Many of our apartments have gravity ventilation, and in some also flue gas ducts discharging fumes from gas-fired devices, e.g.. bath heaters. To check the patency of these channels, stick a burning match or a lighter flame to the grates protecting the channels. With a working order, open channel, the flame will be clearly deflected into the grille as a result of the air draft, and with very strong thrust it will even go out. In the absence of draft, the flame will not deflect.