I've been trying to find an answer to this problem for over 2 weeks
now.

Here is the situation:

2 newly installed panasonic 80 cfm bathroom exhaust fans(the kind that
fit in a 2x4 joist space) in two different bathrooms. These fans have
their own internal flapper.

Bathroom 1: 8 x 4 x 8
Bathroom 2: 8 x 5 x 8

I'm using 3" sheet meal ducting(the kind you have to "snap" together)
going horizontally out the side walls instead of going straight out
the roof. They duct out to exhaust hoods which also have a flapper on
them. I also have put insulation on the whole duct run.(not tried
putting insulation on the fan housing yet due to mold concerns.)
The duct runs are about 4' each from the exhaust fan and slope
downward from the start of exhaust fan to the wall, so you would think
condensation would just drip out towards the wall. You can go outside
while the fan is running and seems like its pushing out plently of
air. The fans are mounted in between the shower and the vanity right
above the toilet. The are mounted at a downward angle since the
sheetrock ceiling has a downward shape that matches my open beam
ceiling slope patter.
My whole house is all open bean ceilings, there is no attic. The duct
run is made above newly installed sheetrock and the open beam
ceilings, with a 4" space between the sheetrock and the open beam
ceiling. My open beam ceilings are vaulted going downward towards the
side wall, so the ducting runs down the similar open beam downward
slope. The ducting is not bent at all in the larger of the bathrooms,
and has a slight bend in the smaller of the two bathrooms going out
the side wall.
When I take a shower in the smaller of the two bathrooms, after about
5-6 minutes, condensation starts leaking from the fan housing thru the
grille. I thought it was due to it being pretty cold outside, but
today was fairly warmer and the same results happended.
In the larger of the two bathrooms, it takes slightly longer to
condensate when taking a hot shower since its a bigger room, but it
happens in that bathroom as well.

Can anyone shed some light as to how I should go about solving this
issue, as I am out of ideas.

thanks,
Isaac

On Feb 4, 7:55 pm, wrote:
I've been trying to find an answer to this problem for over 2 weeks
now.

Here is the situation:

2 newly installed panasonic 80 cfm bathroom exhaust fans(the kind that
fit in a 2x4 joist space) in two different bathrooms. These fans have
their own internal flapper.

Bathroom 1: 8 x 4 x 8
Bathroom 2: 8 x 5 x 8

I'm using 3" sheet meal ducting(the kind you have to "snap" together)
going horizontally out the side walls instead of going straight out
the roof. They duct out to exhaust hoods which also have a flapper on
them. I also have put insulation on the whole duct run.(not tried
putting insulation on the fan housing yet due to mold concerns.)
The duct runs are about 4' each from the exhaust fan and slope
downward from the start of exhaust fan to the wall, so you would think
condensation would just drip out towards the wall. You can go outside
while the fan is running and seems like its pushing out plently of
air. The fans are mounted in between the shower and the vanity right
above the toilet. The are mounted at a downward angle since the
sheetrock ceiling has a downward shape that matches my open beam
ceiling slope patter.
My whole house is all open bean ceilings, there is no attic. The duct
run is made above newly installed sheetrock and the open beam
ceilings, with a 4" space between the sheetrock and the open beam
ceiling. My open beam ceilings are vaulted going downward towards the
side wall, so the ducting runs down the similar open beam downward
slope. The ducting is not bent at all in the larger of the bathrooms,
and has a slight bend in the smaller of the two bathrooms going out
the side wall.
When I take a shower in the smaller of the two bathrooms, after about
5-6 minutes, condensation starts leaking from the fan housing thru the
grille. I thought it was due to it being pretty cold outside, but
today was fairly warmer and the same results happended.
In the larger of the two bathrooms, it takes slightly longer to
condensate when taking a hot shower since its a bigger room, but it
happens in that bathroom as well.

Can anyone shed some light as to how I should go about solving this
issue, as I am out of ideas.

thanks,
Isaac

Every output needs an input. You need an air makeup grill in the wall,
door or wherever. The makeup air can come from any house area that is
dryer than the bath. You will be amazed at how much more output your
fans will have and I doubt you will see any more condensation.
Reminds me of the many times I've seen techs fight a wiring problem in
a car for hours only to find after all the struggle that it was a
problem in the ground, not the positive side. Remember moving air goes
in circuits, too. HTH

Joe

On Feb 4, 6:18 pm, "Joe" wrote:
On Feb 4, 7:55 pm, wrote:





I've been trying to find an answer to this problem for over 2 weeks
now.

