Width of flues for hothouse furnaces

2113. The size of flues is seldom less than 9 in. wide, by 14 in. or 18 in. high inside measure, which suits a furnace for good coal, whose floor or chamber is 2 ft. long, 18 in. wide, and 18 in. high. According as the object varies, so must the proportion both of furnaces and flues.