Hello Everyone. I just installed a 110cfm bath vent to replace an 80 cfm bath vent because of condensation issues and they seem to have gotten worse. The ducting vents to the outside with insulated ducting and water has been leaking in from the fan and the walls are running with moisture. I placed a piece of paper over the vent to see if it working and it is so I dont know what to do. Any ideas would be greatly appreciated
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Not enough info.
Need to know size of room, construction/insulation details, route of ductwork, windows, climate, occupant habits, etc.
Heard one interesting thing a few days ago, someone remarked that they had gotten a low-flow showerhead but didn't like it, someone else remarked that it probably atomized the water too much, causing it to get cold by the time it hit you AND steaming up the room more than other types of heads.
Need to know a bit ... more about your situation. Are you venting from a ceiling up through the roof? (it sounds like it). Is the exhaust port on the fan out the side or straight up?
Generally, it's working ... what is happening is you are venting moist air out. It hits the cold duct and the moisture condenses on the cold duct surface even though the air is still going out ... then that runs back to the fan and drips down.
One thing to help is to insulate the duct to the exit. Another might be to elbow the duct over and down a bit allowing condesation to settle in the low point and eventually evaporate and be carried out. Some installs recommend sloping down from the fan to the exit. Either of these later options, you need to ensure the duct is sealed so that the condensate doesn't leak out into the attic.
Did you remove insulation when you put in the knew? Since you are venting more air volume, you are also potentially condensing more water out ... which makes it worse.
Be glad you see your problem; many situations would have the problem, but you wouldn't see it until e.g. a wall fell apart from the excess moisture.
this is a 10x12 bathroom with the fan venting to the outside through the side of the house not the roof. I left the door open and just took a shower and seemed to be working fine. Is the fan not getting enough return air to work properly? What can be done to fix this. Thanks for all the responses
Yeah, what route does the duct take, how long is it, and what is it made of?
Does the fan clear the air? 110 cfm should keep the air fairly clear, and if not then you have blockage either going out or coming in. (Could be that you need to undercut the door a bit.)
duct is insulated ducting and goes out on the side of my house. It is omnly about 6 feet long. It clears the air somewhat but does a better job when the door is left open. I dont want to leave my door open when I take a shower so there must be some remedy. thanks you for your responses.
Return air and run time of your fan
Adam,
There needs to be a source for air to replace that which the fan sucks out. Most often undercutting the door is all that's done. At 3/4 to 1 inch, it probably isn't enough to allow the volume that is necessary to properly clear the air of moisture while showering.
So, what should be done is enable a longer run time once the door is opened. While you can do this manually by leaving the fan on for say 15 minutes, then returning and shutting it off, there is another way. Replace the fan switch with a timer. I use a push button switch that has varying options for run time, along with a positive "on/off" button. A couple variations are available-5 min to 1/2 hr. / 15 or so to an hour.
You may have to add some wiring and / or a box to separate a light from the fan if you have a combination and want a fan on / light off option.
So, yes, the fan needs enough airflow into the room for it to do its job ... otherwise, if the room is tight, the flow will be low.
You could simple leave the door open after you're done with your shower. Using a crank timer is nice ... then you simply leave and it shuts off later. A dehumidistat can work, too.
I'm a little surprised that you still have issues even though the duct is insulated. Is it well insulated or maybe it has some weak points in the insulation. I think the duct should slope downhill from the fan. Not sure of your config. You vent out the side wall ... but the fan is in the ceiling (e.g. an attic condition where you vent out the gable end wall?)? I think some energy codes have dictated sloping duct downward to a wall vent so there's no tendency for condensate to run back.
Should I replace the insulated ducting with non insulated ducting. I never had a problem before with flexible ducting. It just doesn"t make sense to me that I have to leave a door open to the bathroom for the fan to work. Is this normal?
Air
Let's assume your bathroom door is a 2/6 door with a 1/4" gap from the bottom of the door to the floor.
At 110 cfm the required velocity of air through that gap would be x mph. Through a 7.5 sq in opening. Somehow, I doubt your fan is producing that flow....
With a 1" gap the required velocity of air through that gap would only be 1/4 x mph. Though a 30 sq. in opening.
The fan will move 110 cfm with no intake restriction (unless the manufacturer lied). The greater the available vent area (opening) the closer the fan will come to actually moving it's "rated" 110 cfm.
One could pass 110 cfm through a 7.5 sq in opening at a velocity of 24 mph, but the required fan motor would certainly not be quiet, cheap or practical.
This is why your fan seems to work so much better with the door open - for a given horsepower of the fan motor, less intake restriction = more exhaust flow.
Undercut the door a bit and I think the fan performance will improve. If so inclined, you could experiment to determine the required undercut by blocking the door open a bit - 1/16", 1/8", 3/16", etc until you find a venting level which works for you - then do the math and cut the door.
