Each of these pressures and the associated volumetric flow through each of the three dampers (outdoor, recirculation, and relief air) was modelled, assuming a tracking return fan which maintained a constant offset relative to the supply. The second phase of the analysis involved the study of a commercial building in six geographic locations to determine the hours of operation vs. supply air quantity during times when the economiser cycle control would call for minimum air. This phase predicted a substantial number of hours with deficient quantities of OA in all six climate zones. The conclusion of this research clearly points to the need for modulating control of just the recirculated air damper (not the relief and outdoor dampers) in systems with economiser cycles when in the minimum air mode. Systems that use the cooling tower to produce free cooling also need modulating control of the recirculated air damper.

KEYWORDS: air flow, variable volume air conditioning, air conditioning, dampers, controls, outdoor air, cooling towers, free cooling, economisers, recirculating.