This is a theoretical study on how to improve the optimisation of the all-air system. The economizer cycle was designed to distribute heat and cold with the supply air. To reduce the investment in heating and cooling capacity the outdoor air rate was reduced to the minimum outdoor air rate during high and low temperatures. This was as close to the economical optimum as possible with the control equipment available in the 1920-thies. An improved optimisation gives that the optimal outdoor air rate in the winter is higher than in the summer, because cooling is more expensive than heating. The optimal outdoor air rate is higher than the free outdoor air rate during free-cooling and the optimal outdoor air rate is higher than the minimum or acceptable outdoor air rate, except at the hot outdoor temperature. It is also possible to calculate better design temperatures, than the two and three design temperatures given in ASHRAE Handbook, if the derived equations are solved.