Jim
Thank you for taking the time to explain everything to me. I really appreciate the advice that everyone has given me. Thanks again Adam
Not necessarily replace w/ uninsulated. But you imply that insulate = flex duct which is not nec the case. I almost always encourage people to never use flex duct unless they have to. Always use rigid as much as possible. Generally speaking, IMO, using flex is dumb. It restricts air flow significantly. It is often installed like a sidewinder ready to strike. That makes it even worse. Any contractor who uses it is simply being flat lazy.
You should have rigid duct sloping to the exterior (make sure you orient the joints right to prevent the condensation from dripping out). If you need flex, minimize it and install it w/ minimal turns ... be tidy.
As the other poster elluded to ... air flow needs to be unrestricted; the less, the more easy flow (is that English?). If the door open helps, that answers part of the question. Air needs to flow easy into the room AND out (that's where the flex duct affects the situation) to be affective. Both have to be there, one won't do.
Huh??
>>Any contractor who uses it is simply being flat lazy.
That's BS. Insulated flex can be installed cleanly and effectively and I have used it many times for bath fans. I've seen plenty of lousy rigid installs.
Respectfully, I stand corrected. There is a time and place for everything. Assuming rigid would work, any contractor who uses flex is lazy ... I don't care how carefully it is installed (which it needs to be when it is needed), it still reduces air flow over rigid.
Yes, even rigid can be installed badly, you are right.
IMHO ... rigid should always be used whenever possible and flex only used in those areas where rigid cannot be used ... which generally shouldn't be very often. And just because you need flex in one place, doesn't mean the whole install should be flex.
I don't care how carefully you install it, if you are using it where rigid could be used, the system will suffer from reduced air flow.
I don't agree.
The last place I installed ducting, I ran about 12 feet of flex from a fan to a sidewall outlet. The fan was about 4 feet below the outlet, about 6 feet horizontally from the wall with the outlet in it, and about 4 feet horizontally offset from the line of the hole. In other words, not aligned in any of the three dimensions, and the discharge faced perpendicular to the outlet wall. I was able to run the flex in a nice big arc right over to the outlet without any tight bends. It hardly poses a flow problem for the fan. Due to a post in the attic the same run in metal would have taken 3 or 4 elbows and more footage of pipe. In this case flex was a lot better than metal.
Making blanket statements about stuff like this is not wise.
It's OK to disagree. It happens frequently here. Everyone has an opinion that may have value.
The situation you describe really sounds just fine for a good rigid installation. You don't have to have any perfect alignment to use rigid duct! That's what the adjustable elbows are for! Using 12 ft of flex to go 6-10 feet sounds to say it mildly ... dumb. Not knowing your specifics, but based on what you described, I'm guessing you could have done that w/ 2-3 elbows and 8 ft of rigid.
IMO unless you have a highly unusual geometry that makes using even adjustable elbows impossible, you shouldn't use flex. In your case as you describe it ... you should at minimum run the rigid between two points and used minimal flex ONLY where you need it.
As you describe it ... your situation is EXACTLY what I was referring to, to begin with. Too lazy to take a couple of adjustable elbows and create the geometry you need. Having a fan install that isn't a straight shot isn't that unusual.
The static pressure loss in a flex duct compared to rigid is substantial. An 80 cfm fan may easily be reduced to 20 cfm using flex. I've measured this before where the seemingly adequate fan simply didn't flow much air because of the details of the install.
Making blanket assumptions in your situation that because you have offsets in two dimensions automatically makes using flex as the only solution is simply preposterous. I would encourage your next install, that you take a fresh look at what you can do w/ an adjustable elbow.
I used flex in my new house, but only where I felt I couldn't use rigid ... but I did run rigid up to and from the awkward spot.
Fan flow ... particularly in many of the tighter newer houses can be critical. Avoiding restrictions can be important. 12 feet of flex is a lot of resistance (relative to rigid) even if installed neatly.
I respect you disagreeing, but I have to continue to disagree w/ you and I stand by my original statements ... even more so w/ your example (understanding of course I don't know ALL the details, but only what you described).
There is a time and place for just about everything ... including flex. I'm saying that you should limit its use to a bare minimum if you care about air flow performance.
I could easily
have run the same thing in rigid. I have installed a lot of rigid pipe, obviously it's standard for any dryer vent or range hood, and some bath fans as well. It just wouldn't be any benefit to the customer to have run this particular vent in rigid, it functions perfectly well as built. The length of flex used is the absolute minimum, it would have taken more footage of rigid and quite a few elbows to come close to the sweep that's easy to attain with flex. What are you going to do, go in the attic with 10 adjustable elbows and chain them together with a few degrees of bend in each one? It just isn't done. I'm accountable to customers for a huge dollar amount of work every year (perhaps you are too?) and have to determine every day how to do quality work without getting ridiculous.
