Bathroom ventilation fan question

[Exterous]

In our master bath there is no ventilation fan above the shower. Instead there is a dinky little Broan 50cfm fan about 10' away above the toilet which is having significant difficulties in removing moisture from the bathroom

Can I put something like this fan above the shower and tie it into the existing duct line? I know there are limits to the amount of air movement but its a short run and the other fan is anemic so I was thinking that might be ok.
https://www.amazon.com/Panasonic-FV...&qid=1476321806&sr=1-14&keywords=bathroom+fan

The line is a 4" flex line that runs about 10' to its vent in the roof. No 90s. I'd have both on the same switch so so I am thinking that alleviate any concerns of just venting the air from above the shower over to the toilet.

Alternatively I could just replace the fan above the toilet with a better fan but it would still be 10' away from the moisture concentration. If that doesn't work I could get an in-line fan but looks like it would be less fun to install. My attic is not a fun place to work in

 

[boomerang]

I don't think 10' away is going to be a problem as long as the fan is sized correctly for the square footage. That's the mistake that is made most often and from what you've written, it appears to be the case in your situation. As a secondary issue, I wonder at what point a bathroom fan starts moving enough cfm to require larger ducting? That might be something that would need to be addressed. I'm posing that from the perspective of you potentially tying a second fan into the existing ducting.

Not much help I know but they are my thoughts.

 

[nathanddrews]

4" ductwork should be enough for most full bathrooms to vent a lot of air, but I think what's causing you grief is the loss of efficiency in the flex duct. If you do stick with flex duct, just make sure your flex duct is pulled as tight as possible to avoid losing too much efficiency. If you go with rigid duct, you'll be much better off overall, especially with a better fan.
http://www.energyvanguard.com/blog-...bpi/the-science-of-sag-flex-duct-and-air-flow

Your bathroom fan/duct combo sounds a lot like mine was - flex duct in the attic to a vent in the roof (about 15 feet). I changed mine out to rigid duct, which isn't as easy to work with in the attic space, but I think helped move more air (the old fan was much quieter after). I have since bought a new fan as well - I bought this thing on a whim at Home Depot and am actually really impressed. Much quieter and blows a lot more air. I guess fans can improve after 50 years. LOL
http://www.homedepot.com/p/Home-Net...-Bath-Fan-with-LED-Light-7130-02-BT/205429483

One other thing I learned from a friend in the HVAC business - the efficient movement of air is just like a computer case - you've got to have air coming in from a window or from under the door in order to maximize airflow. Think of the bathroom as a computer case with passive intake and active exhaust. If there's no air source from outside the bathroom, then you're making your fan work extra hard.

 

[NetWareHead]

Agreed on the 4 in duct is usually sufficient to move a good quantity of air. Read the fan manual for recommended duct sizing and maximum length of run. Also agree that flex duct imposes a significant restriction on air movement and rigid duct is the best way to go. The attic will be much colder in the winter than the house and when moist warm air from the shower hits the cold environment, it will condense the water out of the air and you may get leaking water from the fan or accumulation in the ductwork. They sell insulated round duct at home depot that will work. If there any any horizontal runs, pitch the ductwork to run the water out.

Also, if you install the fan over a shower (aka a wet environment) the circuit powering the fan must be GFCI protected.

 

[Aikouka]

Hm, that's a pretty interesting fan... mostly for its ease of mounting. Although, it seems like it's slightly smaller size does give it reduced performance compared to the standard WhisperCeiling fan (that's what I had installed in my main bathroom). Well, by "reduced performance", I mean that it's louder at the same CFM. Although, I can tell you that being louder may not be the worst thing. There have been a few times where I'm in the shower, and I think, "I can't hear the fan... did I turn it on?"

 

[Exterous]

I did read the manual and while they specify only rigid distances they say 80' so I would think 10' of flex would be ok given thats its pretty tight. Sounds like maybe I should try out the higher cfm fan first. FWIW they used the same fan in the 7.5'x8' bathroom as the master which is 12.5'x15'

They sell insulated round duct at home depot that will work. If there any any horizontal runs, pitch the ductwork to run the water out.

It is, at least, already wrapped in insulation

 

[Aikouka]

I recall the manual for my WhisperQuiet (the old name for the WhisperCeiling) having imagery denoting very small runs of flex tubing used to connect to nearby rigid tubing. I'm pretty sure their point was to use as little flex tubing as you could.

 

[boomerang]

I did read the manual and while they specify only rigid distances they say 80' so I would think 10' of flex would be ok given thats its pretty tight. Sounds like maybe I should try out the higher cfm fan first. FWIW they used the same fan in the 7.5'x8' bathroom as the master which is 12.5'x15'



It is, at least, already wrapped in insulation

You should be able to switch to a fan that is appropriately sized and do it from underneath. I replaced the fan in our master bath and it wouldn't have been a bad job if not for all the blown-in insulation on top of the batts. I had a lot of that stuff on the floor when I was done.

FWIW, I think the flex ducting is fine for that short run. It's not your problem, the undersize fan is.

 

[mindless1]

If you tie the two together you're going to make the weaker one much less effective. I'd do a separate run for the new one. You'll lose some flow with flex but still have over double the ventilation and more importantly, remove the moisture before as much of it spreads out into the room.

 

[stormkroe]

While you should definitely replace your fan with 90+cfm, make sure you have plenty of gap below your bathroom door. If there are no cold air returns and your door is tight or obstructed at the bottom, not only will your fan struggle, but will actually try to pull air through your vent stack and may dry out your p traps. Smells great.

 

[Exterous]

New fan has been installed. We'll see how it goes (and there is a gap at the bottom and top of the door)
https://forums.anandtech.com/threads/exterous-home-project-thread.2476432/page-2#post-38525018

 

[Binky]

You could also use a humidity sensing switch that could remove the need for the timer. They run $30-$50, so definitely more expensive than the simple timer used.

 

[mindless1]

^ Motion sensing with timer built in would be better. Humidity sensing doesn't get rid of certain bathroom odors, or of course, do both.