This is a more sophisticated use of the on/off controls of all the air- handling units in a building, typical for a dormitory type of building. Instead of a complete cutoff of power to a unit, the thermostat setting is set back 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 (i.e., the coldest temperature of the day) often occurs shortly after the building 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.
In conjunction with this method of operation, termed night setback, the controls typically have a morning warmup cycle. This feature turns the equipment on an hour or so before wakeup so that it’s comfortable then. For equipment with damper controls for the outside-air intake to the unit, this damper is closed during the warmup cycle.
Another frequent use of the central control of air handlers is to turn the equipment off—or to 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, maintains space temperatures within the comfort zone for the balance of the day. This effect 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, makes the space more comfortable for the occupants. Systems that serve low-occupancy spaces can also close the outside-air dampers during the end-of-day shutdown period. This reduces the latent load on the coils, and makes it possible for temperature to be maintained for a longer time. The reduction in fresh air should not affect the occupants, either. Any deficit is made up during the night from natural infiltration.
This level of sophistication, encompassing the outside-air intakes for the air handlers, makes another feature available in the cooling season. When the ambient temperature at night drops below the temperature of air in the building, the air handlers can be run for an hour or two early in the morning to replace the complete volume of air in the facility with colder fresh air. This should be done with caution, however, because if the air is more humid than the building air, the net effect is an increase in cooling load. It’s best to control this option with an enthalpy sensor to ensure energy is conserved. (Enthalpy is a psychometric quantity that combines temperature and humidity.)
Another advantage a central EMS system provides for optimum start/ stop of air-handling units is in the scheduling of the equipment through the year. As the outside-air ambient temperature changes, the length of the morning warmup cycle needed to prepare the building for occupancy changes. In very cold weather, for example, the equipment must be turned on sooner—even with a night setback— than during mild weather. The EMS system can change the night setback temperature according to the outside temperature to eke out a little more downtime for the HVAC system, and increase utility savings for the customer.
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Excerpt from Energy Conservation Projects.