I installed a Panasonic FV11VQ3 110 CFM bathroom vent fan in a small
bathroom. Works like a champ. No steam on the mirror and within minutes
(with door or window open) majority of steam is cleared.
My one problem is this; I get standing water inside the fan. I wrapped
the duct work and unit with insulation; still condensation turning into
small pool of water. I check for leaks in the roof cap... none. I
switch the duct work from the stiff flexible/expanding type to the
smooth normal duct work; still get pool of water.
I researched google groups and some people recommended running the fan
before starting the shower to warm it up. I don't think this is the
issue because I did some "condensation" tests in the middle of the
afternoon and it was not cold in the attic. I will still try this
though. Also another person recommended having a door slightly open or
a window. I will do this as well.
Would anyone have any other suggestions in case this does not work? One
thing I will mention the instructions ask for 2-3 feet of horizontal
pipe before going to the roof cap and I have 1 foot due to space
issues. I mentioned this to some people with building experience and
said this should not cause a problem.
Thank in advance.
If the water collects only during/after the shower operation, so that
you know that leaking from the outside is not the source of the water,
then it is simply a matter of water from the shower condensing in the
duct and running back to a spot where it collects. The advice to turn
the fan on just before turning on the water is good. Hot water
generates a lot of water vapor, which moves rapidly through the room
and starts condensing on anything cooler, including and especially the
duct going through the attic. Moving air through the duct from the
moment the water is turned on moves much of the moisture generated
straight outside and to some extent localizes the humidity to the
shower stall itself. You also want to leave the fan on for several
minutes after the water is turned off, to give the duct a chance to dry
out. If you wait until you are done with the shower, then you're too
late; water has condensed in a film on the mirrors, walls, duct, and
even inside the walls if the bathroom doesn't have a really good vapor
barrier installed. Trying to revaporize water condensed on the walls
after the fact is harder than simply moving the vapor outside before
much condensation takes place. You also need to make sure that the air
is flowing, which can be done by going up on the roof while the fan is
blowing. If the flapper is stuck, and little air is flowing, then the
only thing drying out the room is the open door and window.
It's clear you discovered the reason for the recommendation of having a
short horizontal run. The run should be close to the fan not close to the
Just because the air in the attic in the afternoon is warm, dosen't
necessarily mean the vent pipe and anything it is resting on/against is also
warm. All you need is to have something below the dew point to get
condensation. Since the dew point is relative to the RH, a steamy room
condenses on fairly warm surfaces. At near 100% RH, water will condensce
on just about anything.
Even with a 150W heat lamp in the vent (my setup) I still get condensation
from overflow from my (new) steam room. I may need to move the vent pipe to
mitigate also but I think it does have a horizontal section near the vent
already. I need to look more closely to see how much is condenscing in the
fan housing itself before the pipe.
I may try to put a dip in the vent pipe (using metal flex pipe) to see if I
can make some sort of P trap and poke a hole for drainage to a pan. I don't
expect more than a few ounces per day so it should evaporate without any
further draining (never reaches freezing at my house in CA). Still thinking
of that solution as it could make for a bacterial breeding ground if the
water does not evaporate daily.
Another solution I am considering is to snake a short piece of wire in the
vent to encourage the condensate to drain where I want it (not dripping from
the middle of the heat lamp). I can probably collect it into a sponge which
will be dried by the adjascent heat lamp. ( I will try this first as it
does not require access to the attic)
Thanks for the quick input. To be clear the condensation is happening
inside the fan itself, the duct work is bone dry except where the
"male" duct part of the fan receives the "female part" of the duct
work. The water inside the fan will drip onto the duct connection,
other than that dry. As far as inulation the fan unit is wrapped and
the duct work as well.
Also I have not sealed the joints with water proof tape due to the
constant removal and installing for troubleshooting. Can this be an
issue as well?
Pipedown, if all else fails I will move the fan 1 1/2 feet to meet the
specs. I'm hoping after that undertaking that the problem will no
So it seems these are the standar remedies for condensation in the
- Insulation is needed around pipes and fan
- Open a window or door slighty to allow better air flow.
