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Old 04-25-2008, 11:54 AM   #1
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Basement Gas Furnace Flue

We had a new HE gas furnace installed a couple of years ago to replace the existing oil furnace. A brick flue is available a couple of feet from the furnace, yet the installers ran pvc about 20 feet to the outside. The pvc goes from a 2" exhaust and immediately converts to 4" pvc for the 20 foot run which has a minimal slope.

Question: Why was the 2" converted to 4"?

Question: Why not keep it to 2" and go directly into the brick flue? No problem also running the pvc inside the flue.


Last edited by tjeieio; 04-25-2008 at 11:54 AM. Reason: misworded
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Old 05-03-2008, 08:18 PM   #2
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QuestionWhy was the 2" converted to 4"?

QuestionWhy not keep it to 2" and go directly into the brick flue? No problem also running the pvc inside the flue. 
The size of pvc is based on manufactuer of equip. If you have the install manuel, they normally have a chart that gives specific guidelines. It's based on btu's of furnace, length of pipe, and number of 90's and 45's in the run of pvc. Although, generally speaking, 3" is used on most furnaces over 80,000 btu's.


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Old 05-03-2008, 10:13 PM   #3
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The high-efficiency units aren't designed or listed to vent into or through a chimney in most cases. Venting it that way would likely get highly scrutinized if the inspector knows what he's doing. There is a chance that it would work, but I doubt that the manufacturer would support such an installation.

Your installer did it right.
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