different kind of bathroom vent question, two fans ONE room one duct


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Old 11-03-14, 04:01 PM
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different kind of bathroom vent question, two fans ONE room one duct

Hello all, I have used this forum religiously and have always found my answers without the need to post until now so here I am

I have researched this question for days and see the topic of two fans on one duct has been beat to death. I cannot agree more that two fans in tworking separate rooms should never share one duct. I actually spent the week in my attic redoing the previous ownears mess and I installed a fan in each bathroom with its own dedicated 4" duct to a gable side vent. It's a 30' run but only one elbow and the last 10 feet is duct pipe not flex like the first 20.

Anyway. Both fans seem to be working great and I placed panasonic 110 cfm fans in rooms only requiring 70cfm or less. Our larger bathroom however is stI'll pretty wet on the walls after we both take showers in the morning so I am thinking I may place a second fan in the master bathroom.

My question is, can a place a second fan in my master and run it through the same 4" duct as the primary fan in the same room? Essentially two fans, one room, one duct. Back draft I do not believe would be an issue as both fans would come on at the same time as I would wire them together on the same switch. Has anyone done this, what's your thoughts? I have already run two ducts and really don't want to run a 3rd full run. I am thinking I would run the first 20 feet on its own run in 4" flex then y pipe it in at the 4" pipe duCT that runs less ththan 10' before exiting outside.

Thanks for your thoughts all....
 
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Old 11-03-14, 04:20 PM
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When combining 2 vents, you can use a switched remote fan (inline past the Y), and use a simple grille like this one. practically silent venting.
 
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Old 11-03-14, 04:51 PM
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Hello all, I have used this forum religiously and have always found my answers without the need to post until now so here I am
I say post away...... You will always find help, even if it has been answered before.
 
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Old 11-04-14, 05:30 AM
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Originally Posted by Handyone View Post
I say post away...... You will always find help, even if it has been answered before.
Thanks, I certainly plan to.

The in line fan option would have been a wise choice however, I have already installed one panasonic whisper green in the room that has a light and such that I would like to retain.
 
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Old 11-04-14, 06:14 AM
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My question is, can a place a second fan in my master and run it through the same 4" duct as the primary fan in the same room?
Absolutely not. You would have to increase the exhaust duct to at least six inch at the point where you connect the second fan exhaust or you will increase the back pressure in the last ten feet of the duct and THAT will reduce the airflow from each fan a significant amount.

My question is why did you make a horizontal run of thirty feet when the roof was much closer? That twenty feet of flexible duct along with the additional ten feet of rigid duct is quite a load for the fan. If you had no choice it would have been better to increase to at least five inch duct and run rigid all the way except for a small piece of flex at the fan to provide a vibration insulator.

Also, that duct in the attic should all be insulated.

One more question. Is there a window in your master bath and if yes do you and your wife have it slightly open when running the exhaust fan? Or do you leave the door open when showering? If you keep the door closed is there a significant gap, at least an inch and bigger is better, between the bottom of the door and the floor? All the exhaust fans in the world won't exhaust any air if there is no provision for air getting IN to the room.
 
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Old 11-04-14, 10:15 AM
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I suppose I didn't realize there was really that much air moving thru that 4" metal duct that would actually create back pressure.

As for the 30' run, it was a few things. The first being, I just had a ridge vent installed to increase air flow in the attic but also to clean up the roof aesthetically. I am trying to reduce all the pipes and vents we had coming out of the old roof. The second was, with this new soffit/ridge vent model, I was instructed to close off the gable vent to block its now reverse airflow given the ridge vent. I saw this as an ample opportunity to use a large existing hole in the house to run my bath fans to.

What is the the insulation on the duct for? Will the flex duct condensate that badly in an Ohio insulated attic? What do you insulate it with? I know they have insulated flex duct however it looks more restricting on the inside that regular flex.

We have a window, the door does not have much gap with bedrooms carpet. Our process is to keep the door ever so slightly cracked when the fan is on. How do others overcome this issue?

