Got a chimney to do in 3weeks when I do a chinney I like to do a proper job don't whant any come back up ther. Mistake so many builders do is block the flues up I always believe you should leave them open. You ither put a elephant foot that are about 40 50 pound or brick vent. But I've seen this idea sink waste pipe which coming through the flue whith bend to stop water going down cheap way of doing it and you get air circulation at the end of day nobody going to see it only the seagles and people going on holiday. Has anybody used this method before I do it lol by the way if it a brick chimney il put a brick vent no problem. if it stone difrent story.