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Panasonic Bath Fan Install - Rigid or Flex Duct

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  • Dec 20th, 2018 3:48 pm
[OP]
Jr. Member
Mar 28, 2007
157 posts
92 upvotes

Panasonic Bath Fan Install - Rigid or Flex Duct

I'm upgrading our builder grade fan to a Panasonic FV-08-11VF5

The new Panasonic's duct is on the opposite side from where the original fans was. Would I be okay to use a flex duct to compensate for the misalignment or am I better off using rigid duct with some elbows. The run is very short to the rigid bend that goes vertical into the roof.

I could also cut drywall on the ceiling to make the opening wider and align the duct work correctly but then I'll have to do patch work to cover up the excess space in the opening.

The current system is set up like the picture below with all rigid duct work.

Image

I also want to avoid having go into the attic but I might end up having to if I can't secure the bracket to both joists from below.

The original duct is 3" wide from the fan and 5" at the elbow that takes it vertical through the roof.

The Panasonic has a 4" duct with a 3" adapter.
Last edited by phantom_911 on Dec 20th, 2018 11:36 am, edited 1 time in total.
6 replies
Deal Guru
User avatar
Mar 13, 2004
12627 posts
4236 upvotes
Ontario
For best airflow rigid ducting should be used however the flex one can be used and still work (thats what we did) However I would keep it at the 4" duct size, dont go down to 3" as that will decrease airflow even more.
[OP]
Jr. Member
Mar 28, 2007
157 posts
92 upvotes
That's what I'm finding too, that Rigid is always better and preferred. I'm just wondering if rigid with sharper bends is still better than flex if the flex has a more direct path.
Jr. Member
Feb 9, 2006
164 posts
50 upvotes
Use rigid pipe and use 4" for either the 110 or 80 cfm setting. I think at 110 cfm with 4" you're getting 1.2 sones and at 80cfm with 4" you're getting .8 sones. I would probably go all 5" to my roof vent and also really insulate the pipe.
Deal Guru
Jan 25, 2007
11451 posts
6635 upvotes
Paris
I dont know exhaust fans but I do know dust collectors and the rule there is 1’ of flex is equal to 3’ of ridgid. Also sharp corners are bad. Ideally for a setup like that maybe 2 45 degree bends instead of a 90?
[OP]
Jr. Member
Mar 28, 2007
157 posts
92 upvotes
So I bought a combination of everything to see what will work.

There's not enough space between the fan housing and duct work to add too many fittings.

By the time I add the 5" coupler and 5"-4" reducer it's already at housing.

So I'm going with rigid 5“ coupler and rigid 5" to 4" reducer and then a very short peice of rigid flex between the fan and reducer.

The fan and reducer are practically touching inside the coupler.

If this doesn't work then it's best I just cut the drywall to align the fan directly.

Rigid Flex
Last edited by phantom_911 on Dec 21st, 2018 9:00 am, edited 2 times in total.
Deal Addict
Jun 26, 2009
4948 posts
4522 upvotes
GTA
I basically went through the same thing last week. Had to make the reducer using old 3 to 5" coupling and new 4 to 5" new piece. Used the snips and rivet gun to make new 4to5" reducer. Had no choice but go swim in fibreglass as old connectors were screwed in Loudly Crying Face

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