A parallel redundant fan system comprising: at least first and second fans located adjacent each other and each comprising: (i) a fan housing defining a fan chamber in which an impeller is supported for rotation; (ii) a first chamber outlet in communication with said fan chamber and defined by an opening in a first wall of said fan housing; (iii) a second chamber outlet in communication with said fan chamber and defined by an opening in a second wall of said fan housing; (iii) first and second backflow prevention dampers (40a, 40b) respectively associated with said first and second chamber outlets; said first backflow prevention damper comprising: (i) a first damper plate located outside said fan chamber and pivotally connected to said first wall of said fan housing, said first damper plate pivotable about a first pivot axis between a closed position where it is positioned in covering relation with said first chamber outlet and an opened position where it is pivoted away from said first chamber outlet to allow exhaust air flow through said first chamber outlet from said fan chamber; and (ii) a first counterweight connected to said first damper plate that exerts a closing force on said first damper plate that urges said first damper plate toward its closed position; said second backflow prevention damper comprising: (i) a second damper plate located outside said fan chamber and pivotally connected to said second wall of said fan housing, said second damper plate pivotable about a second pivot axis between a closed position where it is positioned in covering relation with said second chamber outlet and an opened position where it is pivoted away from said second chamber outlet to allow exhaust air flow through said second chamber outlet from said fan chamber; and (ii) a second counterweight connected to said second damper plate that exerts a closing force on said second damper plate that urges said second damper plate toward its closed position; wherein said first and second damper plates selectively pivot from their closed positions toward their opened positions against the closing force of the first and second counterweights, respectively, in response to exhaust air flow pressure exerted on the first and second damper plates by exhaust air flowing from said fan chamber through said first and second chamber outlets, wherein said first and second pivot axes are located vertically below a lower edge of the first and second chamber outlets, and wherein said first and second backflow prevention dampers define respective centers of gravity that are located vertically above said first and second pivot axes and that are located horizontally between said impeller and first and second vertical planes that respectively include said first and second pivot axes. at least first and second fans located adjacent each other and each comprising: (i) a fan housing defining a fan chamber in which an impeller is supported for rotation; (ii) a first chamber outlet in communication with said fan chamber and defined by an opening in a first wall of said fan housing; (iii) a second chamber outlet in communication with said fan chamber and defined by an opening in a second wall of said fan housing; (iii) first and second backflow prevention dampers (40a, 40b) respectively associated with said first and second chamber outlets; said first backflow prevention damper comprising: (i) a first damper plate located outside said fan chamber and pivotally connected to said first wall of said fan housing, said first damper plate pivotable about a first pivot axis between a closed position where it is positioned in covering relation with said first chamber outlet and an opened position where it is pivoted away from said first chamber outlet to allow exhaust air flow through said first chamber outlet from said fan chamber; and (ii) a first counterweight connected to said first damper plate that exerts a closing force on said first damper plate that urges said first damper plate toward its closed position; said second backflow prevention damper comprising: (i) a second damper plate located outside said fan chamber and pivotally connected to said second wall of said fan housing, said second damper plate pivotable about a second pivot axis between a closed position where it is positioned in covering relation with said second chamber outlet and an opened position where it is pivoted away from said second chamber outlet to allow exhaust air flow through said second chamber outlet from said fan chamber; and (ii) a second counterweight connected to said second damper plate that exerts a closing force on said second damper plate that urges said second damper plate toward its closed position; wherein said first and second damper plates selectively pivot from their closed positions toward their opened positions against the closing force of the first and second counterweights, respectively, in response to exhaust air flow pressure exerted on the first and second damper plates by exhaust air flowing from said fan chamber through said first and second chamber outlets, wherein said first and second pivot axes are located vertically below a lower edge of the first and second chamber outlets, and wherein said first and second backflow prevention dampers define respective centers of gravity that are located vertically above said first and second pivot axes and that are located horizontally between said impeller and first and second vertical planes that respectively include said first and second pivot axes.