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??
Unused flue
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kennyp2339
Minister of Fire
I'd cap it, prevent bee's and other animals from eventually entering it. I would be worried about moisture build, masonry sweats and contracts just like everything else, Think of your foundation below grade.
I just have a bent piece of sheet metal over the top of an unused flue. It's not sealed completely, and I'm not concerned about any condensation because I think it is a dummy flue and doesn't get any air from inside the house, as far as I know.
Is your chimney interior or on an outside wall? If it's interior, I wouldn't think condensation would be a problem.
I don't know much about this issue, just guessing here...🤔
Is your chimney interior or on an outside wall? If it's interior, I wouldn't think condensation would be a problem.
I don't know much about this issue, just guessing here...🤔
So, I’ve done a little research and put some thought into this. I’m thinking it might be a good idea to leave the screened cap on the top, so it can vent. And not seal it completely air tight. That way if moisture gets in through the brick, mortar, or rain through the mesh cap (sideways blowing), it has a place to vent out through the top. It’s sealed from the bottom, but I would imagine that’s no different than having a fireplace with the damper shut at the bottom.
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