counterflux ventilation
Ventilation in Victorian chimneys is complicated. The chimney is often interconnected with the other chimneys in the house, meeting at the first floor, rather than at the stack. Because of the presence of dead chimney urchins, the downdraft can impact the convex heat distribution. For this reason, counterflux ventilation is required.
Oh Tarquin, we simply must get that chap round to sort the counterflux ventilation