In the past, heat gain and loss calculations have been done with only small allowances for time delays involved, storage effect of building structures, and the natural daily variations in outdoor air conditions. Minimum outside air has been provided during all occupied periods. No allowances have been made for the different energy inputs required for cooling. Control operating strategies were to provide minimum or maximum air and water temperatures required for the small period of time at design conditions and due largely to the throttling ranges required (30C (5 F) - 100C (18 F)) for stability of control, temperatures provided for most periods therefore resulted in energy waste. The use of linear reset controllers partially reduces this energy loss but usually results in control cycling and the reset required is normally not linear.

The maintaining of high space temperatures during the morning results in providing maximum cooling during the afternoon when the energy input per unit of cooling is greatest. The maximum daily effective heat gains in both summer and winter occur in the afternoon when the highest outdoor air temperature occurs. Therefore in winter these may be used for ventilating the building, providing humidification and storing heating energy in the stucture and contents. In summer, ventilation may be reduced at this time and cooling storage by the building structure and contents may be done in the morning.

If energy input to buildings is controlled by outside air conditions, simple control systems may be used to operate buildings, dynamically by varying, ventilation rate in accordance with surplus energy available.