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Old 08-23-2007, 07:39 AM   #1
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Multiple bathroom exhaust fans


Hi,
I have 2 bathrooms, and their exhaust fans ducts join in the attic craul space into one ridged foil pipe about 19 feet long. One of the fans is over the toilet rather than over the shower where it belongs. I have few questions.
1) Does the fact that the two fans use one pipe result in significant loss of power?
2) If so, would it be a good idea to run 2 separate pipes which would require 2 openings in the attic wall?
I am thinking about adding another fan over the shower in the bathroom that has a fan over the toilet (there is a light over the bath tub, so it should not be hard to use its wires for it).
3) Would it be a good idea, knowing that it would require probably the 3rd 19 feet long pipe and the 3rd opening in the attic wall?

Thank you in advance,
Dmitriy

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Old 08-23-2007, 11:11 AM   #2
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Multiple bathroom exhaust fans


You should be able to go to Broan fan web site and find a fan that has a separate intake port to add a separate duct to go over the shower. Use this replacement fan to duct from both ceiling registers. If I were doing this I would use metal exhaust pipe from the fan to exterior sloped continuously to the exterior wall so that the water that will condense inside the duct can run our at the end. Seal all exhaust pipe joints tightly. Other possible web site is Greenheck.

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Old 08-23-2007, 11:36 AM   #3
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Multiple bathroom exhaust fans


Thank you for your reply. But I am not sure I understand this:

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Originally Posted by yudamann View Post
find a fan that has a separate intake port to add a separate duct to go over the shower. Use this replacement fan to duct from both ceiling registers.
Are you suggesting using one fan for several places, including both my bathrooms? If yes, don't you think it will ventilate even the bathroom that is not being used, vasting power?
Quote:
Originally Posted by yudamann View Post
If I were doing this I would use metal exhaust pipe
Is it better than a pvc pipe? Because I have already purchased one.

Thanks.

Last edited by dpreznik; 08-24-2007 at 01:02 PM.
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Old 08-26-2007, 06:19 PM   #4
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Multiple bathroom exhaust fans


Maybe somebody could at least say if it is a good practice in general to join pipes from two fans into one pipe? Would it cause too much resistance? Or should I run separate pipes for each exhaust fan?
It is very important for me.

Thanks.
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Old 08-27-2007, 08:56 AM   #5
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Multiple bathroom exhaust fans


Quote:
Originally Posted by dpreznik View Post
Maybe somebody could at least say if it is a good practice in general to join pipes from two fans into one pipe? Would it cause too much resistance? Or should I run separate pipes for each exhaust fan?
It is very important for me.

Thanks.
I used flex pipe and tied the two together but used check dampers to prevent backflow. If I were to do it again I would pipe them separately with hard pipe and insulate. I will probably do that someday when I don't have anything else to do.
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Old 08-27-2007, 09:45 AM   #6
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Multiple bathroom exhaust fans


Duct size counts only if you run both fans at the same time. If vent duct is connected with a Y u should not have a need for a damper. ( if you run both fans all the time then after the Y go to 6" duct assuming fan has 3"duct.)

Be sure your fans are clean for best performance.

Should not be a problem (fan not over tub) if cfm rate of fan is enough for the bathroom.
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Old 08-27-2007, 10:48 AM   #7
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Thanks for the answers.
There is another problem though. I read that any elbows right next to the fan should be avoided. But because of the rafters, the 90 degrees elbow is inevitable:

Do you have any ideas what can be done about it?
Thanks.
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Old 08-27-2007, 11:07 AM   #8
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Multiple bathroom exhaust fans


2) 45 degree duct elbows or flex duct / notch the nailer not the joist if you feel you must.
This risks ceiling damage if not done with care.

By the way...what problem are you trying to solve?
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Old 08-27-2007, 12:00 PM   #9
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Quote:
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By the way...what problem are you trying to solve?
The problem has a long history... But to make it short, last winter I noticed lot of condensation in my insulation in my attic craul space. And now I am trying to reduce it. I suspect the wet air from at least one of the showers has not enough time to be removed by the fan. So I want to change the ventilation system, and you can look in my messages above what my system is.
So you believe I can use one pipe for two fans. If the fans have 4" duct, does it mean the main pipe should be 8"? I wonder if I can even find that large dryer-hood type vent...
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Old 08-27-2007, 12:56 PM   #10
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Multiple bathroom exhaust fans


No sir: you really should not need to go bigger. Your only problem might be when both fans are on at the same time. Air spirals thru the round duct. When both are on the air currents may be in conflict resulting in (choking). solution: try not to run both fans at once.

Have you tried leaving the fan on for 5 min after the shower? (do this in the bath with the longest duct run) This should rid the room and duct of unwanted humidity. Does the duct vent clearly to your exterior?

You may have attic venting problems ( check other threads Ed the Roofer)

Your insulating values are hurt by (high Rh in insulation)

Good luck
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Old 08-27-2007, 01:07 PM   #11
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Thank you very much.
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Old 08-27-2007, 01:11 PM   #12
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you are very welcome
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Old 08-27-2007, 01:38 PM   #13
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BTW, I am going to use a PVC 4" pipe (is it a good idea?). But the dryer hood like vent which I have now and which is also 4" is made of metal, and has a little bit smaller diameter than the pvc pipe. Is it a problem, and how would you recommend to connect these two parts together?
Thanks.
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Old 08-27-2007, 02:23 PM   #14
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put the pvc over the metal flange and duct tape it.
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Old 08-30-2007, 01:05 AM   #15
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Quote:
Originally Posted by dpreznik View Post
last winter I noticed lot of condensation in my insulation in my attic craul space. And now I am trying to reduce it. I suspect the wet air from at least one of the showers has not enough time to be removed by the fan.
You may have more of a problem than humidity from one shower. In fact, the shower may not even be the problem. The best thing I can suggest is do a search in these forums and elsewhere on the Internet about attic ventilation, attic insulation, and humidity. You most likely have a problem with high humidity in general in the house during the winter coupled along with inadequate attic ventilation and a leaky ceiling. The moisture in the house air gets into the attic where it condenses on a cold surface. That cold surface is probably the top of the insulation. Before you spend time and money redoing your bathroom fan study your attic ventilation situation first.

On the other hand, 19 feet is a fairly long run for a duct. Make sure the fan you already have is pushing air like it should. Make sure the duct is clear all the way and that whatever back draft damper is in there isn't stuck shut. You may just need to replace the current fan with a bit bigger one. It's location in the bathroom shouldn't matter much at all. I don't think fan location is your problem.

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