A venting system for gas fired heating plants including a diverter box, conduit means for conducting gas from one heating plant upwardly in one leg of an inverted U pattern and downwardly into the second leg of such pattern while admitting air at the bottom of the box to form a heat lock to restrict the flow of air in this pattern, and a second conduit means for conducting gas from a second heating plant upwardly and then downwardly of the second leg of the pattern for the first heating plant to a level at which gas may be passed within the box from the inlet to the outlet of the box, thus providing a structure which avoids the blowing out of the pilot flame in one or another of the heating plants.