what size flue

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Billy123

Member
Mar 5, 2011
91
PA
What are the standard flue sizes? My furnace and fireplace are side by side going up. The furnace looks square but the fireplace looks rectangle.
 
Depending on where you live, sizes vary, out west they use a lot of round and oval, in the east, square and rectangle. I believe you'll find that the OD will run about 8" x 8", 8" x 13", 13" x 13", 13" x 18", and 18" x 18". All of those numbers can vary a little. My house has a 12" x 12" and the ID is about 10.5".
It's not unusual to have different sizes, and have the fireplace with a larger flue than the furnace.
 
You're going to need to measure. The 'standard' was not very standard over time and across regions.

While you are up there shine a bright light down there and verify that it is in fact straight and there are no broken sections of flue liner in the way.
 
Depends on the size of the furnace and fireplace, but an 8x8 tile is common for furnace installations.
 
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