Abstract: Supplemental ventilation apparatus is provided for manufactured housing whose window and door openings are tightly sealed when closed. Exterior air is drawn in by the furnace's room air circulating blower through a flexible duct flattened wideningly at its base to fit removably on the upstanding flange of a dampered inlet to the furnace's upper air inlet chamber, which is installed in the narrow top wall surface of the furnace forward of its accessory-mounting portion. When combined with an adequately powered attic exhaust system, negative pressure, which such exhaust system creates in the attic, acts through openings in the ceiling to draw off positive room air pressure. Thus stale room air is withdrawn and exhausted through the attic, without any increase in capacity of the room air circulating blower.