Building automation and controls

Mechanical equipment does nothing useful on its own. A chiller, an air handler or a pump needs to be told when to run, how hard to work and what to do when something goes wrong. Building automation systems (BAS), also called building management systems or direct digital control (DDC) systems, do this. They read thousands of sensors, run control sequences, start and stop equipment, raise alarms, log data and give operators a window into the building. They are also where many energy savings are won or lost.

Controls are a frequent weak point at handover. They sit between trades, rely on software as much as hardware, and are often the last system finished, squeezed by a late schedule. A building can have excellent equipment and still perform badly because sequences were never implemented as designed, sensors were never calibrated or alarms were never set up. This lesson explains how controls work and how the constructor can manage them well.

The control loop

Every automatic control has the same basic parts. A sensor measures a variable, such as room temperature, duct pressure or water flow. A controller compares the measurement with a setpoint and calculates an output. An actuator, such as a valve, damper motor or variable frequency drive, changes the process. The process responds, the sensor measures again, and the loop repeats. This is closed-loop or feedback control.

The simplest controllers switch equipment on and off (two-position control), as a house thermostat does. Most commercial controls are modulating: they adjust output gradually. Proportional control sets the output in proportion to the error between measurement and setpoint; adding integral action removes the steady offset that proportional control leaves; adding derivative action responds to the rate of change. Together these are called PID control, and loop tuning, setting the gains so loops are stable but responsive, is a skilled part of controls start-up.

Sequences of operation

A sequence of operation is a written description of how a system should behave in every mode: occupied, unoccupied, warm-up, cool-down, economizer, fire alarm, power failure and equipment failure. It is the most important controls document, because it tells the controls contractor what to program and tells the commissioning team what to test. Vague sequences, such as 'control to maintain comfort', lead to inconsistent programming and disputes.

ASHRAE Guideline 36, High-Performance Sequences of Operation for HVAC Systems, provides standard, tested sequences for common systems such as VAV air handlers and terminal units. Specifying Guideline 36 reduces design effort and makes programming and commissioning more predictable, and it is increasingly referenced in both countries.

Typical sequence elements that the constructor will see tested include:

• Occupancy schedules and optimal start, which brings the building to temperature just before people arrive.
• Economizer control, which uses cool outdoor air for free cooling when conditions allow.
• Supply air temperature and duct static pressure reset, which reduce fan and cooling energy at part load.
• Demand-controlled ventilation, which varies outdoor air with occupancy measured by CO₂ sensors or people counts.
• Safety interlocks: freeze protection, high-pressure limits, smoke detector shutdown and fire alarm responses.

BAS architecture and protocols

A modern BAS is a layered network.

• Field level: sensors, actuators, valves, dampers and variable frequency drives wired to controllers.
• Controller level: programmable controllers for each air handler, plant and terminal unit, running sequences locally so they keep working if the network fails.
• Supervisory level: network controllers or servers that coordinate schedules, trends and alarms across many controllers.
• Management level: the operator workstation or web interface, often with analytics, dashboards and remote access.

Devices talk through communication protocols. BACnet, developed by ASHRAE (Standard 135) and also published as ISO 16484-5, is the main open protocol for building automation in North America. Modbus is common for meters, drives and packaged equipment, and older buildings may have LonWorks or proprietary systems. Open protocols matter to owners because they reduce dependence on one vendor for future service and expansion; specifications often require BACnet with a protocol implementation conformance statement (PICS) for each device.

Control wiring is low voltage and is usually installed as Class 2 circuits under Section 16 of the Canadian Electrical Code, with line-voltage power to controllers and actuators installed by electricians. Who supplies and installs control power is a classic scope gap between mechanical, controls and electrical subcontracts.

Points and points lists

A point is a single input or output: a temperature sensor, a valve command, a fan status. Points are classified as analog inputs (measured values), analog outputs (modulating commands), binary inputs (status) and binary outputs (on/off commands), plus software or network points read from other systems. The points list, usually a schedule in the specification, is the basis for controls pricing, hardware quantities and checkout.

Integration with other systems

The BAS often connects to other building systems: lighting controls, electrical meters and power monitoring, generators, elevators, access control and the fire alarm system. Integration with the fire alarm is a special case. Fire alarm systems are life safety systems with their own codes and listing requirements, so the fire alarm panel usually sends signals to the BAS or directly to equipment (for example to shut down air handlers or run smoke control fans), and the BAS cannot override the fire alarm. Smoke control systems must be tested with the fire department or authority having jurisdiction, as lesson 7 explains.

Meters and submeters for electricity, gas, water and thermal energy feed energy dashboards and, in many jurisdictions, mandatory energy benchmarking and reporting programs. Getting meters integrated and trending correctly at handover avoids months of missing data.

Controls cybersecurity

A connected BAS is also a network that can be attacked. Building systems have been used as entry points to corporate networks, and an attacker who controls HVAC or access control can disrupt operations. Good practice includes separating the operational technology (OT) network from the business network, changing default passwords, limiting and logging remote access, keeping firmware patched, and documenting the network for the owner's IT team.

In Canada, guidance from the Canadian Centre for Cyber Security applies to building systems, and owners of critical infrastructure may face new obligations under the Critical Cyber Systems Protection Act enacted through Bill C-8 in 2026. BACnet Secure Connect (BACnet/SC) adds encrypted communication to the protocol.

Installing and commissioning controls

Controls work runs through the whole project but concentrates at the end. A practical sequence is:

• Review controls submittals: hardware, network riser, points list, graphics and written sequences.
• Install devices and wiring during rough-in, coordinated with mechanical and electrical trades.
• Perform point-to-point checkout: verify that each sensor reads correctly and each output moves the right device.
• Program and tune loops, set up schedules, trends and alarms.
• Support functional performance testing by the commissioning provider, then train operators.

Functional testing of many identical units, such as VAV boxes, is often done by sampling. If too many samples fail, the specification usually requires testing all units at the contractor's cost.

Monitoring-based commissioning and analytics

Buildings drift from their design performance as sensors fail, overrides accumulate and sequences are changed. Fault detection and diagnostics software examines BAS trend data to find problems such as simultaneous heating and cooling, valves leaking, stuck dampers or schedules overridden. Monitoring-based commissioning uses this continuously after handover, and many owners now specify that the BAS be ready to feed such tools at turnover.