You can continue to call me lazy, Maybe I'll assume that you're milking the job by installing unnecessary material and taking too long? For the sake of the debate I will see if I can come up with some real numbers on airflow loss in the two types of pipe.
I'm not calling you lazy ... just anyone who uses flex when they could have used rigid. Based on your description, I'm guessing 3 elbows would have been the max you would have needed.
10 adjustable elbows chained together ... just isn't done. You are right ... you shouldn't need 10 to do what you described. You are over exagerating to make a point. You aren't doing a service to your customers to run flex as you describe. It functions perfecly well as built ... and you confirmed that with a flow hood, right?
Anyone that does quality work would minimize the length of flex they use, period.
The length of flex you used was the "absolute minimum" ... then you left something out of your description. Seems to me you could have put a very short length of flex on either end and connected the two with rigid rather than run flex the entire distance.
The benefit to the customer is maximizing air flow ... getting as much out of the 110 cfm that was installed and doing so with reasonable effort. It takes a bit more effort to install the rigid, but you have a quality job when you are done and I don't consider that 'ridiculous' as the additional effort isn't that big of a deal.
You are entitled to your opinion, and I mine.
A few references ...
http://www.steelduct.org/uploads/060601_CC-KW_DuctTechPaper.pdf
This discusses that previous values of SP losses in flex are not very reliable and analyzes more 'real world' type situations in flex duct SP losses.
The following implies that flex duct will increase SP by 25-50% depending on the material used
http://www.greenrinc.com/pan/size.htm
Again, to stress my point ... generally speaking and the way flex is more often than not used and installed, using flex duct is highly detrimental for maintaining good air flow. Flex is frequently NOT installed in a manner that will maximize air flow. With many exhaust fans used in residential applications, they have very little excess power for overcoming SP losses caused by flex duct. I'd be surprised that any fan manuf. worth their salt would suggest that their product be installed with either flex or rigid as acceptable alternatives to each other.
The first reference does indicate reasonable performance of flex ... if stretched tight ... which is rarely the case. Anything but that becomes problematic.
Maybe in your case, you installed it with a high degree of quality ... stretched tight and supported properly ... which I suspect would easily take as much effort as installing 2-3 elbows and rigid. So you would have had to put at least as much effort into something that still cannot perform as well as rigid.
Using your argument, the HVAC industry would use flex everywhere to minimize install time. Any good HVAC install will always maximize rigid duct use in all but the most necessary locations (which by current practice is usually limited to the last few feat of supply registers, but many think that is even too much).
http://www.thermaflex.net/pdfs/Air_Flow_Air_Friction_Brochure.pdf
Thermaflex also talks about proper install of its product and reinforces that to get the SP performance they claim, that proper install is critical.
There is a lot of literature available on this topic and it has become well known (by those in the industry it has always been well known), that flex duct is generally highly detrimental to proper air flow.
Again, I'm obviously not familiar w/ the example you provided other than the words you used. You gave us no indication of any particularly unusual geometries of your situation that would beg the use of 12 feet of flex to get from a fan to an exterior wall poin that is 6 ft away, 4 ft higher, and 4 ft offset (e.g. in an open attic/space). Assuming really the only significant obstruction might be a nearby ceiling joist, this seems to me like a pretty straight shot from just above the fan to the exit point. Even w/ e.g. trusses, I'm guessing that you still wouldn't be going through many girations ... but you didn't give any indication of such difficulties.
Just to continue the thread drift
I am arranging to have the fan flow measured on the install I described. I will also post a photo of the ductwork, although I'm not sure I have a len wide enough to show the whole thing at once.
Now back to your regularly scheduled programming.
yo David.................
In the midst of your discussion............
I'm wondering why I haven't heard any answer to the "running the fan longer and making more makeup air available" question.
I think I read that leaving the door open made the situation better.
Could that mean that the existing piping is OK?
I wish when someone asked a question here they would exhaust the answers so in a week or so someone might be able to pull some sensible fix outta it. But that's just me, a dumb carpenter that didn't hit the golf ball too bad this evening. There was something about cause and effect in there somewhere.
I'm too beat
to re-read his replies to our questions, but I think an almost guaranteed solution is to verify adequate flow thru the fan under normal conditions. If normal conditions mean the door and window(s) are closed then test that way, not with the door open. If the door has to be open, or undercut a little more, do that. I can't remember if he told us where he lives or how cold his attic is, but it's pretty much spring at this point, and unless he's way north I doubt it's so cold as to cause significant condensation ASSUMING adequate flow thru the duct. If his 110CFM fan is so poorly ducted as to only run 25CFM then condensation could be a problem even in marginal conditions, i.e. not very cold.
