We recently stopped using one of our chimney flues, since we removed an oil boiler. On the inside, it was capped with a galvanized cap and furnace cement. My question is, should we completely cap off the top making it shut, or leave the rain cap on, which still allows it to breathe as when it was in use? My concern about capping it completely off is the fact that any moisture that develops inside the clay liner (however it may occur), can’t escape. Of course, my concern about leaving it open is water intrusion during heavy rains. Normally my concerns about any rain intrusion were nullified since the boiler ran throughout the day, heating the flu and drying it out. Now that doesn’t occur. Any thoughts??