The following overview offers a quick reference to key considerations with some of the most effective technologies. As with lighting, trial installations are a good idea; so is working with manufacturers and distributors.
Getting the most from HVAC controls
Because a building’s performance can be dramatically improved by installing and fully using HVAC controls, it is essential to understand and correctly use those controls. The place to start is with a close look at what is really transpiring in your building, 24 hours a day, seven days a week.
What is happening with each piece of equipment? On holidays? Weekends? As the seasons change, do your operations change? It is important to understand where and how energy is being consumed in order to identify where waste is occurring and where improvements can be implemented. Then it is imperative to ask, “What exactly do I want these controls to do?”
Energy management systems (EMS) are designed to run individual pieces of equipment more efficiently and to permit integration of equipment, enhancing performance of the system. In a typical EMS, sensors monitor parameters such as air and water temperatures, pressures, humidity levels, flow rates, and power consumption. From those performance points, electrical and mechanical equipment run times and setpoints are controlled.
Seven-day scheduling provides hour-to-hour and day-to-day control of HVAC and lighting systems and can account for holidays and seasonal changes. As the name implies, night temperature setback allows for less cooling in summer and less heating in winter during unoccupied hours.
Optimal start/stop enables the entire system to look ahead several hours and, relative to current conditions, make decisions about how to proceed; this allows the system to ramp up slowly, avoiding morning demand spikes or unnecessary run times.
Peak electrical demand can be controlled by sequencing fans and pumps to start up one by one rather than all at once and by shutting off pieces of HVAC equipment for short periods (up to 30 minutes), which should only minimally affect space temperature. Economizers reduce cooling costs by taking advantage of cool outdoor air. Supply-air temperature-reset can prevent excessive reheat and help reduce chiller load.
An EMS can provide an abundance of information about building performance, but someone has to figure out what they want the EMS to do and then give it directions. Calibrating controls, testing and balancing are key to any well-maintained HVAC system, but are especially critical to optimize control efforts.
Variable speed drives and energy-efficient motors
Variable speed drives (VSDs) are nearly always recommended as a reliable and cost-effective upgrade.
VSDs are profitable where equipment is oversized or frequently operates at part-load conditions. Savings of up to 70 percent can be achieved by installing VSDs on fan motors operating at part-load conditions. They may be applied to compressor or pump motors and are generally used in variable air volume (VAV) systems. They are also cost effective in water-side applications. Backward-inclined and airfoiled fans are the best VSD candidates.
Air-handler configurations controlled by variable inlet vanes or outlet dampers squander energy at part-load conditions. Using throttle valves to reduce flow for smaller pumping loads is also inefficient. The efficiency of motors begins to drop off steeply when they run at less than 75 percent of full load; they can consume over twice as much power as the load requires. VSDs operate electronically and continually adjust motor speed to match load.
The power to run the VSD is proportional to the cube of the speed (or flow), which is why this technology is so efficient. If the speed is reduced by just 10 percent, a 27 percent drop in power consumption should result. A VSD pilot study performed by EPA found that VSD retrofits realized an annual average energy savings of 52 percent, an average demand savings of 27 percent and a 2.5-year simple payback.
Perform harmonic, power factor, electric load, and torsional analyses before selecting a VSD. Though harmonic and power factor problems are not common in VSD applications, VSDs should generally be equipped with integral harmonic filters (or a three-phase AC line reactor) and internal power factor correction capacitors (or a single capacitor on the VSDs’ main power line). In general, this equipment is not standard and must be specified.
Improved design and better materials enhance the performance of energy-efficient motors, which use 3 to 8 percent less energy than standard motors; units with efficiencies of 95 percent are available.
To achieve maximum savings, the motor must also be properly matched with its load, increasing run time at peak efficiency. Motors operate best when running at 75 to 100 percent of their fully rated load; motors routinely operating below 60 percent of rated capacity are prime candidates for retrofit. For motors whose loads fluctuate, VSDs should also be considered.
Smaller, more efficient motors are integral to a system downsizing stratagem; downsizing a 75 horsepower standard motor to a 40 horsepower energy-efficient model will result in energy savings of 15 percent.
Some energy-efficient motors have less “slip” than standard-efficiency motors, causing energy-efficient motors to run at slightly higher speeds; consider a larger pulley to compensate for the higher speed and to maximize energy savings. Installing a new pulley or adjusting the existing one can also be an alternative to a VSD when the cost for the VSD is prohibitive or the load has been reduced.
Improving fan system performance
A common way to improve the efficiency of the air distribution system is to convert constant air volume (CAV) systems to VAV. One authority on energy issues, E-Source, reports that “typical (VAV) air flow requirements are only about 60 percent of full CAV flow.”
VAVs respond to load requirements by varying the volume of the air through a combination of pressure controls and dampers rather than by varying the air’s temperature. According to the air pressure, fan power and volume of conditioned air are reduced, thus increasing energy efficiency. Of course, it is crucial to maintain indoor air quality (IAQ) when altering air handling systems.
To maximize savings, VAV components such as VSDs, variable-pitch fan blades, diffusers, mixers, and VAV boxes must be operating properly; careful zoning is also required to achieve VAV optimization.
E-Source recommends considering the following VAV retrofit procedures:
• complete load reduction measures and calculate the maximum and minimum air flow requirements,
• measure existing fan performance; examine duct system for possible improvements,
• stage fans that are in parallel configurations,
• commission the system thoroughly,
• optimize static pressure setpoint and implement reset control, and
• possibly remove return air fans.
Energy-efficient and properly sized motors are also recommended along with careful control strategies. Installing a self-contained, thermally powered device to each diffuser can add greater control to VAV systems by controlling individual spaces, rather than entire zones, and eliminate the need for VAV boxes. Such a device also offers VAV-style capabilities to CAV systems. Hvac supplies near me