# Bath fan: can you oversize?



## gizzygone (Jul 26, 2012)

I'm not sure if this is the correct forum for this (I was torn between electrical and HVAC), but here goes:

My 50cfm nutone bath fan isn't cutting it. It's moving air, but not nearly enough to keep up with my wife's apparent boiling hot showers....

I added a timer to the fan last year, but despite a 30 minute run time, I'm starting to get some mold growth (it doesn't help that the walls are dripping after every shower). 

I know there are calculations for CFM, and duct length... But I'm wondering if you can go too big on a bath fan?

The bathroom is maybe 7'Wx 7'L x8'H, with no more than 10' of duct length... So I feel an 80cfm fan would nicely fit the size of the bathroom.... But is there any harm in going for the big gun, and getting a 110cfm fan?


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## joecaption (Nov 30, 2011)

Before replacing the fan look to see if there's a gap under the door to the room.
No gap, not enough make up air avalible and any fan will not work right.
I like to see a 1" gap.
As far a "hot showers" that water does not need to be above 120 Deg. any hotter and your just wasting energy.


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## MushCreek (Aug 10, 2012)

As noted above, check to make sure make-up air can get into the bathroom. Going from 50 to 80 cfm probably won't make much difference. I would go for the 110, as long is it isn't too noisy.

In most areas, 120 F. is the max temperature allowed by code for a shower. If you a have a single handle anti-scald valve (also code) you can adjust the stops to limit the water temp if the source is hotter than 120.


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## gizzygone (Jul 26, 2012)

MushCreek said:


> As noted above, check to make sure make-up air can get into the bathroom. Going from 50 to 80 cfm probably won't make much difference. I would go for the 110, as long is it isn't too noisy.
> 
> In most areas, 120 F. is the max temperature allowed by code for a shower. If you a have a single handle anti-scald valve (also code) you can adjust the stops to limit the water temp if the source is hotter than 120.







joecaption said:


> Before replacing the fan look to see if there's a gap under the door to the room.
> No gap, not enough make up air avalible and any fan will not work right.
> I like to see a 1" gap.
> As far a "hot showers" that water does not need to be above 120 Deg. any hotter and your just wasting energy.


There's a good gap between the door and threshold. At least an inch I'd say. 

I used a little artistic license when describing her showers as boiling.... She just tends to like them hot.... You'd think you were walking into a steam room after her showers! 

I'm not sure what the shower valve is set at, but we have the water heater set to 140 I believe. It's what the dishwasher calls for.


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## ddawg16 (Aug 15, 2011)

To confirm it's an airflow problem, leave the door open a fraction. If 'steam' comes out, then your fan it not moving enough.

Have you inspected the vent line to make sure there is not an obstruction? 

Have you cleaned the fan? 

I used rigid 4" pipe on mine. And I periodically check on the roof to make sure it's blowing air out. 

If you decide to go bigger.....look into some of the integrated light/fan setups. You pay a few more $$, but they look great and are a lot quieter.

Instead of a timer, I use an occupancy sensor. No one in my house besides me knows how to turn off a light. So far I have about 8 of them installed. As soon as I get a ground wire installed to my little girls room, she is getting one. 

Nice thing about the sensors, you can set the time delay to 5-10-15 min or so. Real flexible. Touch to turn on or off. If no activity, turns off after the set time.

All my lights are LED or CFL so the load is small.


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## cjm94 (Sep 25, 2011)

My wife is the same way with showers. I have been so tempted to go in the basement and turn off the hot water valve after 20 minutes.


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## oh'mike (Sep 18, 2009)

I tend to install high volume fans in small rooms---I never heard any complaints.

Bear in mind--the big boys need 4 inch or larger ducting--so figure that into your plans.


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## MrElectricianTV (Nov 13, 2014)

gizzygone said:


> I'm not sure if this is the correct forum for this (I was torn between electrical and HVAC), but here goes:
> 
> My 50cfm nutone bath fan isn't cutting it. It's moving air, but not nearly enough to keep up with my wife's apparent boiling hot showers....
> 
> ...


