Building Automation Systems

A building automation system (BAS) is a computer-based control system that monitors and runs the mechanical equipment in a large building from one place. Instead of a thermostat wired to a single furnace, hundreds of sensors feed controllers that decide what fans, valves, dampers and chillers should do. This is high-value work, and it rewards people who like both wiring and logic.

What You Will Be Able to Do

• Explain what a BAS does and how it differs from a thermostat.
• Tell inputs from outputs on a controller.
• Read a sequence of operation.
• Explain BACnet and Modbus communication at a basic level.
• Work through common sensor, output and communication faults.

Direct Digital Control

Most BAS equipment today uses direct digital control (DDC). A controller is a small computer. It reads inputs, runs programmed logic and sends out commands. DDC replaced pneumatic controls, which used compressed air signals. Many older buildings still have some pneumatic parts, so you may see both.

Every BAS loop follows the same closed-loop logic: sense, compare, act and feedback.

Types of Controllers

• Application-specific controllers come pre-programmed for one type of equipment, such as a VAV box or a rooftop unit. They are simple to set up.
• General-purpose controllers are programmed for custom sequences, such as a central plant or a large air handler.
• Supervisory controllers connect groups of controllers to a network and to the operator's computer.

The operator's screen, often called the front end, shows graphics, alarms, schedules and trends.

Inputs and Outputs

Common sensors include thermistors for temperature, CO2 sensors for occupancy, current switches to prove a motor is running, and differential pressure switches for filters and airflow.

Reading a Sequence of Operation

The sequence of operation is the written description of how the system should behave. It tells you what the controller is supposed to do, so read it first when you troubleshoot. Common items include:

• Occupied and unoccupied modes. Different setpoints and outdoor air based on the schedule.
• Economizer. Uses outdoor air for free cooling when outdoor air is cooler and drier than return air.
• Demand-controlled ventilation. Adjusts outdoor air based on CO2 levels, which rise and fall with the number of people.
• Supply air temperature reset. Raises or lowers the supply temperature to save energy.
• Static pressure control. Changes fan speed to hold a duct pressure setpoint, common with variable air volume (VAV) systems.
• Staging. The order of heating and cooling steps.
• Safeties and alarms. What to do on a freeze stat trip, a failed fan or a bad sensor.

Communication Networks

Controllers talk to each other using a protocol, which is a shared language.

BACnet is the most common open building automation protocol, so devices from different makers can share data.

• BACnet/IP runs on standard Ethernet networks.
• BACnet MS/TP runs on a two-wire RS-485 serial bus, daisy-chained from device to device.

On an MS/TP bus, each device needs a unique address, and all devices must use the same baud rate. The bus needs termination resistors at each end. Stars and long branches cause problems, so keep to a daisy chain.

Modbus is an older protocol, common with meters, drives and boilers. It uses numbered registers rather than named objects. Modbus RTU runs on serial lines, and Modbus TCP runs on Ethernet.

LonWorks appears in some older buildings. New systems use it less.

Networks in buildings are also a cybersecurity concern. Follow the site's IT rules, and do not connect a laptop or remote tool without permission.

Troubleshooting

Work from the outside in. First, is the problem real? Then ask whether the input, the logic or the output is at fault.

Sensor Reads Wrong

1. Compare the reading with an independent meter.
2. Check that the sensor type in the controller matches the actual sensor.
3. Check wiring for opens, shorts and reversed polarity.
4. Check the location. A sensor in sun, or next to a diffuser, reads false.

Output Does Not Move the Device

1. Check for an operator override in the software.
2. Check the logic and setpoint.
3. Measure the signal at the controller output.
4. Check power at the actuator.
5. Check that the actuator is mounted and stroking fully.

Controller Offline

1. Check the controller's power, usually 24 volts AC or DC.
2. Check the network cable and connections.
3. Check for duplicate addresses or mismatched baud rates.
4. Check termination at each end of an MS/TP bus.
5. Look at nearby devices. If several are offline, the issue is likely the bus.

Warning: BAS panels can contain line voltage as well as low voltage. Turn off power and lock it out as required. Never run equipment in manual or override without telling the building's operators, and always remove overrides when you are done. An override left in place can hide a problem or leave equipment off.

Good Habits

• Back up the controller's program before changing anything.
• Change one setting at a time, and note the old value.
• Use trend logs, which show how a value changed over time. Trends can show problems a single reading cannot.
• Clear overrides, and record your work in the service log.

Common Mistakes

Check Your Understanding

A VAV air handler's fan speed swings up and down. What might you look at? The static pressure sensor, its tubing and the control loop's tuning, using a trend log.

Three controllers on the same MS/TP run went offline after an electrician worked nearby. What is your first suspicion? A damaged or disconnected bus cable, or a termination problem.

Outdoor air is 58°F and dry, and return air is 74°F. The economizer stays closed during a cooling call. What do you check? The outdoor air sensor, the damper and actuator, and the sequence's economizer enable.

Why does a CO2 sensor help ventilation? CO2 rises with occupancy, so it lets the system bring in outdoor air based on how many people are present.

Key Takeaways

• A BAS is a centralized computer system that monitors and controls building equipment.
• Controllers use inputs, logic and outputs in a closed loop.
• The sequence of operation is the reference for what the system should do.
• BACnet is the common open protocol, and MS/TP needs unique addresses and end-of-line termination.
• Clear overrides, back up programs and use trends.