Rooftop packaged systems are normally designed to handle a building's total cooling and heating loads at design weather conditions, which are based on climate extremes at the building's location. Unfortunately, according to the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE), design conditions rarely occur during the year - only 2.5% to 5% of the time, making many rooftop systems inefficient.
The challenge, then, is selecting a packaged rooftop unit that can handle the peak design cooling load and also maintain control of indoor comfort during the non-peak times of the year.