HVAC zone control

Some rooms in commercial buildings, such as the university’s laboratory, may require that the temperature be a setpoint that isn’t designed for human comfort, but is a controlled environment for experiments to thrive. Whereas others, like classrooms, would require an entirely different environment.

As a result, large HVAC systems are designed to supply heating or cooling throughout a building to several zones while separately control the setpoints for each one.

Greg has some options for addressing this issue. He can:

• Use the On/Off method if his application requires basic course control with an acceptable window of tolerance. This is more than sufficient and is the least expensive option
• Apply Pulse Width Modulation (PWM) where more precise control than the On/Off method is needed. This is a mid-range price option.
• Apply Proportional, Integral, Derivative (PID) control where precision and accuracy are critical. This is the most expensive, but most commonly used option

What Greg chooses to do is dependent upon what’s happening with the furnace. Let’s dig a bit deeper into these methods.

When the temperature drops below a bandwidth setpoint, the power will turn back on until the setpoint is reached. This cycle of turning on and off repeats continuously to control the temperature within the desired bandwidth over time.

PID control is a closed loop mathematical function.

It works by:

• Applying gains to the error, derivative of the error, and integral of the error between the setpoint and feedback
• It sums those values together, and then outputs the sum to the system as a reference
• By adjusting the gains, you can adjust the response time, overshoot, and the amount and size of steady state oscillations

An example of how a nano PLC can implement PID control is in its ability to control the position of a damper based on temperature feedback:

• The error between the setpoint and the feedback from an analog device can be fed into a PID function
• The output of the PID can then directly control an analog output to a damper position actuator
• The further the damper is open, the more hot or cold air flows into the room depending on whether you are heating or cooling the room
• As the temperature fluctuates in a zone, the damper position will dynamically adjust to automatically regulate the temperature