and no updates.

Keeping a BAS healthy

1. Review overrides regularly. Produce a list of points in manual or override and clear any that are no longer needed.
2. Check schedules seasonally against actual occupancy, including holidays.
3. Calibrate key sensors on a schedule. A wrong outdoor air sensor can upset the whole building.
4. Use trends. Set up trends on key values and look at them. Equipment running overnight is easy to spot in a trend.
5. Manage alarms. Reduce nuisance alarms so the important ones get attention.
6. Keep documentation current: sequences, points lists, network diagrams and graphics.
7. Secure it. Treat the BAS as part of your IT environment: unique accounts, updated software, secure remote access and separation from public networks.

Energy and the BAS

Because the BAS decides when and how hard equipment runs, it has a large effect on energy use. Many no-cost or low-cost savings found in energy audits are control changes: tightening schedules, resetting setpoints with outdoor conditions, and fixing economizers. These are worth checking before investing in new equipment.

How ehZee can help

Our mechanical system inspections include a look at controls: schedules, overrides, key sensors and how the equipment is actually behaving compared with what it should be doing. We report findings in plain language so you and your controls contractor can act on them. We also design and build network infrastructure, so we can help with the network side of a BAS.

Common questions