below about 0 4 to 0 45 inches water column while there continues to be no bypass flow with the bypass dampers 28 closed in this condition which is shown in fig2 there is obtained minimum delivery of outdoor air of for example about 40 percent with the outside dampers 24 almost closed and the return dampers almost fully open while the bypass dampers remain fully closed then to supply additional outside air the outside air recirculated air and the bypass air are all simultaneously modulated with the outside air return air and bypass air dampers 24 30 and 28 so that the predetermined pressure drop is maintained by not permitting the pressure drop to fall below a slightly higher figure of about 0 45 to 0 55 inches water column when there is both bypass flow and recirculation in this mode which is shown in fig3 there is then available anywhere from about 50 to 90 percent outdoor air thus giving greater flexibility to matching the outside air needs to the amount of outside air that can be cost effectively delivered when heating of the outside air and building space is required it will also be appreciated that in each of the above modes the air may be chilled downstream of the burner by operation of the water cooler 36 to provide cooling as well as heating and ventilation moreover it will be appreciated that since all the operating conditions are determined empirically from the outside bypass and return dampers there is no need for sensors and feedback control to maintain the desired pressure drop across the burner and.