duct 39 s inner surface 14 and the seal 39 s periphery 30 in the partially sealed position air flow past the seal 24 is partially obstructed but air flow is substantially greater compared to any flow that may occur past seal 24 when door 18 and seal 24 are in their sealed position similarly shaft 20 can rotate doors 18 19 about 100 degrees away from the sealed position of door 18 to the sealed position of door 19 at which door 19 and seal 24 substantially obstruct and prevent air flow in duct 18 past door 19 and seal 24 when shaft 20 rotates about axis 22 in either rotary direction to a position at which door 19 and seal 24 are spaced angularly about five degrees from the duct opening door 19 and seal 24 are in their partially sealed position when door 19 and seal 24 are in their partially sealed position air can flow in duct 12 through a small gap between the duct 39 s inner surface 14 and the seal 39 s periphery 30 in the partially sealed position air flow past the seal 24 is partially obstructed but air flow is substantially greater compared to any flow that may occur when door 19 and seal 24 are in their sealed position fig2 is side view showing door 18 in its partially sealed position i e spaced a short distance from the inner surface 14 of duct 12 and door 19 in its sealed position contacting surface 14 fig3 is a side view showing door 18 in its sealed position i e contacting the inner surface 14 of.