AC Control

Tips & Tricks

Controls
The ultimate objective of any serious energy conservation program for a sizeable facility is a central, computer automated electronic controls system. This integrated system of remote sensors and control devices permits the optimum use of energy in all areas while simultaneously providing the best environment for building occupants.

• Optimized start/stop of air handling units
  This is a more sophisticated use of the on/off controls of all the air handling units in a building. Instead of a complete cut off of power to a unit the thermostat setting is setback at night and on weekends. The advantage over full on/off controls is that when units are turned off all night they must work extra hard to return the space to the comfort zone in the morning. This is especially important during the heating season, when the peak load often occurs shortly after the office opens in the morning.
  
  Operation of the equipment to maintain a nominal space temperature all night reduces the energy needed to start up, so the equipment can be sized to satisfy a smaller peak load. This, in turn, translates into less expensive air handlers and ones that operate nearer their most efficient, full load condition.
  
  Another common use of the central control of air handlers is to turn the equipment off, or initiate the night setback sequence, an hour or two before the end of the day. The thermal momentum of the building mass and the volume of air already conditioned will maintain space temperatures within the comfort zone for the balance of the day. This affect is especially useful when the supply and return air fans continue to circulate air after the heating or cooling system is disabled, thereby extracting any residual heat from the circulating fluids or the building mass. The continued movement of air, even as the temperature floats away from the setpoint, will make the space more comfortable to the occupants.
  
  
• Demand limiting
  The demand limiting philosophy is to begin turning off pieces of equipment as the electrical use approaches the peak. The software, already programmed with a prioritized list of items to be turned off, simply follows the list until the energy use curve is leveled off and the peak load passes. Clever programmers will make use of the building mass to provide some thermal momentum during these periods, extracting or rejecting heat to the building while the HVAC is turned off to always maintain a comfortable environment.
  
  
• Peak load shifting
  Some systems accomplish demand limiting by shifting the building load to off peak hours. One way to do this is to run the chillers during the night to chill water that is stored in large tanks on the premises. Then during the peak building load the following day the chillers are turned off and the ready-made chilled water is circulated to the building loop. Other systems make ice in the night and melt it later to chill the loop water.
  
  This same sort of technique can be used on a smaller scale, using simple controls. There are several thermal masses that can be used to store energy during off-peak hours: the building mass, the volume of fluid in the chilled water loop, the volume of cooled air within the building and the humidity of the cooled air in the building. An hour or two before the peak load is expected (based on an average of previous days, or other criteria) the building and its systems are allowed to float below the set point, storing energy that is released for the next few hours until the peak is passed.
  
  The immediate demand on the system is met by the existing air volume, then by the chilled water resident in the closed loop. The air humidity then drifts upward as the warmer air is able to absorb thermal energy from the building environment. Finally, the building mass contributes to the system.
  
  Keep in mind that the peak load that the system is designed to handle typically lasts only a couple of hours. Use of the aforementioned dynamic elements, eg letting the temperature and humidity drift upward in the process, will greatly reduce the daily peak load. Also, since this load is usually at the end of the work day the entire system will be shut down soon and no additional energy will have to be input to make up for the excesses permitted, since the building will equalize with its environment through the night, possibly aided by artificial circulation of outside air.
  
  
• Load leveling
  A plot of a typical day will indicate several peaks and spikes. A peak will show a gradual increase to maximum of, for example, the HVAC system as the building approaches the maximum load. Spikes may indicate the operation of the laundry or kitchen for an hour of intense activity, when copious quantities of hot water are used and generated rapidly, and electric equipment operates at high loading conditions.
  
  The use of energy to complete the necessary daily functions at the facility cannot be avoided. However, the timing is often flexible. Instead of operating the laundry in the middle of the afternoon, for example, when the HVAC is approaching its peak, the laundry can be done earlier in the day. This will not affect the actual energy used, but it will reduce the peak load because the baseline is lower. The lower daily peak, in turn, will reduce the demand fee charged by the utility.
  
  The ability to apply this principle of load leveling depends on a thorough understanding of the energy using equipment at a facility, plus knowledge of the daily routines that happen in every department. The best way for the engineering staff to attain this level of competence is to first document information on each major item of equipment, then follow an explicit maintenance program. Once a strong technical understanding is accomplished, then the facility management can be approached with the load leveling concept. If sufficient support is presented no doubt the decision will be made to reschedule certain activities to reduce the peak usage by shifting a part of the load to off peak hours.
  
  
• Two stage controls
  There are numerous applications for two levels of controls. One example is a large room served by two air handlers. Instead of having both controlled by a single thermostat (which will resort to short cycling and excess energy use) or controlled by separate thermostats, a single controller will activate one unit, then both, as the space load demands. Many manufacturers have programmable thermostats with this function built in, for control of two stage compressors.
  
  Another common application for this simple device is to control a two speed motor of an air handler. The controlling function can be static pressure in the discharge duct to a variable air volume system. This is an inexpensive option to inlet vanes or a variable speed drive, and is a good compromise for system retrofits when the VFD is too costly. If the new two speed motor is a high efficiency model, there may be nearly as much savings as from installation of a VFD anyway, depending on the number of exterior and interior zones served by the air handler.
  
  If an air handler serves only a few zones, then the two speed motor can be interlocked with a space temperature sensor; that is, if the air distribution is not overly affected at the new air flow rate for a constant volume system. An ideal application of this method is a system serving, for example, two operating rooms in a hospital or two classrooms in an academic building. If only one of the spaces is in use the air ducted to the other can be dampered off, the motor put on low speed and the system operates at half capacity to adequately condition the one room.

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