Here is the situation:

2 newly installed panasonic 80 cfm bathroom exhaust fans(the kind that
fit in a 2x4 joist space) in two different bathrooms. These fans have
their own internal flapper.

Bathroom 1: 8 x 4 x 8
Bathroom 2: 8 x 5 x 8

I'm using 3" sheet meal ducting(the kind you have to "snap" together)
going horizontally out the side walls instead of going straight out
the roof. They duct out to exhaust hoods which also have a flapper on
them. I also have put insulation on the whole duct run.(not tried
putting insulation on the fan housing yet due to mold concerns.)
The duct runs are about 4' each from the exhaust fan and slope
downward from the start of exhaust fan to the wall, so you would think
condensation would just drip out towards the wall. You can go outside
while the fan is running and seems like its pushing out plently of
air. The fans are mounted in between the shower and the vanity right
above the toilet. The are mounted at a downward angle since the
sheetrock ceiling has a downward shape that matches my open beam
ceiling slope patter.
My whole house is all open bean ceilings, there is no attic. The duct
run is made above newly installed sheetrock and the open beam
ceilings, with a 4" space between the sheetrock and the open beam
ceiling. My open beam ceilings are vaulted going downward towards the
side wall, so the ducting runs down the similar open beam downward
slope. The ducting is not bent at all in the larger of the bathrooms,
and has a slight bend in the smaller of the two bathrooms going out
the side wall.
When I take a shower in the smaller of the two bathrooms, after about
5-6 minutes, condensation starts leaking from the fan housing thru the
grille. I thought it was due to it being pretty cold outside, but
today was fairly warmer and the same results happended.
In the larger of the two bathrooms, it takes slightly longer to
condensate when taking a hot shower since its a bigger room, but it
happens in that bathroom as well.

Can anyone shed some light as to how I should go about solving this
issue, as I am out of ideas.

thanks,
Isaac

Every output needs an input. You need an air makeup grill in the wall,
door or wherever. The makeup air can come from any house area that is
dryer than the bath. You will be amazed at how much more output your
fans will have and I doubt you will see any more condensation.
Reminds me of the many times I've seen techs fight a wiring problem in
a car for hours only to find after all the struggle that it was a
problem in the ground, not the positive side. Remember moving air goes
in circuits, too. HTH

Joe- Hide quoted text -

- Show quoted text -

So you don't think its too cold in the space between open beam celing
and sheetrock?

The one thing I did which made it not leak at all, was to leave a
space heater on while taking a shower....the fan did operate much
better not even letting the bathroom mirror to fog up. So in this case
your theory proves correct except it wasn't just any air....it was
WARM air I was adding into the bathroom. Any thoughts about this as
well?

thanks so much for the prompt response

The one thing I did which made it not leak at all, was to leave a
space heater on while taking a shower....the fan did operate much
better not even letting the bathroom mirror to fog up. So in this case
your theory proves correct except it wasn't just any air....it was
WARM air I was adding into the bathroom. Any thoughts about this as
well?

thanks so much for the prompt response

Pondering your duct sizing, could there be a mismatch? 3 x 14's are
42" sq,, 4 x 10's are 40"sq., 7" round 38.5" sq., Many bath fans use
4" round outlets (12.6 sq.") which prompted the question. Might be
your moist exhaust air is meandering down the ductwork and being over-
cooled on the trip.This suggests a) increasing the velocity with
smaller ducting, b) using fiberglass or insulated ductwork or, c) all
of the above and paying atterntion to makeup air, of course. HTH

Joe

On Feb 5, 9:43 am, "Joe" wrote:
The one thing I did which made it not leak at all, was to leave a
space heater on while taking a shower....the fan did operate much
better not even letting thebathroommirror to fog up. So in this case
your theory proves correct except it wasn't just any air....it was
WARM air I was adding into thebathroom. Any thoughts about this as
well?

