What should be the correct chimney flue diameter?

Home Maintenance Questions and AnswersCategory: HeatingWhat should be the correct chimney flue diameter?
Question by Joey

I have a question that I hope you can help me with. My neighbor and I share a chimney my heat is gas forced hot air with a power blower exhaust on it. I’m getting water spots through the chimney wall on the 3rd floor. I was told to put a toper on the chimney. The out side dimensions of the chimney are 30in x 39in The contracture wants to fabricate a flat piece of aluminum with two 8in vent caps in the center of the chimney instead of one big cap over the whole chimney. My vent off my heating unit is a 3in so the contractor told me that will be more then sufficient venting. The issue I’m having with doing this is that my neighbor dues not use there side of the chimney anymore they switched to electric if the two 8” round isn’t enough will my exhaust go up my side around the brick divider and down there side of there chimney or will the natural gas exhaust go out the top. The neighbor douse not want me to close off there side of the chimney. Thanks for any help

1 Answers
Dariusz Rudnicki Staff answered 9 months ago

Hi,
If the diameter of the chimney flue is 8” and your furnace is the only appliance venting through it, the chimney may be too large. To give you that answer I’d need your furnace’s BTU’s rating, vent pipe horizontal length (from furnace to chimney wall), and approximate (as much as possible) height from the top of the furnace to the top of the chimney (your HVAC contractor should be able to calculate that).
Oversized chimney might be responsible for condensation and moisture / water spots on walls. It is also possible that its liner (if it has a liner) has cracks, has deteriorated, and the chimney requires re-lining to stop leakage.
Assuming the diameter is correct, liner in a good condition, chimney walls and roof flashings around it are fine, it might be sufficient just to install a rain cap on your side of the chimney. However, if the top part of the chimney (its crown) and any of the other 3 things require attention, it might not be enough to correct only your ½ of the problems to stop leaking.
I would get a certified chimney sweep opinion before investing in any repairs.
If your neighbors chimney has been sealed at the bottom (inside their home) there should be no problems with proper draft (it might still cause leaks on your side). If it’s still open, ask them to seal it or have your side of the flue extended one foot higher than it is right know and a rain cap installed. Maybe your neighbor would go just for a rain cap instead of completely sealing his chimney off.

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