Blocking a chimney flue is convenient that the width of the mouth of the home is 1/5 or 1/6 greater than the height of the same. This does not apply to small stoves that may have a square shaped mouth. Maximum irradiation of the heat towards the outside: it is achieved giving the sidewalls of the home, an oblique direction, of 27 degrees more or less, until touching the point of the upper wall.
Blocking chimney flue,
The rear wall shall rise vertically to a height equal to one third of the height of the blocking a chimney flue mouth. From that height the back wall will be inclined towards the front, forming with the horizontal a 60 degree angle and will continue to surpass 10 or 15 centimeters the level of the lintel of the hearth. This does not apply to small stoves that may have a square shaped mouth.
Blocking a chimney flue – Considerations to take into account when building a stove or fireplace to firewood. Great combustion: it is achieved taking into account certain measures that relate the dimensions of the home with those of the smoke duct (chimney). The surface or area of the mouth will generally be ten times greater than that of the chimney, although this ratio may be increased twelve times when the chimney reaches a height of more than nine meters.