In addition to what others have said, the location of the fan can also make a difference. I moved mine over the shower and there is no longer fog on the mirror. Customers that I relocated bath fans for have also said that it makes a difference.

Bath fans located over the shower usually require GFCI protection.

Panasonic makes the best fans for retrofit. There are some photos in my gallery.


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## gizzygone (Jul 26, 2012)

ddawg16 said:


> To confirm it's an airflow problem, leave the door open a fraction. If 'steam' comes out, then your fan it not moving enough.
> 
> Have you inspected the vent line to make sure there is not an obstruction?
> 
> ...


The fan is clean. It's moving some air (my simple test was to hold a paper towel up to the grate and see if it held... The vent itself is on the second floor soffit, so there isn't an easy way to feel for exhaust, but it certainly sounds like it's working (whistle). 

The occupancy sensor is something I installed in my garage and it works great, but the application wouldn't work in our bathroom (our dog tends to prefer the cool bathroom floor for sleeping instead of her nice $60 dog bed.... So when she gets up and adjusts in the middle of the night shed turn the light on). Plus, a timer shuts the light/fan off anyways, so it works fine for our application. 

As far as the integrated bulb/fan fixtures, they're nice.... But there's already a square cutout in the ceiling so I'd rather just fill it up with a new fan instead of plastering the ceiling over!



oh'mike said:


> I tend to install high volume fans in small rooms---I never heard any complaints.
> 
> Bear in mind--the big boys need 4 inch or larger ducting--so figure that into your plans.


That's good to know. My main concern would be too much airflow would pull too much heat out of the house. I'm honestly not sure it's a valid concern, but the forced air heat duct is about 10" away from the vent. 

Plus I wasnt sure if too much airflow would lead to other issues? (Like pulling on the door, causing it to make rock on the striker... I assume if the gap below the door is large enough it wouldn't be an issue but again these are things I worry about for some reason!). 

I guess I looked at it from the perspective of an HVAC engineer: where oversizing a whole home furnace IS possible. 

As far as ducting, I took a quick peek in the attic last night and it appears to be 4" already.... Although if be tempted to replace it with 4" insulated duct. 



cjm94 said:


> My wife is the same way with showers. I have been so tempted to go in the basement and turn off the hot water valve after 20 minutes.


Maybe you're onto something here: perhaps a device that rations hot water in the home? I'm sure homes with wives like ours, and teenagers would eat it up! Heck, make it wifi enabled and you can change rations on the go! Hahahaha


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## gizzygone (Jul 26, 2012)

MrElectricianTV said:


> gizzygone said:
> 
> 
> > I'm not sure if this is the correct forum for this (I was torn between electrical and HVAC), but here goes:
> ...


Thanks, I'll check out the gallery when I get home (this mobile website doesn't seem to let me). 

A member of the family is an electrician and swears by Panasonic, which is the route I'll likely go. Apparently they're the quietest models around (although I hear their models with an integrated light tends to put off unattractive bright white light) 

I'm not interested in moving the fan at this time, due to the extra work it entails. The shower is in a nook in the bathroom, and the fan sits right in front of it... So any air leaving the shower has to pass by the fan anyways; so I'm thinking if I up the CFM, I should be able to suck more vapor out before it makes it to the mirror.


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## 123pugsy (Oct 6, 2012)

110 CFM's.


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## Arrow3030 (Apr 19, 2015)

You can go big as you want AFAIK. Get a humidistat to control the fan. They replace a standard switch. There are ceiling fans with built in wifi speakers if you want to impress someone.


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## Wildbill7145 (Sep 26, 2014)

I was painting for a woman who was having renovations done on her bathroom at the same time. She had the contractor install this huge bathroom fan. If you turned it on with the bathroom door open, other doors in the house would slam shut! It was incredible.


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## micromind (Mar 9, 2008)

Maybe you should install one like I hooked up at an asphalt plant a couple of weeks ago......400HP, 60" ducts.

I'm pretty sure that baby would remove the steam..........lol.

Seriously though, if the duct is 3". 80 CFM is about all you'll get. If it's 4", I'd go with a 110 CFM model. 

If a 110 CFM fan is installed with 3" ducts, it'll likely make more noise because the airflow will be less than it's designed for. But it will vent more than an 80 CFM one.


