The ventilation outlet system for the Glyn Pits was by way of an air duct some 6 ft below the surface of the upcast shaft, which came out of the pit head into a brick-lined air duct, some thirty feet long and approximately five feet cross-section, and which rises out of the ground toward the fan. Foul air was exhausted from the mine through this duct by a small 'Scirocco' type fan, originally steam-driven, and exhausted to atmosphere. In the early days before the introduction of fans, things were much different, for in order to induce clean air into a  mine a furnace had to be lit near the bottom of the upcast shaft. This caused the warm air to rise and the fresh air to enter the mine by way of the downcast shaft. After traversing the mine workings the foul air would come within 40 feet of the ventilation furnace at the bottom of the upcast shaft. Here, a 'Dumb Drift' would be used. A dumb drift was a tunnel which rose at an angle of about 45 degrees, to emerge about 40 or so feet above in the upcast shaft, thus minimizing any chance of the gases coming into contact with the furnace near pit bottom. There is little doubt that this type of ventilation system, or something very similar, was used in the early days at the Glyn. In some mines, in order to prevent hot gases and smoke rising in the up-cast shaft, a separate brick or stone stack was built, usually a  short rather squat construction.