Supply ventilation system design: outside air duct to return side of central fan.

A number of systems exist for ventilation of residential buildings. In general, they can be categorized as supply, exhaust, or balanced ventilation systems. Subcategories include integration into central air distribution ducts, or single- or multi-point air distribution. This effort focused on establishing a design methodology for centrally integrated supply ventilation systems using an outside air duct to the return side of a central air distribution fan. A measurement protocol was developed, and air flow measurements were taken for 25' lengths of 5" through 9" diameter flexible ducts, with a 6" wall-cap air inlet, at duct pressures of -10 Pa to -120 Pa. Based on these measurements and other charts, a four-step design method was developed as a guide for correctly sizing the components of the ventilation system. The steps include: l) establishing the required continuous ventilation air flow; 2) establishing a fan duty-cycle; 3) converting the continuous air flow requirement to an intermittent air flow requ:1rement based on the selected duty-cycle; and 4) establishing the correct size outside air duct and duct pressure k give that intermittent ventilation air flow. It was found that an effective ventilation system can be achieved using a fan recycling control and a 5" to 9" diameter insulated duct from outdoors to the return side of a central air distribution fan.