Thanks for all the feedback so far. So it sounds like if I go back and do 6" flex duct I could run the two fans in this one room with one duct?
 
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Old 11-04-14, 11:11 AM
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Of course there is back pressure and the longer the duct or the more convoluted (flex duct) the more back pressure. That's why the shorter and more direct the duct the better will be the airflow.

What is the the insulation on the duct for? Will the flex duct condensate that badly in an Ohio insulated attic? What do you insulate it with? I know they have insulated flex duct however it looks more restricting on the inside that regular flex.
Your attic is NOT insulated, your living quarters are insulated. With the combination of soffit vents (you DO have soffit vents, don't you) and the ridge vent your attic should closely approach the outside temperature and that will be cold during the winter in Ohio. The exhaust fan will be moving hot, humid air through the cold attic so of course you will get condensation in the duct if it is not insulated. The duct should have no dips (flex duct) and the duct should be run with a slight downward slant towards the exit. Obviously with its convolutions you can see how flex duct will be far worse in trapping the condensed water and that is why you should use rigid as much as possible. For insulating the duct it can be done with unfaced fiberglass wrapped around the duct with as little compression as possible yet tightly enough to close any openings to the duct itself. They DO make insulation sleeves to fit over round duct but they are a bit hard to find and also expensive.

You really need to open that door farther or else cut an inch or more off the bottom to allow a free flow of air into the room when the exhaust fan is running. Opening the window is ideal but I have never met a woman that was willing to do so because of drafts. Unless air can get IN the room you can't exhaust any of the hot humid air. If your home's construction is more airtight than usual you may not be able to get enough air in even with the door wide open. Without incoming air to replace the exhausted air the fan stops moving air and simply becomes a noise-making machine.
 
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Old 11-04-14, 03:50 PM
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The duct should be run "downward" to the exit or did you mean upward? I thought is was a slight incline towards the exit given hot air rises?

So to do this right we are saying I need to rip my flex pipe back out and install 4. Rigid pipe for the one bathroom and 6" rigid for my master to accommodate the two fans. Insulate the piping and exit accordingly?

With 6" I will not have any issues running both fans?

Is 30' feet going to still be an issue with larger Rigid duct?


I appreciate the feedback so far. Very helpful. Disappointing that I messed this all up but helpful.
 
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Old 11-05-14, 01:48 AM
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Run downward to allow any condensate to run outside the wall louver rather than run back to the fan and into the bathroom.

Not absolutely necessary to remove the existing flex duct but rigid makes a better installation without the ridges of the flex to trap condensation. Yes, insulate the entire run of duct from the fan to the exit louver.

Shorter duct is always better but using 6 inch a 30 foot length should not cause any problems.

You didn't "mess up", you did the best you knew how. Now you know a better way to do the job.
 
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Old 11-05-14, 05:38 AM
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Few more questions if you don't mind.

How does one go about running it downward if the vent is way higher than the fan? Especially if it was a roof vent.

if I redo the duct, is it best to go with the full rigid, next best is semi rigid or are they about equal? Would any version of pvc work? I feel pvc would help with condensation.

Last, if I do 6" duct will the two fans in the one room be fine with sharing that? (my original question)

Thanks again for your help. I can't express my appreciation. I did notice a few drips from my new Panasonic today. I pulled the vent and the fan shrowed was a little wet. I wonder if a few drops is the nature of the beast issue tho given it is sucking up wet air regardless?
 
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Old 11-05-14, 06:47 AM
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If you can't (or it would be very difficult) to run the duct downhill then you can't, it's that simple. In such a case you have to be prepared for the condensate to run out the fan housing or else be extra careful with insulating the duct. Leaving the fan to run for a time period after the shower will also help and this is where a delay timer on the fan is most helpful.

Rigid duct is always best but others are acceptable. PVC may develop a static charge which may or may not be a problem.

Yes, two four-inch fans exhausting to a single six-inch duct would be acceptable. Best to have both fans run at the same time but probably not an absolute as the back draft damper integral to the fan will stop most backflow through an idle fan.
 
 

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