4. The 1:8 rule applied

In the diagram at right, A. is an open fire prone to smoke problems and 'blow-back' because at a ratio of 1:10 the aperture is too large for the chimney.
XXIn B. a convector fire of the Jetmaster or Rutland type has been installed. bringing the ratio down to 1:8 The smoke problem has disappeared and the fire now lights easily, draws well and suffers little or no blow-down.
XXIn C. a small stove has been installed. With the door of the stove wide open during refuelling the ratio is only 1:6 with the result that air rushes into the stove and up the chimney with considerable force.
XXIn all three examples, the same chimney is being used but its performance alters dramatically according to the size of opening it serves.