The feed duct (1) for a straw firing furnace comprises a slide valve (10) with a valve member (13) for closing the duct, a sprinkler (21) in a wall of the duct upstream of the slide valve, a water supply pipe (22) for the sprinkler, and means (14) for control and activation of the opening and closing of the valve member. The sprinker (2) is positioned close to the valve member and adapted to sprinkle said valve member by water, and the bottom (23) of the housing (12) of the slide valve (10) is positioned lower than the bottom (9) of the feed duct and is provided with drain openings (24) for draining off water supplied through the sprinkler.