Bathroom exhaust ducting plan

[OZ324]

Not sure if this is the right forum to post this, but you all have been so good with everything else I thought it worth a shot.

I'm in the process of roughing in a new bathroom in my basement. I'm looking for feedback on my exhaust fan plan. Bathroom is 5x11, eight foot walls. The fan will sit in the joist bay (obviously) and the duct will run about one foot in the bay to the wall, where I then need to drop the duct below the joist to run it along the wall in a chase for about 18 feet. At that point I hit a plumbing obstruction (main stack). I plan to use flexible duct for the first two feet, PVC for the next 15ish and flexible again at the end to get around the stack. There's an existing vent cap in the rim joist which means the duct will need to go back up at the very end to connect to it. To sum up, it will go down, over and back up for a total run of around 18-20'. No 90s if i use the flex stuff, but will use PVC in the middle. I plan on insulating the entire thing. Fan will be minimum 110CFM (probably Panasonic).

Am I missing anything? Are there any problems I'm not noticing?

Thank you.

 

[PorcelainVacation]

I have that Panasonic fan. The instructions say you are supposed to slope the ducting such that you don't get moisture condensing and collecting in a low point on the pipe- it needs to be able to drip out. A run that long almost certainly will cool the air enough to condense water out.

 

[Sponsor]

 

[OZ324]

Drip out? I thought it was to be slopped away from the fan so can't come back towards it. I assumed water in the low spot would evaporate over time. It won't be a heavily used bathroom either.

 

[CountryBumkin]

I think the point is that there should not be a dip ("U") or sag in the exhaust piping that could allow moisture to accumulate and sit there. If you had to change directions it would be best to have a continuous slope (say 1/4" per slope "up" from fan to max height above ceiling as needed, then you could change directions maintaining a continuous slope downward to the outside wall exhaust port. Any moisture in either section of pipe could then flow freely out.

 

[PorcelainVacation]

The Panasonic fan is designed for vertical stack mounting too, so it's acceptable to drip back into the fan housing. It's not as good as out, but it won't hurt the fan. The point is mainly to have some slope and no U traps.