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However, HVAC systems are sized to meet design heating and cooling conditions that historically occur only 1% to 2.5% of the time.Thus, HVAC systems are intentionally oversized at least 97.5% to 99% of the time.Energy-efficient, climate responsive construction requires a whole building perspective that integrates architectural and engineering concerns early in the design process.For example, the evaluation of a building envelope design must consider its effect on cooling loads and daylighting.Whenever possible, design systems to take advantage of this situation.For example, energy management systems can shed non-critical loads at peak periods to prevent short duration electrical demands from affecting energy bills for the entire year.A highly energy-efficient building using conventional comfort could have an energy use of 40,000 Btu//yr. A building designed and operated with extended comfort strategies might only use 20,000 to 30,000 Btu//yr.However, note that highly energy-efficient design utilizing high-performance HVAC equipment often requires more effort and more collaboration from the design team than a conventional, sequential approach.

If the comfort zone is extended through natural ventilation and air movement in summer, and through lower air temperatures in winter (made possible by highly-insulated and, therefore, warmer wall and window surfaces), even higher savings can be achieved.In addition, most equipment is further oversized to handle pick-up loads and to provide a factor of safety.Therefore, systems almost never operate at full load.Part-load performance of equipment is a critical consideration for HVAC sizing.Most heating and cooling equipment only operate at their rated, peak efficiency when fully loaded (that is, working near their maximum output).

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