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I replaced my hvac and noticed that my flue is dripping water. Wondering what might cause it? I assume some sort of condensation, but I don’t know what should I do to fix it.

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  • Dripping from inside the pipe to the outside of your home? Or inside your home? If it's a horizontal flue to a high-efficiency furnace, that may be normal. – spuck Dec 4 '19 at 16:10
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If you installed a high efficiency sealed combustion chamber system the flue should have been changed to pvc. The old school ducting should not have passed inspection as it is not designed to be pressurized, there is not much pressure but it is there. Actually there should be both a supply line and the exhaust, normally connected to a coaxial roof jack so the system has fresh air and can exhaust without creating pressure differentials. In my state this install would be a code violation on several levels.

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  • I assumed it was not high efficiency furnace because it would be so easy to put PVC through the wall near the furnace ( pitched to the outside). My brother had one years ago and the odd thing was that a "purring" sound like a engine came from the pipe, along with the water. – blacksmith37 Dec 4 '19 at 17:55
  • It is not high efficiency – Andrew Dec 5 '19 at 6:33
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Condensation of water from the combustion gases , the flue is too cool. The flue pipe needs to be insulated to keep it hot, either replace with double wall pipe ( type B flue may be enough) or wrapped with a FIRE PROOF insulation.You could change the pitch ( raise the close end) but that will just run the water back down to the furnace, not likely to be good. The new looking pipe indicates a new furnace; newer furnaces are more efficient - get more heat out so cool the flue gas more. And most newer furnaces have a forced draft fan which also cools the flue gas more. I just got a new furnace ; the old one had a draft fan but the new one has a bigger fan ( the contractor said it was required to be larger).

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