Controls Strategies


SchedulingScheduling is the practice of turning equipment on or off depending on time of day or day of the week. Improving equipment schedules is one of the most effective measures for saving energy in commercial buildings. A feature called "Optimum Start," can increase energy savings by automatically starting a system no earlier than necessary based on daily variations in the weather.


ResetsHVAC systems typically use less energy when their operating parameters are adjusted to meet the building load. Because this load varies with the weather, a BAS can help equipment to operate at greater efficiency levels by automatically varying these operating parameters. The simplest approach is to use a proportional reset schedule based on outdoor temperature. Although that method works reasonably well, a more effective method is to base resets directly on building loads. Examples of building control parameters that can be reset include supply-air and discharge-air temperature for fan systems that use terminal reheat, hot-deck and cold-deck temperatures for multizone HVAC systems, and heating-water supply temperature.


Demand limiting- Because electrical demand charges can comprise 40 percent or more of a utility bill, many building automation systems can benefit from demand-limiting or load-shedding functions. For example, when the demand on a building meter or piece of equipment, such as a chiller, approaches a predetermined setpoint, the BAS does not allow the equipment to load up any further. In buildings with electric heat, electrical demand charges can be reduced if the heat is staged on in the morning over a several-hour period starting with the coldest spaces first. Other demand-limiting strategies are expected to be developed as utilities implement time-of-day or real-time electrical pricing in their rate structures.


Direct digital control (DDC)Direct digital control is provided by a BAS that directly controls valves, dampers, and other system components for building temperature control. The advantage of this approach is that a more advanced control algorithm called PID can be implemented in the BAS's computer code. If applied properly, PID can both save energy and provide improved comfort.