Leaving the door open made the situation better. Ergo ... that IS making more make-up air available.
The problem w/ this forum format is sifting through all the responses and trying to formulate a synopsis of 'what the answer is'. The trouble is ... there is often no real answer ... lots of opinions, lots of food for thought, lots of solutions. The answer is up to the poster to sift the information and figure out what is right for his particular situation.
ergo, no kidding.
.There's no problem with this forum format as far as the disemmination of information goes.
Several good plausible answers were given right away. You come back to the post (not hard to find, there's not many threads, read them, try them.
There were simple-check the fan discharge operation (look for the flapper flapping............
Open the door for more makeup air.
Let the fan run longer to take the moisture out.
He tried one (leaving the door open) and didn't like the idea of showering in public. But it worked (helped). The answer is still correct-more makeup air. Louvered door, add a good looking (?) grill to the door, undercut it more.
Leaving the fan on longer-simple math on that one. Worried about power usage-by forgetting to turn it off? Timer switch-also suggested.
What's the problem. Seems to me there were real answers given-would take all of about two showers to come up with the info on how to fix the situation.
Sorry, I was responding really to calvin who I thought made a comment about a tidy compact compilation of answers vs. having to wade throught misc resposes and side conversations (like this one!) ... but I may have misunderstood his comment, too.
I kind of read his and your posts and maybe thought they were one in the same ...
Clew, you really got me goin'
you got me goin' so I can't sleep at night................
To take a line from an old rock and roll tune.
Sorry, I was responding really to calvin
I am calvin.
Then help the navigator.
Dan
?
Come, now, you're old enough to know that one.
Dan, I'm sorry to say.
While I might have known it, I certainly am having trouble remembering it.
Yosarian was the bombadier, no?
Yep.
Come on Dan, is this 20 questions?
Perhaps clewless could come around enough to help me figure this one out.
Was the navigator reference affiliated in any way to Catch 22?
Now I'm really confused ... oh well ... no offense intended to anyone anyway. This makes the train of the post difficult to follow. Hey such is life, I guess. We'll pick this up again when Calvin or David or whoever was going to put a flow hood on their exhaust fan.
Should be interesting to say the least.
How can i have the duct slop[e towards the outside if the exhast point is higher than the fan. Will i have to bring the duct up over the fan then down to make the downward slope
Bring it up and then go down. But it's most important to have the last 2-3 feet slope down, in case rainwater gets blown in. (Don't ask me how I know.)
Yes. The more you have sloping down to outside, the better (i.e. the less condensate available to run back).
Is There a Window?
You never mentioned if there were a window in the bathroom. Codes usually require an exhaust fan if there's no window, but if you should have both a window and a fan, cracking the window a little can allow for more air to enter the bathroom without having to open the door. I live in Wisconsin and do this all winter with no bad results. My wife once complained about moisture on the walls after her shower, and I asked her if she had opened the window a bit. I got a " you dumb s---" stare from her and " Like it's winter, guy. You don't open windows in the winter."
I just asked my wife to open the door while she showers and got the response that she was too cold. Now mind u it is 70 degrees here in RI today so I have no shot of getting her to open a window while she showers
I saw something interesting the other day.
I was driving by my girlfriend's condo and I saw a starling enter a vent opening in the buildings wall. I believe it was one of those aluminum caps with a door that swings open when the fan blows the air out. The bird flew up to it and was able to quickly pry open the door and get in. I wondered if there was a bird's nest in there?
I believe that they sell plastic grates to put over the door opening to protect against this sort of thing.
Back to YOUR problem
hey there Adam22.....so does the dripping water from the exhaust fan occur only in the cold season? If so then it is condensation as some have told you.
the other stuff mentioned about the door and such is another topic..... but the condensation is most likely the cold air from the outside hitting the warm air being blown out of the house.....where the two meet you get condensation. So yes like the one person said try and slope as much sloping down to the outside vent. Insulate it all.
Condensation turns to water and water runs downhill. It is simple physics.
Is the area where this duct runs all finished off? Is there any access to the ducting? If not, then you might not now it is slopes or not.If you removed the exterioe vent you might get a view?
One thing about sloping the flex stuff, if you get a low spot some sagging, and if there is a little rip, it could still drip on the top of your ceiling...or if the metal wires are exposed, they could rot. If it is rigid metal ducting, it might rust eventually, I'd use PVC piping for the dust.
If the problem is worse with the bigger CFM fan, maybe it is because you are moving more warm air.
Is this a basement bathroom?
Good luck these can be tricky.
other stuff mentioned about the door and such is another topic..
So what's the answer for the water "dripping" down his walls in this bath?
The door..........
leaving the fan run ...............
other?