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Old 08-11-2010, 11:16 AM   #1
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How should I pitch my pipe?

Installing a bathroom exhaust.
My duct is running horizontal for 7 feet and elbowing up 2 1/2 feet through the roof? The entire run is insulated.
If I slope away from the fan I'm afraid I'll have water/condensate sitting at the elbow. If I slope towards the fan I'm afraid I'll get dripping out of the fan.
Which direction should I pitch my horizontal duct?


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Old 08-11-2010, 07:21 PM   #2
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Use 4", not 3". Use a timer. Insulate the fan box and ducting, with a vapor barrier. Use 45* elbows instead of 90*s. Use straight pipe, not flex. If flex- go 1 size bigger. Foil tape all joints, even the 45* individual ones against air. Read about the thicker walled abs pipe. Flex at the end of run has twice as much surface volume to trap water and reduces the air-flow by causing turbulence depositing more moisture on the pipe walls. Be sure the fan is rated for the bathroom. Be sure a back-draft damper is on the duct termination, not just the fan housing to prevent outside air into the pipe with moisture to condense even when fan is not operating.
I would pitch it straight from low to high. The farther from the fan, the less steep the angle because the air pressure is weakest there, most likely to deposit the moisture on the pipe walls.

Be safe, Gary


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17,000 dryer fires a year, when did you last clean the inside of the dryer near motor or the exhaust ducting?
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Old 08-11-2010, 09:10 PM   #3
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I plan on doing it as you outlined - without all the insulation. The entire run is in interior (heated) space. Are you saying I should use ABS pipe instead of galvanized duct? What about PVC pipe? (I've never used abs pipe)

Last edited by buildjack; 08-11-2010 at 09:13 PM.
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