SD-95-02-1 -- Minimum Outside Air Control Methods for VAV Systems ✓ Most Recent

Why the concern in recent years about minimum outside air control when it never used to be an issue? Since 1973, various editions of ASHRAE Standard 62 (and before 1973, ASHRAE Guide and Data Books) have established criteria for determining the amount of outside air ventilation necessary to achieve acceptable indoor air quantity. That amount is the "minimum outside air quantity". In a constant-volume system, once that quantity has been properly determined and the system has been set up to take in that minimum outside air quantity, there was always enough outside air ventilation to dilute contaminants generated with the building - unless some sort of mechanical failure occurred or someone changed the controls. That is because the laws of physics dictate that flow always goes from a high pressure to a low pressure and that, for a fixed geometry, the flow will always be proportional to the square root of the pressure difference. But the growing popularity of variable-air-volume (VAV) systems has introduced new factors into the equation and new challenges for designers. Considers only the minimum outside air quantity for ventilation and does not address system operation during the outside air economiser cycle. When the system is on an outside economiser, the outside air quantity is based on the need for cooling and is generally greater than the minimum required for ventilation. If, during very cold weather, satisfying the temperature requirement would mean less outside air than the minimum ventilation quantity, then whatever minimum outside air control strategy is selected would come into play. For simplicity and clarity, the discussion completely ignores the economiser cycle since the concern is with maintaining minimum ventilation. Similarly, the discussion is in terms of systems with return fans. A relief fan that provides power exhaust to outdoors may be used instead of a return fan, or there may be a single supply fan with neither a return fan nor a relief fan. Generally, the issues of stable measurement and control are the same as those discussed with a return fan.

KEYWORDS: year 1995, minimum, outdoor air, controls, variable volume air conditioning, ventilation, fans, air flow, economisers, air quality, indoor