What is the purpose of the 'relief slot' at the bottom of an open-hearth gas fire? I've recently had my chimney swept and noticed that the backplate had no opening at the bottom. I subsequently read the instructions for my Cannon K 26 Misermatic (for the first time) where it says the relief slot must not be obstructed.The chimney had not been swept for at least twenty years and nobody seems to have suffered. It must have been by installed by a competent person. Is there anything to worry about or is the slot just to improve the draw (which seems to be good anyway)?