- Turn fan on a few minutes before shower to warm up fan and minimize
condensation inside of unit.
- Let fan run for a while after to evaporate all steam in unit and
I will try #2 and #3. If this fails I will reposition the fan to allow
for a longer horizontal run as recommended by the manufacturer.
I'm wondering if you have good air flow? Have you checked the outside vent
to see if air is actually coming out when the fan is on. It is possible
that the vent flap is stuck. If you are using flex duct check to make sure
that it is not crushed anywhere. Are you able to see the fan blade spinning
at high speed? Make sure that the damper on the fan is opening when the fan
Well the weekend has past and here are the results of my
troubleshooting. For the sake of being brief I will use bullet points:
- With the Window open for more air flow I still get condensation
insided the vent fan.
- With insulation around the air duct and bathroom fan I still get
condensation inside the vent fan.
- I switched to the smooth metal air duct instead of the flexible type;
- I ran the fan for 5 minutes before running the shower and for 5
minutes after and still condenstaion.
- When I turn the fan on I can hold a piece of paper by the fan and the
paper will stick to the grille.
Also just to be clear the condensation will build up over 2 days to
form standing water and ends up dripping out of the fan.
The vent flap on the roof vent opens when the fan turns on, the flap on
the fan is opens as well. The condesation is not coming back from the
vents because I have a slight angle going down, away from the fan.
A couple of other items for thought. My bathroom fan is in the attic
about 3-4 feet away from a soffit vent. It's not directly by it but
it's in the area. Also I am using aircondition/heater vent insulation;
about 3/4" thick. Could these things be an issue?
The next thing I am going to try is remove the flap from the fan, to
see if this allows faster airflow. Also someone mentioned to me that
it's possible that the climate in my house and attic is just going to
produce this affect of condenstaion so I might need to figure out a way
to get the condensation out. Possibly there is a fan made with some
sort of drip that I can attach pipe to to channel the water out;
similar to how A/C. Any thoughts on this.
Insulating the fan box then preheating it before running the fan might help
but if you read my earlier post, I suggest it's a lost cause since very
humid air will still condensce on even warmish surfaces because the dew
point is raised so high when steam is present.
I suggest instead of preventing the water, deal with it instead, give it a
place to collect and drain or better yet, evaporate. A sponge or cloth
inside the housing will absorb the water and with its large surface area,
evaporate that water more efficiently than standing in a puddle.
Checking the Broan website they suggest the same advice you already have,
namely the condensation is from cold duct work. My observation and
seemingly yours is that most of the water is forming on the fan metal
housing moreso than the duct. In this case insulate more directly on top of
the fan and make sure the flapper is closing so cold outside air does not
chill the housing more than necessary
I don't think that I have ever heard of this problem before. A few things
come to mind such as the CFM and the size of the duct. I have seen
instances where a 4" fan outlet is reduced to 3" duct. I am also wondering
if maybe your bathroom is too cold. Perhaps a bathroom heater might help.
I am just guessing here as I thought that the insulation would have
corrected the problem. This is a long shot, but are there any water pipes
in the vicinity of the fan?
Before you use the shower and turn on the fan does it feel cold? Do you
feel cold air coming out of it? What would happen if you used the fan
without the decorative cover on? Where is the fan in relation to the
I have seen this kind of problem when the fan is not sealed to the
ceiling. Where the fan housing penetrates the ceiling, seal around it
in the attic with expanding foam. This keeps the attic air from being
drawn into the fan housing along with the room air when the fan is
runnind, as the inside of the housing is under a vacuum. When the
humid room air mixes with the cold attic air, it causes condensation
inside the fan housing and on the surface of the fan grille. You could
also use silicone. Make sure you leave the grille off while the foam
or silicone is curing, other the grille will be glued to the fan
housing. Note this can also happen in the summer with humid air in the
attic and the room air conditioned.
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