Innovative Energy Engineering



HVAC Controls and Operating Strategies

Proper controls are essential to reduce energy consumption and provide comfort. In all but the simplest applications a Building Automation system (BAS) compatible with the Niagara Framework should be used. HVAC controls also can integrate with other building systems. For example an activated fire alarm can require AHU to be turned off, or exhaust fans to be on. Most equipment is offered with manufacturer's control, which is problematic. Those proprietary controls don't know what the rest of the system is doing and require expensive proprietary maintenance devices. It is best to specify all equipment with minimum control and have the BAS control the rest.

Basics

A "controlled variable" is measured and controlled (i.e. room temperature). A "sensor" measures the variable and sends an electric signal to the controller (i.e. temperature sensor). The "controller" is the brain making decisions based on a "control loop" and sends a signal to a "controlled device" (i.e. valve) that manipulates the "controlled agent" (i.e. heating water).

Control Loops can be "open" without a direct feedback. For example a hot water coil valve is controlled by outside air temperature. This assumes a fixed relation ship (i.e. colder ambient temperature requires more heating). The problem is that heating load may not be directly related to outside ambient temperature. A "closed loop" with direct feedback would control the valve based on meeting a discharge air temperature setpoint or a space temperature setpoint. A "Control Reset" can be added to change the setpoint based on other parameters. For example discharge air temperature is reset based on ambient temperature.

Controllers receive (input) signals from a sensor or send (output) signals to an actuator. Digital (or binary) signals are on/off (often 24 V). Examples include switches (Digital Input - DI) and On/off actuators (Digital Output - DO). Analog signals provide 0-100% ranges in voltage (0-10V, 2-10V) or current (4-20 mA). Examples include Temperature sensors (Analog Input - AI) and modulating actuators (Analog Output - AO). Actuators (for valves or dampers) also have a feedback (endswitch for DI, and resistor for AI). Communicating many signals between devices using standard digital and analog signals would be too complex. Imagine a chiller submitting hundreds of data points and the wiring required. In such cases information is transmitted via a signal cable and a standard protocol similar to the Internet Protocol (IP). Most common are LonTalk, BACnet, and ModBus.

A controller receiving an input signal uses some logic to decide on which output signal to send. A simple proportional (P) loop will compare measured variable (i.e. room temperature) to to a setpoint (i.e. desired temperature). Difference between the two is the "Offset" and the corrective output signal is based on offset value. With thermal mass, reaction times and other delays this often leads to overshoot of the measured variable and instable loops. Even when loops can be stabilized, the actual variable will always differ a bit from setpoint. Integral (I) control takes into account how long a deviation existed to create a corrective output signal. Derivative (D) control generates corrective output signals based on the rate of change of the measured variable (slope or derivative). All corrective signals are added in a Proportional-Integral-Dervative (PID) loop. This stabilizes the loop and allows the measured variable to actually achieve setpoint. In an attempt to simulate modulating control with cheaper digital equipment fast acting sensors and slow moving actuators were used to create a floating control. This is obsolete nowadays as it is more complex and not cheaper than modulating equipment.

Typically controllers control a device (i.e. a VAV terminal unit and radiator to condition a room) on "unit-control level". In that case the controller will modulate air and water flows, but won't influence the condition air or the water. In this example airflow of cold air will increased when cooling is required, and hot water flow to a radiator will be increased when heating is required. On a "system Level" AHU, boiler, chiller and other devices will be controlled to also change temperature of the air and the water as required. For example if the zone setpoint can't be achieved and outside temperatures are low, the boilers are commanded to produce warmer water.

Opportunities for Improvement

In zones with operable windows magnetic contacts detect when a window is open. The zone can go into a "freeze protection" mode by disabling all ventilation, cooling and heating above 5°C)

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