thanks so much for the prompt response

Pondering your duct sizing, could there be a mismatch? 3 x 14's are
42" sq,, 4 x 10's are 40"sq., 7" round 38.5" sq., Many bath fans use
4" round outlets (12.6 sq.") which prompted the question. Might be
your moist exhaust air is meandering down the ductwork and being over-
cooled on the trip.This suggests a) increasing the velocity with
smaller ducting, b) using fiberglass or insulated ductwork or, c) all
of the above and paying atterntion to makeup air, of course. HTH

Joe

Nope this fan uses a 3" duct and says so in the manual. its a
panasonic model FV-08VS1,,,,and for some reason they don't have it on
their website. Since it is a smaller fan(fits in a 2 x4 joist instead
of a 2x6 joist), that is the reason for the smaller ducting. The
makeup air does not seem to help, as leaving the bathroom door wide
open still results in a good amount of condensation. The condensation
is definitely coming from the fan housing itself as the duct slopes
downward AWAY from fan and has fiberglass insulation wrapped around
the ducting. Are you saying that by using even smaller ducting, that
it will increase the overall velocity?

I should also mention that we had the same problem using a nutone
model 695 which had 70cfm.

Perhaps we just need a stronger vent like 110 cfm?

wrote:
On Feb 5, 9:43 am, "Joe" wrote:

The one thing I did which made it not leak at all, was to leave a
space heater on while taking a shower....the fan did operate much
better not even letting thebathroommirror to fog up. So in this case
your theory proves correct except it wasn't just any air....it was
WARM air I was adding into thebathroom. Any thoughts about this as
well?

thanks so much for the prompt response

Pondering your duct sizing, could there be a mismatch? 3 x 14's are
42" sq,, 4 x 10's are 40"sq., 7" round 38.5" sq., Many bath fans use
4" round outlets (12.6 sq.") which prompted the question. Might be
your moist exhaust air is meandering down the ductwork and being over-
cooled on the trip.This suggests a) increasing the velocity with
smaller ducting, b) using fiberglass or insulated ductwork or, c) all
of the above and paying atterntion to makeup air, of course. HTH

Joe


Nope this fan uses a 3" duct and says so in the manual. its a
panasonic model FV-08VS1,,,,and for some reason they don't have it on
their website. Since it is a smaller fan(fits in a 2 x4 joist instead
of a 2x6 joist), that is the reason for the smaller ducting. The
makeup air does not seem to help, as leaving the bathroom door wide
open still results in a good amount of condensation. The condensation
is definitely coming from the fan housing itself as the duct slopes
downward AWAY from fan and has fiberglass insulation wrapped around
the ducting. Are you saying that by using even smaller ducting, that
it will increase the overall velocity?

I should also mention that we had the same problem using a nutone
model 695 which had 70cfm.

Perhaps we just need a stronger vent like 110 cfm?

It would appear that since the duct slopes away from the fan, the fan
housing is cold enough to condense the moist air right away. If you
cannot insulate around it you may just have to deal with it.

BTW, I noticed my bathroom fogs much less with the door closed than
open. It is my thought that the easily available [dry] air from the
bedroom gets to the fan before all the air in the bathroom. Closing the
door makes it draw from the room more, perhaps.

On Feb 5, 11:01 am, yourname wrote:
wrote:
On Feb 5, 9:43 am, "Joe" wrote:

The one thing I did which made it not leak at all, was to leave a
space heater on while taking a shower....thefandid operate much
better not even letting thebathroommirror to fog up. So in this case
your theory proves correct except it wasn't just any air....it was
WARM air I was adding into thebathroom. Any thoughts about this as
well?

thanks so much for the prompt response

Pondering your duct sizing, could there be a mismatch? 3 x 14's are
42" sq,, 4 x 10's are 40"sq., 7" round 38.5" sq., Many bath fans use
4" round outlets (12.6 sq.") which prompted the question. Might be
your moist exhaust air is meandering down the ductwork and being over-
cooled on the trip.This suggests a) increasing the velocity with
smaller ducting, b) using fiberglass or insulated ductwork or, c) all
of the above and paying atterntion to makeup air, of course. HTH

Joe

Nope thisfanuses a 3" duct and says so in the manual. its a
panasonic model FV-08VS1,,,,and for some reason they don't have it on
their website. Since it is a smallerfan(fits in a 2 x4 joist instead
of a 2x6 joist), that is the reason for the smaller ducting. The
makeup air does not seem to help, as leaving the bathroom door wide
open still results in a good amount ofcondensation. Thecondensation
is definitely coming from thefanhousing itself as the duct slopes
downward AWAY fromfanand has fiberglass insulation wrapped around
the ducting. Are you saying that by using even smaller ducting, that
it will increase the overall velocity?

