Improving existing bathroom venting


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Old 11-17-16, 08:32 PM
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Improving existing bathroom venting

Folks:

I have a rambler build in 2001; 9 foot basement ceilings (finished) and vaulted upstairs. 2 bathrooms with showers that get heavy use (1 down and 1 upstairs); master bathroom isn't an issue. I've got several kids who shower daily and my wife has asked if we can upgrade existing ceiling fans to try to keep humidity down. I just don't know what current fan's capacity is to compare to what a new one could do. I could use help with is:

1) I've confirmed my current fan is the Broan 686-J. 50 CFM and loud as a jet at takeoff. It's only 7 1/2 by 8 1/2. Small unit, clearly.

I confirmed via the manufacture's website that this piece of trash uses a 3" duct. Would anyone guess that the unit is connected to a 4" in duct? Or do some builders use 3"?

2) Let's say my current ducting is only 3" but I want a new unit that moves 110 or 150 CFM, but requires a 4 inch duct, will going down to a 3 educe my new fan's capabilities a great deal? I would think yes.

2) How hard would it be to run a 4 inch duct in place of a 3? For my upstairs rooms, not a problem. Just pull to the attic from below. However, basement on a rambler up to the attic? I don't know if duct is tied to any studs, as it runs between walls. etc. Do builders do that? I wonder if I could, like running wire, attach 4" to my existing 3" and pull it up from the basement to my attic. Thoughts?
 
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Last edited by MRTkH; 11-17-16 at 10:28 PM.
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Old 11-18-16, 04:27 AM
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Pull the cover off the outlet wherever it exhausts outside and you'll be able to see the size ducting. Chances are it's 4". You can also remove the fan motor assembly and stick your hand/finger into the outlet. Usually there is a plastic square to round transition piece and you'll be able to feel for it's size.

While the size of duct does have an impact on airflow so does the tubing used. The cheap plastic flexible stuff with a coily wire is pretty bad as it has a very rough interior shape that reduced air flow. The length of run also has a big impact on flow with longer being worse. And lastly are bends. Bends in the duct, especially sharp 90 degree turns, are very bad on flow. So, even if you only have 3" duct if it's a short, straight run it may flow better than a longer 4" that has a sharp bend or two.
 
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Old 11-18-16, 07:37 AM
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I would also guess the duct is rigid 4".
Either way I think replacing the fan will help a lot. Once you get the old fan out you can check what size duct you're dealing with.
 
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Old 11-18-16, 08:31 AM
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Pulled the old fan out. Unfortunately, it is a 3 inch flex junk. I can't tell where it goes, since I can see it runs 2 feet then hangs a left into a hole cut into the floor joist. I guess I'm stuck. Either I run with a new fan and use the 3 inch pipe or i tear out lots of ceiling to run a 4 somewhere outside. Yuck.

My question now is would it be possible to put a unit that works with 4 or 6 inch on a 3 inch system? I know the flow will be reduced a great deal; however, would it hurt the unit over time? I think a stronger blower will move more air--just not as much as the vendor says it can.
 

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Old 11-18-16, 09:41 AM
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A restriction on the outlet side like smaller ducting will not hurt the motor or blower assembly. Like you guessed you'll just get less air flow. Look around at different fans. There are some that can handle back pressure much better than others. Those having a traditional fan "propeller" need a very free flowing exhaust. Units with a squirrel cage blower (looks more like a hamster wheel) can generate higher pressure and work better with a restricted outlet.
 
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Old 11-18-16, 10:28 AM
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or i tear out lots of ceiling to run a 4 somewhere outside. Yuck.
I would tear out the ceiling if you think it's possible to run a duct to the outside.

I'm in a high priced area and I could get the ceiling fixed like new for maybe $180.
You don't have to be careful ripping out the ceiling, the drywaller can patch and texture quickly.
 
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Old 11-18-16, 11:51 AM
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So this is what I'm going to do.

I'll replace the fan with a much stronger one and see just how much flow happens. If is substantially better and we don't get fogging issues post shower. DONE.

My question is this. I have two fans that are available at my local big box store. I'm picking between the:

WhisperCeiling 110 CFM Ceiling Exhaust Bath Fan, ENERGY STAR*
or
WhisperCeiling 150 CFM Ceiling Exhaust Bath Fan, ENERGY STAR*

40 more CFM for $22 more. Not bad. Same dimensions, so its just a more powerful motor.

My only worry is could I have an issue with too much CFM behind a 3 inch pipe. I assume not.

If, say with the 150 CFM, I still get fogging and humidity doesn't drop, I will have to go the route of new duct work and cutting holes.
 
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Old 11-18-16, 04:02 PM
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Well, the 110 has an adapter for 4" or 6" ducting while the 150 only goes to 6". Do you have a plan or adapter to make it down to 3"?
 
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Old 11-18-16, 06:23 PM
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The 110 was fitted for 4" out of the box. So at HD I bought a 4" to 3" adapter. We will see how it works in the AM. One thing is sure, it makes much less noise. Just makes me wish the original construction guys would have run a 6 inch pipe the right way.
 
 

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