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## 123pugsy (Oct 6, 2012)

micromind said:


> Maybe you should install one like I hooked up at an asphalt plant a couple of weeks ago......400HP, 60" ducts.
> 
> .


Pretty sure it would take care of my MIL problem too.....


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## SPS-1 (Oct 21, 2008)

gizzygone said:


> The shower is in a nook in the bathroom,


 That could be a part of the problem. Does the shower have an open top, or is it "built in". Is any air actually flowing through the shower? Air flow will take the path of least resistance.


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## MushCreek (Aug 10, 2012)

I stayed at a campground where you had to feed the shower with quarters. You learn to wash up quick.


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## gizzygone (Jul 26, 2012)

There's a glass door for the shower, and about a foot between the ceiling and the top of the glass


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## SPS-1 (Oct 21, 2008)

A foot at the should be enough for air to flow out. I have zero at the top (actually about a foot of "fascia") and I added a circulation fan to get airflow into the shower.


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## Bud9051 (Nov 11, 2015)

A quick read so apologies if I missed something or duplicate others.
Some bath fans are designed with two speeds so instead of turning off they switch to low and run 24/7 as part of a fresh air solution for a house. That feature might be something you could incorporate with another timer to shut it down after a long run so it is less than the 24/7.

A low flow shower head might (?) reduce the steam while providing an acceptable scalding. I tried 120° but the wife overruled and went back to 140°.

What is the RH of the rest of the house? Since that is the replacement air, if the house is humid, then the bathroom will have difficulty drying.


Is this moisture issue all months or mostly the cooler months?


Bud


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## GrayHair (Apr 9, 2015)

Wildbill7145 said:


> I was painting for a woman who was having renovations done on her bathroom at the same time. She had the contractor install this huge bathroom fan. If you turned it on with the bathroom door open, other doors in the house would slam shut! It was incredible.


Sounds like she should rethink what she feeds her hubby!! (Or herself.)



micromind said:


> Maybe you should install one like I hooked up at an asphalt plant a couple of weeks ago......400HP, 60" ducts.
> 
> I'm pretty sure that baby would remove the steam..........lol.


Probably also remove the bath cloths, towels, toilet tissue and maybe the floor covering!



Arrow3030 said:


> There are ceiling fans with built in wifi speakers if you want to impress someone.


Now there's an idea! Install a wifi fan and when she's in the shower, play the theme from _Jeapordy!_


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## gizzygone (Jul 26, 2012)

Bud9051 said:


> A quick read so apologies if I missed something or duplicate others.
> Some bath fans are designed with two speeds so instead of turning off they switch to low and run 24/7 as part of a fresh air solution for a house. That feature might be something you could incorporate with another timer to shut it down after a long run so it is less than the 24/7.
> 
> A low flow shower head might (?) reduce the steam while providing an acceptable scalding. I tried 120° but the wife overruled and went back to 140°.
> ...


I'll look into the shower head. 

The basement has a humidity right now of 50%, so I imagine the rest of the home is drier. 

It's mainly an issue in the winter months: when the house is buttoned up. I'm in New England, so in the spring and fall the Windows are open, and in the summer, the central air likely takes care of the humidity. The house tends to get real dry in the winter, though... But I'm guessing because of a lack of airflow it just builds up in the bathroom.


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## MrElectricianTV (Nov 13, 2014)

gizzygone said:


> Thanks, I'll check out the gallery when I get home (this mobile website doesn't seem to let me).
> 
> A member of the family is an electrician and swears by Panasonic, which is the route I'll likely go. Apparently they're the quietest models around (although I hear their models with an integrated light tends to put off unattractive bright white light)
> 
> I'm not interested in moving the fan at this time, due to the extra work it entails. The shower is in a nook in the bathroom, and the fan sits right in front of it... So any air leaving the shower has to pass by the fan anyways; so I'm thinking if I up the CFM, I should be able to suck more vapor out before it makes it to the mirror.


The fan will be pulling in air from its make up source which is probably far from the shower. Even with a higher CFM the air movement will be between the fan and wherever the makeup air comes from. 