I should also mention that we had the same problem using a nutone
model 695 which had 70cfm.

Perhaps we just need a stronger vent like 110 cfm?

It would appear that since the duct slopes away from thefan, thefan
housing is cold enough to condense the moist air right away. If you
cannot insulate around it you may just have to deal with it.

BTW, I noticed my bathroom fogs much less with the door closed than
open. It is my thought that the easily available [dry] air from the
bedroom gets to thefanbefore all the air in the bathroom. Closing the
door makes it draw from the room more, perhaps.- Hide quoted text -

- Show quoted text -

I can try insulating around it, but am fearful that the insulation
will just absorb moisture leading to mold.
I also notice that the bathroom fogs a tad less with the door and
window closed, but still fogs and the fan still condensates big time(a
few tablespoons actually).

I am pulling my hair out trying to find an answer to this tough puzzle.

I am pulling my hair out trying to find an answer to this tough puzzle.

From your data (3 " outlet, 3" x 14" duct)) you have a grossly
oversized duct for a small fan. The ratio, sizewise, is an amazing 6
to 1. Your output velocity therefore is so slow as to allow a
backdraft of cold air to flow back to the fan unit and cause
condensation there. Your options may be simple, like use a 3" PVC pipe
for a vent to take advantage of less heat loss than a 3" metal duct
would have. Theoretically you could add 5 more Panasonics, obviously
not the practical thing to do. Vertical ducting may be a solution,
since warm air rises the system would have some advantage from a
natural draft. IMO this could be the best way to get a workable
system, one that doesn't flout the laws of physics. Good luck.

Joe

On Feb 5, 12:53 pm, wrote:
On Feb 5, 11:01 am, yourname wrote:





wrote:
On Feb 5, 9:43 am, "Joe" wrote:

The one thing I did which made it not leak at all, was to leave a
space heater on while taking a shower....thefandid operate much
better not even letting thebathroommirror to fog up. So in this case
your theory proves correct except it wasn't just any air....it was
WARM air I was adding into thebathroom. Any thoughts about this as
well?

thanks so much for the prompt response

Pondering your duct sizing, could there be a mismatch? 3 x 14's are
42" sq,, 4 x 10's are 40"sq., 7" round 38.5" sq., Many bath fans use
4" round outlets (12.6 sq.") which prompted the question. Might be
your moist exhaust air is meandering down the ductwork and being over-
cooled on the trip.This suggests a) increasing the velocity with
smaller ducting, b) using fiberglass or insulated ductwork or, c) all
of the above and paying atterntion to makeup air, of course. HTH

Joe

Nope thisfanuses a 3" duct and says so in the manual. its a
panasonic model FV-08VS1,,,,and for some reason they don't have it on
their website. Since it is a smallerfan(fits in a 2 x4 joist instead
of a 2x6 joist), that is the reason for the smaller ducting. The
makeup air does not seem to help, as leaving thebathroomdoor wide
open still results in a good amount ofcondensation. Thecondensation
is definitely coming from thefanhousing itself as the duct slopes
downward AWAY fromfanand has fiberglass insulation wrapped around
the ducting. Are you saying that by using even smaller ducting, that
it will increase the overall velocity?

I should also mention that we had the same problem using a nutone
model 695 which had 70cfm.

Perhaps we just need a stronger vent like 110 cfm?

It would appear that since the duct slopes away from thefan, thefan
housing is cold enough to condense the moist air right away. If you
cannot insulate around it you may just have to deal with it.

