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Central Illinois | My new furnace also has a 2 inch outlet but goes to 3 inches right away.
Looking at my manual, it looks like altitude plays a part along with furnace size and number of elbows.
Example: 100,00 BTU unit at 2000 feet altitude , 2 inch pipe with NO elbows ( straight) is 20 feet. Between 4000 and 7000 feet it's 10 feet.*
Edit: (at 2000 feet altitude) 2.5 pipe is 80 feet, and 3 inch pipe is 175 feet , again NO elbows. 2.5 and 3 inch pipe don't lose as much capacity. Edit:(at 4000-7000 feet altitude) 2.5is 60-70 feet, 3 inch is approx. 155- 135 feet.
*That's the way I read the chart anyway. All I know is what I read in the manual.
Mine is 100,000 BTU with 3 elbows and 2 45 degrees and total length is around 30 feet.
Edited by Ron (Cen. IL.) 1/13/2019 18:47
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