Bath Fan Duct Questions - Kitchen & Bath Remodeling - DIY Chatroom Home Improvement Forum


Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > Remodeling > Kitchen & Bath Remodeling

CLICK HERE AND JOIN OUR COMMUNITY TODAY...IT'S FREE!

Reply
 
Thread Tools Display Modes
Old 12-27-2013, 10:12 AM   #1
Member
 
Join Date: Dec 2013
Location: Monmouth County, NJ
Posts: 47
Rewards Points: 25
Default

Bath fan duct questions


Bathroom fan duct questions:

Setup (pls see attached diagram): Fan w/ 4" duct --> semi-rigid duct pipe (I bought 4" diameter but can buy 3" based on answers) --> rigid duct pipe (slightly less than 3" diameter)

1. Bathroom fan has a 4" duct. I eventually need to reduce it to 3" (existing rigid duct has a diameter slightly less than 3"). Does it make a difference in terms of the static air pressure if I reduce it at the fan or if I put it at the end of a 55" 4-inch-diameter semi-rigid duct pipe?

2. When attaching the duct pipes, I would think that I'd want the pipe that is carrying the air flow to go INSIDE the pipe it's attaching to (as opposed to fitting OUTSIDE the pipe it's going to attach to) in order to avoid leakage of the air (even with foil tape and a clamp). Is this correct or ... does it not matter?

** NOTE: if I were to attach, for example, the 4 to 3" reducer to the INSIDE of the Rigid Duct (see diagram), I'd, of course, get some flexible duct or another solution to ensure that it fit inside the Rigid Duct.

Location: Monmouth Cty, NJ

Thx -
Attached Thumbnails
Bath fan duct questions-duct-reducer.jpg  

Advertisement

metazone is offline   Reply With Quote
Old 12-27-2013, 05:01 PM   #2
Member
 
Join Date: Nov 2011
Location: Hartfield VA
Posts: 27,798
Rewards Points: 712
Default

Bath fan duct questions


The right way would be to run 4" all the way.
Why does it need to be reduced?
Going loose a lot CFM, the fans going to have to work harder, may increase the sound level.

Advertisement

__________________
When posting in forums, letting us know your location will help others give better feedback/advice/solutions to your questions
joecaption is offline   Reply With Quote
The Following User Says Thank You to joecaption For This Useful Post:
metazone (12-27-2013)
Old 12-27-2013, 05:32 PM   #3
Member
 
Join Date: Jan 2009
Location: WV
Posts: 3,026
Rewards Points: 2,232
Default

Bath fan duct questions


There should be no reason you'd need to reduce the duct diameter anywhere. If you think there is, then it should be routed some other way. Most exhaust up and through the roof, though I don't particularly care for that - mine go straight through an outside wall.
md2lgyk is offline   Reply With Quote
The Following User Says Thank You to md2lgyk For This Useful Post:
metazone (12-27-2013)
Old 12-27-2013, 05:53 PM   #4
DIY Enthusiast
 
sublime2's Avatar
 
Join Date: Mar 2012
Location: New Jersey
Posts: 1,110
Rewards Points: 500
Default

Bath fan duct questions


You will be fine adding the reduction at the end of the pipe.
The CFM Lose will be small if any.
As for attaching the pipes, use the shinny silver tape to cover all connections.

Last edited by sublime2; 12-27-2013 at 05:55 PM.
sublime2 is offline   Reply With Quote
The Following User Says Thank You to sublime2 For This Useful Post:
metazone (12-27-2013)
Old 12-27-2013, 07:10 PM   #5
Retired from the grind
 
gregzoll's Avatar
 
Join Date: Dec 2006
Location: Midwest - Central Illinois
Posts: 14,423
Rewards Points: 2,508
Default

Bath fan duct questions


For the bath, you would be better to use insulated duct, to not allow any condensation from when the pipe cools, if there is still any humidity in it. You can get the flex duct with the R-5 insulation on it at your local Big Box.
__________________
Ren: Now listen, Cadet. I've got a job for you. See this button? Ren: Don't touch it! It's the History Eraser button, you fool! Stimpy: So what'll happen? Ren: That's just it. We don't know. Maybe something bad, maybe something good. I guess we'll never know, 'cause you're going to guard it. You won't touch it, will you?
gregzoll is offline   Reply With Quote
The Following User Says Thank You to gregzoll For This Useful Post:
metazone (12-27-2013)
Old 12-27-2013, 08:53 PM   #6
Member
 
Join Date: Dec 2013
Location: Monmouth County, NJ
Posts: 47
Rewards Points: 25
Default

Bath fan duct questions


Thanks, everyone, for the replies.

Goal: replace bath fan w/o changing rigid ducts -- calculated needed CFM for bathroom + EDL & bought fan based on this. Given this, reduction would be needed (not optimal but ...).

My remaining question: To reduce chance of air flow leakage (assuming using metal clamp and shiny silver foil tape): does it matter if the pipe with the air flow source is attached to the rigid duct on the OUTSIDE or the INSIDE? Thinking that pipe w/ the air flow source should go on the INSIDE?

Thanks -
Bill
metazone is offline   Reply With Quote
Old 12-27-2013, 09:18 PM   #7
Registered User
 
Join Date: Mar 2009
Posts: 10,007
Rewards Points: 2,080
Default

Bath fan duct questions


Think of the exhaust air as water from your pump (fan), all should "drip" into the next one, toward the exit hood. Any irregularities inside will give turbulence, like the crimped (smaller) pipe (IF it were pointed at the air flow). With the reducer at the fan, the airflow in the whole duct will be faster, moving the moisture faster, giving less time to condense in the pipe. With the reducer near the exit hood, it will collect more condensation because it is colder there -- being closer to outside air (hood).

Gary

Advertisement

__________________
If any ads are present in my answer above, I do not condone/support/use the product or services listed, they are there against my permission.
Gary in WA is offline   Reply With Quote
The Following User Says Thank You to Gary in WA For This Useful Post:
metazone (12-27-2013)
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Exhaust hood for range - Duct questions BigJimmy Kitchen & Bath Remodeling 0 09-22-2013 10:38 PM
Designing new ductwork. would love some input. Queequeg152 HVAC 8 08-30-2013 07:02 PM
New return duct size questions blackhemi HVAC 1 06-26-2011 07:50 AM
My Duct Sealing Project Cakemeister HVAC 6 06-13-2011 12:45 PM
bath exhaust fan questions rtoni HVAC 3 10-25-2007 04:46 AM




Top of Page | View New Posts