BTW, I noticed mybathroomfogs much less with the door closed than
open. It is my thought that the easily available [dry] air from the
bedroom gets to thefanbefore all the air in thebathroom. Closing the
door makes it draw from the room more, perhaps.- Hide quoted text -

- Show quoted text -

I can try insulating around it, but am fearful that the insulation
will just absorb moisture leading to mold.
I also notice that thebathroomfogs a tad less with the door and
window closed, but still fogs and the fan still condensates big time(a
few tablespoons actually).

I am pulling my hair out trying to find an answer to this tough puzzle.- Hide quoted text -

- Show quoted text -

do you mean my duct run is too long?....i'm not sure where you get the
14" measurement from.

The 3 " duct size fits the opening perfectly on the housing.

On Feb 6, 4:32 am, wrote:
On Feb 5, 12:53 pm, wrote:





On Feb 5, 11:01 am, yourname wrote:

wrote:
On Feb 5, 9:43 am, "Joe" wrote:

The one thing I did which made it not leak at all, was to leave a
space heater on while taking a shower....thefandid operate much
better not even letting thebathroommirror to fog up. So in this case
your theory proves correct except it wasn't just any air....it was
WARM air I was adding into thebathroom. Any thoughts about this as
well?

thanks so much for the prompt response

Pondering your duct sizing, could there be a mismatch? 3 x 14's are
42" sq,, 4 x 10's are 40"sq., 7" round 38.5" sq., Many bath fans use
4" round outlets (12.6 sq.") which prompted the question. Might be
your moist exhaust air is meandering down the ductwork and being over-
cooled on the trip.This suggests a) increasing the velocity with
smaller ducting, b) using fiberglass or insulated ductwork or, c) all
of the above and paying atterntion to makeup air, of course. HTH

Joe

Nope thisfanuses a 3" duct and says so in the manual. its a
panasonic model FV-08VS1,,,,and for some reason they don't have it on
their website. Since it is a smallerfan(fits in a 2 x4 joist instead
of a 2x6 joist), that is the reason for the smaller ducting. The
makeup air does not seem to help, as leaving thebathroomdoor wide
open still results in a good amount ofcondensation. Thecondensation
is definitely coming from thefanhousing itself as the duct slopes
downward AWAY fromfanand has fiberglass insulation wrapped around
the ducting. Are you saying that by using even smaller ducting, that
it will increase the overall velocity?

I should also mention that we had the same problem using a nutone
model 695 which had 70cfm.

Perhaps we just need a stronger vent like 110 cfm?

It would appear that since the duct slopes away from thefan, thefan
housing is cold enough to condense the moist air right away. If you
cannot insulate around it you may just have to deal with it.

BTW, I noticed mybathroomfogs much less with the door closed than
open. It is my thought that the easily available [dry] air from the
bedroom gets to thefanbefore all the air in thebathroom. Closing the
door makes it draw from the room more, perhaps.- Hide quoted text -

- Show quoted text -

I can try insulating around it, but am fearful that the insulation
will just absorb moisture leading to mold.
I also notice that thebathroomfogs a tad less with the door and
window closed, but still fogs and the fan still condensates big time(a
few tablespoons actually).

I am pulling my hair out trying to find an answer to this tough puzzle.- Hide quoted text -

- Show quoted text -

do you mean my duct run is too long?....i'm not sure where you get the
14" measurement from.

The 3 " duct size fits the opening perfectly on the housing.- Hide quoted text -

- Show quoted text -

I don;t see why you;'re reluctant to insulate around the fan
housing. The exhaust pipe slopes down, so we know the water isn't
condensing there and running back. It has to be condensing inside the
fan. Putting insulation around the fan will keep the fan from
getting cold. Any moisture should be reduced and it should be
contained on the inside of the fan housing and will not get the
insulation wet. I would check any electrical rating concerns related
to how close insulation can be to the fan housing.

