each draw approximately 33 of the maximum amount of air that can flow through the outdoor air inlet 12 in this system a switch within the sensor assembly 56 operates to energize the first exhaust fan 48 when the outdoor air damper 16 opens to substantially one third of its extreme open position activation of the first exhaust fan 48 creates a sizeable pressure differential across section 58 of the barometric relief damper 46 see fig2 thereby opening that damper section so that air will be drawn from the return air inlet 36 through the exhaust air outlet 44 as shown by line 64 in fig4 this results in the static air pressure within zone 30 dropping to substantially zero the barrier wall 52 shown in fig2 prevents air from being drawn in through the inactive second exhaust fan 50 by the negative pressure produced between the active exhaust fan 48 and section 58 of the barometric relief damper 46 thus the activated exhaust fan 48 will draw air only from the return air inlet 36 as the outdoor air damper 16 continues to open allowing more outdoor air into the zone and the return air damper 40 continues to close proportionally the static.