Controls for Building HVAC & Lighting
The basics on the all-important “nerve centre”
Controls are the brains of a building and are responsible for the performance of the mechanical and lighting systems. Because these systems are the primary consumers of building energy, the controls must be well designed, commissioned and maintained if energy performance is to meet design expectations.
By Richard Lay, Stan Holko,Tim Dietrich and John Kokko
CONTINUING EDUCATION UNIT ARTICLE
In most cases it is no longer good enough to install a simple on/off switch. A LEED Platinum building with an energy simulation projecting a 70% reduction in energy consumption relative to MNECB, needs these controls to adjust [among other things] the temperatures, air and water flows and equipment operation and lighting according to weather, time of day, use and occupancy on a daily, weekly or annual schedule. For many building owners – and even building designers – control systems remain a mystery. What follows is an attempt to explain the basics.
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Efficiency First
We know that the most efficient way to operate most equipment is to turn it OFF. Or at least just run it slowly. Many energy efficiency controls are fancy ways of turning equipment off when it’s not needed, or running it at the lowest possible capacity to meet the required load. Because conditions change over time, the traditional controls which could do just one operation are too limited; so most modern controls are programmable and therefore electronic.
Many systems need to share information so that their operation can be coordinated; hence networks connecting equipment have supplanted traditional local controls.
How do Control Systems Work?
The fundamental objective of controls is to observe an input and produce an output in order to modify the input to a desired state. This is called a feedback loop and is the basis of all control systems. A feedback loop first requires a physical system that is capable of modifying the state of the input to the controller. A room with a hot water heating radiator is an example of a physical system in which a room temperature sensor or thermostat would provide the input.
The basic feedback loop is made up of a sensor, controller, and output signal and end device. The sensor provides an input to the controller by converting a physical phenomenon, such as temperature or flow, into an electric or electronic signal that can be read by a controller. Typical sensors used in building HVAC control are temperature, humidity and pressure sensors.
Sensors and Signals
Sensors can be analog or digital. An analog sensor converts a varying input into a varying signal output. For example, typical room temperature sensors used in commercial buildings sense temperatures in the range of -25 to 90 degrees C and produce an output signal that varies accordingly. The signal may be active or passive.
An active signal requires a power source in the signal loop between the sensor and controller, whereas a passive signal requires only a marginal amount of power in the signal loop.
An advantage of using an active signal is that the signal typically does not degrade with increased length of wiring so may be wired back to a central control point. Passive signals (when available) use less power and rely on factory calibration.
The signal output is the command given to the physical system to modify the input. As with sensors, end devices can be digital or analog. Examples of analog end devices include water temperature control valves, variable frequency drives, air dampers, and signals to other equipment controllers such as modulating boilers, chillers or rooftop air handlers.
Controllers
Back to our radiator example - a way of controlling the temperature of the room would be to install a modulating, control valve that can incrementally increase or decrease the flow of hot water through the radiator by accepting a variable voltage input. Examples of digital end devices include fan or pump starters, water valves, air dampers, lighting control, and enable/disable signals to other equipment controllers such as boilers, chillers or rooftop air handlers.
A controller is the mechanism by which the feedback loop is processed. It reads the wired electric signal from a sensor, applies programmed logic, and provides an output to a physical device. Controllers are electronic devices that use integrated circuits to activate control loops.
Typically, a controller requires two inputs in order to provide control to a system: one is the sensor input of the system to be controlled and the second is the set-point or desired state that the controller is attempting to maintain.
For our radiator example - the set-point can either be pre-programmed into the controller, or there can be a variable input set by the user. Most simple room temperature controllers come with a default set-point (usually 21o C) but can also accept an external set-point.
Central vs. Local Control
Controllers can come pre-programmed, designed for a single purpose, or they can be programmable to allow for customization. Building automation systems (BAS) fall into the latter category as they can provide central control to the majority of the building’s mechanical systems and are able to process multiple inputs and apply complex logic to optimize the operation of the building’s mechanical equipment. Simple controllers are often used to control isolated systems within the building.
An example of a simple, local control loop would be the radiator valve that is either fully open or closed by a completed circuit from a simple thermostat. In this case, the thermostat provides both the input and control logic to command the unit heater on and off.
Another common function of centralized control is time-of-day scheduling of equipment. This allows equipment to be turned down or off during unoccupied or low occupancy periods in order to save energy. Most building automation systems are able to be programmed for 365 days per year, with customization for weekend and holiday scheduling. Other scheduling strategies include morning warm-up or cool-down, switchover of lead-lag equipment, and ventilation control - all of which can be manipulated to optimize occupant comfort and energy savings.
Scheduled ventilation control refers to the capability to shut off dedicated ventilation equipment such as Energy Recovery Ventilators (ERVs) or close outdoor air dampers based on occupancy schedules to reduce unnecessary heating and cooling loads on the building. Local timers and occupancy sensors can provide similar functionality without centralized programmable controllers.