On Feb 6, 5:33 am, wrote:
On Feb 6, 4:32 am, wrote:





On Feb 5, 12:53 pm, wrote:

On Feb 5, 11:01 am, yourname wrote:

wrote:
On Feb 5, 9:43 am, "Joe" wrote:

The one thing I did which made it not leak at all, was to leave a
space heater on while taking a shower....thefandid operate much
better not even letting thebathroommirror to fog up. So in this case
your theory proves correct except it wasn't just any air....it was
WARM air I was adding into thebathroom. Any thoughts about this as
well?

thanks so much for the prompt response

Pondering your duct sizing, could there be a mismatch? 3 x 14's are
42" sq,, 4 x 10's are 40"sq., 7" round 38.5" sq., Many bath fans use
4" round outlets (12.6 sq.") which prompted the question. Might be
your moist exhaust air is meandering down the ductwork and being over-
cooled on the trip.This suggests a) increasing the velocity with
smaller ducting, b) using fiberglass or insulated ductwork or, c) all
of the above and paying atterntion to makeup air, of course. HTH

Joe

Nope thisfanuses a 3" duct and says so in the manual. its a
panasonic model FV-08VS1,,,,and for some reason they don't have it on
their website. Since it is a smallerfan(fits in a 2 x4 joist instead
of a 2x6 joist), that is the reason for the smaller ducting. The
makeup air does not seem to help, as leaving thebathroomdoor wide
open still results in a good amount ofcondensation. Thecondensation
is definitely coming from thefanhousing itself as the duct slopes
downward AWAY fromfanand has fiberglass insulation wrapped around
the ducting. Are you saying that by using even smaller ducting, that
it will increase the overall velocity?

I should also mention that we had the same problem using a nutone
model 695 which had 70cfm.

Perhaps we just need a stronger vent like 110 cfm?

It would appear that since the duct slopes away from thefan, thefan
housing is cold enough to condense the moist air right away. If you
cannot insulate around it you may just have to deal with it.

BTW, I noticed mybathroomfogs much less with the door closed than
open. It is my thought that the easily available [dry] air from the
bedroom gets to thefanbefore all the air in thebathroom. Closing the
door makes it draw from the room more, perhaps.- Hide quoted text -

- Show quoted text -

I can try insulating around it, but am fearful that the insulation
will just absorb moisture leading to mold.
I also notice that thebathroomfogs a tad less with the door and
window closed, but still fogs and thefanstill condensates big time(a
few tablespoons actually).

I am pulling my hair out trying to find an answer to this tough puzzle.- Hide quoted text -

- Show quoted text -

do you mean my duct run is too long?....i'm not sure where you get the
14" measurement from.

The 3 " duct size fits the opening perfectly on the housing.- Hide quoted text -

- Show quoted text -

I don;t see why you;'re reluctant to insulate around thefan
housing. The exhaust pipe slopes down, so we know the water isn't
condensing there and running back. It has to be condensing inside thefan. Putting insulation around thefanwill keep thefanfrom
getting cold. Any moisture should be reduced and it should be
contained on the inside of thefanhousing and will not get the
insulation wet. I would check any electrical rating concerns related
to how close insulation can be to thefanhousing.- Hide quoted text -

- Show quoted text -

I am going to try both insulating the fan housing itself as well as
insulation my floors of both bathrooms since I know that cool air is
coming up through the walls from the crawlspace.

I'll let you guys know my results.

- Show quoted text -

do you mean my duct run is too long?....i'm not sure where you get the
14" measurement from.

The 3 " duct size fits the opening perfectly on the housing.

My error...your stated 3" duct size led me to believe it was 3" round
ducting. It must therefore follow, that your outlet size is 3" x 14"
rectangular. That being the case, keeping the troubled area warm by
insulation and heat tape may solve the problem. The heat tape
logically would be be powered by the fan circuit. So far as which to
try fist, the heat tape would be much easier to install. If it does
the job, quit there. If you judge that some insulation might help, go
for it.

Je

On Feb 7, 9:55 am, "Joe" wrote:
- Show quoted text -

do you mean my duct run is too long?....i'm not sure where you get the
14" measurement from.

The 3 " duct size fits the opening perfectly on the housing.

My error...your stated 3" duct size led me to believe it was 3" round
ducting. It must therefore follow, that your outlet size is 3" x 14"
rectangular. That being the case, keeping the troubled area warm by
insulation and heat tape may solve the problem. The heat tape
logically would be be powered by thefancircuit. So far as which to
try fist, the heat tape would be much easier to install. If it does
the job, quit there. If you judge that some insulation might help, go
for it.

Je

what do you mean by outlet size??? the outlet of the fan that connects
to the ducting?....that is also a 3" round.