I think that the DeltaBreeze fans are the quietest, but they are made for new construction. I have retrofitted several bath fans with new Panasonic fans. They now furnish a bracket for retrofiting and for new construction. Without the bracket there are still a few other methods to installing a Panasonic fan in place of an old existing fan.


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## gizzygone (Jul 26, 2012)

The wireless music idea is a neat gimmick (and I'm all about gimmicks usually!)

But I think I'm sold on the Panasonic brand; they seem to be well regarded among professionals, but I don't think they have a fan with a Bluetooth feature.


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## r0ckstarr (Jan 8, 2013)

I've been following this thread because my girlfriend takes boiling hot super long showers as well. When she's finished, the walls are wet just as described in the original post. I was considering replacing the old fan with a new one, but did not think to go bigger. +1 to the OP. 

The fan is in the ceiling. I can access it from the attic. The duct is a flexible rubber duct with spiral wrapped wire running the length of it. While up there, I found a 2 small holes in the duct. For the time being, I covered them with tape until I replace it. 

What would be the proper ducting for an exhaust fan? It comes up straight from the ceiling into the attic, and then runs about 10ft across where it vents out of a soffit vent. Can I use straight metal duct like this:










Or is there a better choice since it will be venting hot steam from a shower? What about condensation?

The current duct is just laying on top of insulation in the attic. Should it be suspended up off of the insulation, but oriented in a way that condensation would run out and not back towards the fan?


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## gizzygone (Jul 26, 2012)

r0ckstarr said:


> I've been following this thread because my girlfriend takes boiling hot super long showers as well. When she's finished, the walls are wet just as described in the original post. I was considering replacing the old fan with a new one, but did not think to go bigger. +1 to the OP.
> 
> The fan is in the ceiling. I can access it from the attic. The duct is a flexible rubber duct with spiral wrapped wire running the length of it. While up there, I found a 2 small holes in the duct. For the time being, I covered them with tape until I replace it.
> 
> ...


As far as I know: the flexible duct work is more than sufficient. You don't have to worry about lint and heat like you would in a dryer duct. Plus, most home HVAC systems use the flexible stuff off the trunk too. 

If anything, I'd buy the flexible stuff that has insulation already: it should make the fan even quieter


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## oh'mike (Sep 18, 2009)

Up north,here, we always use insulated duct-----or the humidity can condense inside the duct in the winter---

Down south--I'm not sure it is needed, but it could not hurt.


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## Piedmont (Nov 1, 2007)

When I researched the heck out of bathroom fans, the only reason you size a fan to the bathroom is for energy efficiency. You can put in as big as you want but bathroom fans can move a lot of air out of your house. 

In winter that means reheating, Summer it's cooling, but there's a 3rd component your wife factor. In my house my bathroom has a properly sized fan, but not enough ventilation under the door. If I crack the door open while my wife showers she knows immediately because it creates a cool draft in the shower so she cranks up the hot water to compensate. If I keep the door closed it does turn into a sauna, but she doesn't use as much hot water doing it. Which to choose... 

When I did an energy audit of my house I found 65% of my energy is used heating hot water so I decided to let her have her sauna's and I'll deal with the mold/mildew remediation.


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## user_12345a (Nov 23, 2014)

With flex duct you really have to watch out for sizing, turns, and length.

The fan's literature should give duct sizing into.

4" is fairly small, but it depends on what pressure the fan is made to work against. 5" would flow a lot more air; far more cross section area.

Common trick, at least for range hood fans is to rate them with no duct connected.


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## user_12345a (Nov 23, 2014)

Example:

Expanded performance data for 100 cfm air king fan from the spec sheet...

Static " - cfm

0 - 104
0.1 - 100
0.25 - 86

Using this (https://www.cdicurbs.com/ductcal), to get the 100 cfm you can only have 10 ft of perfect, straight, no-sag flex 4".

Add a single 90 elbow, and you're down to to 2 ft.

Add the roof vent, sags, etc and you're under 2 ft.

Will you realistically get 100 cfm with a 4" pipe? heck no. You'll be lucky to get 70.

Go up to a 5" and you'll get far better performance, even if it means using a 4 to 5" adapter.


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