BMS Control Fundamentals: How to Navigate the Backend of Building Automation

You’ve mastered the mechanical side of HVAC—compressors, motors, refrigerant circuits, and airflow. But when it comes to the digital brains controlling these systems, things get fuzzy. What exactly happens behind those colorful graphics on the BMS screen? How do control sequences actually work? And most importantly, how can you troubleshoot them when things go wrong?

In this article, we’ll peek behind the curtain of building automation and break down the fundamental control concepts in language that makes sense to HVAC technicians. Once you understand these basics, you’ll be able to approach any BMS system with confidence—whether it’s a brand-new installation or a 20-year-old legacy system.

The Core Building Blocks of BMS Control

Every BMS, regardless of manufacturer, operates on the same core principles. Think of these as the fundamental “HVAC laws” of the digital world:

1. Inputs and Outputs: The Controller’s Senses and Muscles

Just like a technician uses their senses to gather information and their hands to make adjustments, a BMS controller has inputs and outputs:

Inputs (The Senses):

• AI (Analog Input): Reads variable values like temperature, humidity, pressure, or CO2. These are your temperature sensors, pressure transducers, etc.
• DI (Digital Input): Reads binary (on/off) states like switch positions, alarms, or status indicators. These are your filter switches, high-limit cutouts, etc.

Outputs (The Muscles):

• AO (Analog Output): Controls modulating devices like valve positions, damper positions, or fan speeds.
• DO (Digital Output): Controls binary devices like relays, contactors, or on/off valves.

Here’s a practical example: A VAV box controller might have an AI for space temperature, a DI for occupancy sensor, an AO for damper position, and a DO for the reheat valve. The controller reads the inputs, runs its control logic, and adjusts the outputs accordingly.

2. Control Loops: Making Decisions

Once a controller has information from its inputs, it needs to decide how to adjust its outputs. This is where control loops come in—the decision-making algorithms that maintain setpoints.

The most common type is the PID loop (Proportional, Integral, Derivative). Don’t let the technical name scare you. Here’s what it means in practical terms:

• Proportional (P): How strongly should the system react to the current error? If the space is 5°F too warm, how much should we open the cooling valve?
• Integral (I): How should the system handle persistent errors over time? If the space has been 2°F too cool for the last hour, we need to reduce heating output.
• Derivative (D): How should the system react to rapid changes? If the temperature is rising quickly, we need to increase cooling before we overshoot.

Think of P as the present, I as the past, and D as the future trend. Together, they provide responsive, stable control that can handle most HVAC applications.

3. Sequences of Operation: The Playbook

A sequence of operation is exactly what it sounds like—a step-by-step playbook for how the system should behave under different conditions. It’s like a detailed job plan for your BMS.

For example, a simple AHU sequence might read:

1. On a call for heating (space temp < heating setpoint):

• Close outdoor air damper to minimum position
• Modulate heating valve to maintain supply air temperature setpoint
• Operate supply fan at minimum speed

1. On a call for cooling (space temp > cooling setpoint):

• Check outdoor air temperature
• If suitable for economizing, modulate outdoor air damper to maintain setpoint
• If mechanical cooling required, open chilled water valve
• Increase fan speed as needed to maintain setpoint

For more advanced BMS applications, sequences get much more complex, handling multiple operating modes, various failure scenarios, and optimization strategies. In large buildings, you might see thousands of lines of sequence documentation.

Practical Application: Troubleshooting Control Issues

Now let’s apply these fundamentals to real-world troubleshooting:

Scenario 1: Zone Temperature Won’t Reach Setpoint

1. Check Inputs: Is the temperature sensor reading correctly? Compare BMS reading with a calibrated thermometer.
2. Check Outputs: Is the system commanding the correct output? Check valve/damper positions or stages of heating/cooling.
3. Check Control Loop: Is the PID loop tuned properly? An aggressive loop might cause hunting, while a sluggish one might never reach setpoint.
4. Check Sequence Logic: Is the system in the correct mode? Verify that it’s calling for heating or cooling as expected.

Scenario 2: System Hunting or Oscillating

If a system constantly overshoots and undershoots its setpoint, the control loop is likely poorly tuned:

1. Reduce the proportional gain to make the system less aggressive
2. Adjust the integral time to slow down the accumulation of error
3. Check for delays in the mechanical system that might be causing feedback issues

For more advanced troubleshooting techniques and detailed BMS network architecture, see our article on BMS Network Communications.

Beyond Basic Control: Smart Building Features

Modern BMS systems go well beyond simple control loops, incorporating advanced features like:

• Trend Logging: Recording historical data for analysis and troubleshooting
• Fault Detection and Diagnostics: Automatically identifying potential issues
• Demand Response: Adjusting operation based on utility grid demands
• Predictive Maintenance: Using data patterns to predict equipment failures
• Energy Optimization: Dynamically adjusting setpoints and schedules to minimize energy use

These advanced features build upon the fundamental control principles we’ve discussed. To dive deeper into the user interface side of BMS, check out our guide on BMS User Interfaces.

Bridging Your HVAC Knowledge to BMS

The best BMS technicians combine deep HVAC knowledge with control system understanding. When you encounter a new BMS, focus on these questions:

1. What are the inputs? (What is the system measuring?)
2. What are the outputs? (What can the system control?)
3. What is the sequence? (How should it behave?)
4. What are the setpoints? (What is it trying to achieve?)

Your HVAC knowledge already helps you understand how the equipment should operate. BMS control fundamentals simply add the layer of how that operation is automated. Once you bridge this gap, you’ll find that BMS work becomes much more intuitive, allowing you to apply your existing expertise to this growing field.

For an introduction to building automation systems, start with our BMS Basics article to get a complete overview of the industry.