an opening area of the inlet hole 101 is preferably formed to be at least 375 of a total opening area of the duct holes 115 or 125 formed at the inlet duct 110 or 120 the relation between the upper and lower length l of the inlet hole 101 and diameter d of the rear plate and the other relation between the opening areas of the inlet hole and duct holes are explained in detail by referring to fig1 and fig1 as follows referring to fig1 a diameter d of a rear plate 100 like a general rear plate applied to a general laundry dryer is set up as 660 mm an inlet hole indicated as u2018 gas type u2019 in fig1 has the same size of the inlet hole used for the laundry dryer according to the background art where a distribution height of the inlet hole duct hole is 200 mm a distance between a lowest end of the rear plate and a lower end of the inlet hole duct hole is a combustion section and should be at least 350 mm being advantageously positioned lower the distance is generally designed to be 350 mm a position of the inlet hole duct hole is about 150 mm from the.