Communication Protocols
Another major consideration when specifying building automation systems is the type of protocol that the controllers will use to communicate with one another. Protocols used in commercial building automation systems are of two basic types: closed and open protocol.
In a closed protocol, the communication among controllers is proprietary to the controller manufacturer. Examples are Siemens P1-P2 and Johnson Controls N1-N2 communication. On the other hand, open protocol controllers can communicate using an external standard which allows controllers manufactured by different companies to be installed on the same network and communicate with one another. Examples of this include Lon and BACnet protocols.
Closed protocol systems typically cost less than open protocol systems, and Canadian contractors tend to be more comfortable installing them. Hence closed systems often perform better initially than open systems. The primary risk of closed protocol installation is single source service and parts for the life of the installation.
Advantages of open protocol systems are exactly the opposite - open protocol allows for open bidding on all service work as most components are interchangeable among the major BAS manufacturers. Disadvantages include increased complexity of initial installation and possibly lower initial performance. A compromise between open and closed protocol systems are “open protocol ready” installations, which use closed protocol sub-communication up to an open protocol controller or gateway that is capable of communicating in an open protocol. This can optimize initial performance - and permit future competitive bidding on maintenance.
The main advantages of a centralized, fully programmable BAS system lie in the flexibility of the system and its ability to control and monitor inter-related systems from a single location. The building operator has full control of both central and remote systems.
The challenges associated with centralized controllers lie in the relatively high cost of installation and maintenance. Also, the complexity requires a high level of sophistication on the part of the specifying engineer, installing sub-contractor, commissioning agent and building operator. Local control systems are often sufficient to provide adequate occupant comfort and energy savings through the use of occupancy sensors and local timers. However, they must be properly specified up front due to their inflexibility once installed. Quite often buildings are specified with a blend of centralized and local control.
Lighting Controls and HVAC Integration
Adding simple local sensor and relay based lighting control is the easiest way to reduce lighting energy consumption. It has been shown that adding simple occupancy control to an existing lighting system can save 40% on average in lighting energy.
There are two basic sensor technologies, passive infrared (PIR) and ultrasonic. PIR sensors track the movement of warm objects in a room and require line of sight for operation. Ultrasonic sensors create a high frequency standing acoustic wave and measure changes in the wave (human movement) to detect occupancy and can operate around corners and office partitions.
In a daylighting control system, photo sensors read the light level in the space and switch off or dim perimeter lights by circuit or fixture in order to harvest available daylight.
Occupancy and daylight harvesting can be taken to the next level by connecting the sensors to a network based lighting control system.
These systems combine the savings of local control with the intelligence and flexibility of a central control system. Sensors and ballasts are given addresses and are uniquely identifiable in the system. Instead of a sensor directly controlling a local relay, it communicates the occupancy status back to the system control and the controller turns the appropriate relays on or off. Changes in office layout can be dealt with by reprogramming software and reconfiguring low voltage wiring rather than through extensive changes to line voltage power circuits.
These systems also incorporate time scheduling, load shedding (reducing lighting output based on high energy prices to lower demand), and even “personal control” which allows individual occupants to control the light level over their desk from a small computer or web application.
Utilizing most or all of these strategies has been shown to provide a 60-70% reduction in lighting energy but at some additional capital cost in equipment and control wiring.
Integrating Control Systems
Occupancy control can be integrated with building HVAC systems, turning off ventilation air, heating and cooling) in spaces with no occupants. The vast majority of low voltage sensors on the market today are equipped with a set of dry relay contacts that can be connected to the HVAC system for this purpose.
Utilizing the input of an existing sensor with another system reduces the cost of installation and duplication. These sensors can be used to control local variable air volume (VAV) boxes reducing fan energy or close local air dampers and reduce the required air delivery from a central air handling unit. Connections to heating and cooling systems allow spaces to be placed into an unoccupied setback temperature for additional savings.
Mandatory Controls
Buildings in Canada must already comply with energy codes which mandate certain automatic control, such as:
- thermostat control for each zone
- no simultaneous heating and cooling within the same zone
- no simultaneous humidification and dehumidification within the same zone
- setback controls - automatically adjust heating and cooling system set points on a programmed occupancy schedule
- automatic shutdown: provide time-of-day start/stop controls for heating/cooling/ ventilation systems
- shut-off dampers: close motorized dampers when ventilation systems stop
- domestic hot water recirculation heating - automatically disable temperature maintenance in hot water pipes when heating is not required
Do your buildings have these control functions? They should.
Conclusion
While control systems can indeed be complex, they are now an essential component of modern buildings and will continue to evolve. Designers and owners should make every effort to understand the technology so they can operate their buildings at their best efficiency.
Enermodal engineering: is one of the largest consulting firms in North America focusing exclusively on the design of sustainable buildings. Among its particular areas of expertise is the design, commissioning and auditing of low energy mechanical and electrical systems.
