I've just upgraded the pathetic nuisance of a bath fan the house came
with....there is now a shiny Nutone Quiettest 110 cfm fan in there. Also
upgraded the crappy plastic flex hose to 4" aluminum flex hose and installed
the hose insulation jacket on it. Couple of Q's on this now...
1. the opening around the fan box - of course the original drywall ceiling
opening was cut jagged and imprecise, and we had to do some more simple
tweaking on the opening to get the new box in there. this has left some
gaps around it for air to pass up through. What is the proper method to
sealing this? I'm assuming I should wrap the box in vapour barrier, and
then pack insulation around it? should I bother with vapour barrier at all
and just just stuff the gaps with insulation?
2. the roof vent - of course the vent on the roof itself is of 3" design,
and the new fan is 4"!! needless to say to get things going this time of
year I picked up a 3" to 4" reducer and connected it as-is. do you
recommend having a roofer upgrade to a better quality 4" vent? am I losing
much by adapting down like this? is it something to even worry about??
otherwise the fan seems to be VERY effective. I can no longer leave the
bathroom window open (its in the shower area) with the fan running because
it draws so much air with the room closed that there is literally a cold
wind being sucked in!! having the hottest shower I could (to create as much
steam as possible), the mirror never got an ounce of moisture on it. and
within 1 or 2 minutes of the shower ending, the room is completely clear.
needless to say I'm happy, I just wanna be sure i finish up the job
Contemplating the same job so I will follow this post. Question for
you: How big is your bathroom and how quiet is the fan you chose?
Sounds efficient. My problem is allergies to perfumes and such. TIA
the bathroom is about 97 sq feet. I'd say the fan isn't brutally quiet like
the high end Panasonic fans in my friends house (they're big cabbage tho).
but its not annoyingly loud either. I'm impressed with how "smooth" the
we have a shower radio we use every day and it doesn't have to be turned up
any louder to hear it than you normally would to just overcome the noise of
the water...if that means anything.
The 3 inch is more than adequate. I'd suggest stopping it down even
more as it is drawing too much of that expensive heated/cooled air out
of your house.
It doesn't take much fan to do the job you need doing.
On Tue, 24 Feb 2004 01:51:21 GMT, "Hamilton Audio"
hang on here....the fan is 110 cfm, and the box stated it was for rooms up
to 120 sq. feet. In speaking
with a few individuals, namely the contractors upgrading my parents bathroom
right now, they say that
I should get a MINIMUM of 100 cfm to perform well. My choice was either 90
or 110, so I got the
larger one (rather have a bit more capacity than not enough, you know?)
the fact that the roof vent is only 3" is a matter of what the previous
owner left me with. I conciously
upgraded to aluminum/insulated pipe with taped and clamped connections and
HAD to deal with
the 3" factor for what was already there. I fully intend to have the vent
upgraded in the summer when
its actually possible (its Feb. and where I live its VERY cold in the
winter) I simply asked if the 4" to 3"
adaptation would imposeany serious issues, and the resounding answer has
been NO. a reduction in potential
I can't see how a 4" to 3" pipe reduction could have ANY impact on the life
of the motor. we're also
not taking into consideration the bends on the way up to the vent (there are
2) and the fact that almost
all flexible duct is corrugated in some way, which is ALSO a hamper to
performance. based on what
logic will the motor's life be compromised? I agree it is NOT moving 110
true cfm right now, but the
life of the motor is compromised??? I'm interested to know....
50 CFM PER PERSON.
So..if you have 3 people, it would need to be a 150CFM fan....forget the
size of the room. You first size to code.
All I was stating to the other poster was that choking it was not a good
So...you change it. Its simple, and takes under an hour normally.
We had to do the same thing today as a matter of fact...change not only a
bathroom fan and vent but also the dryer, since on this remodel the previous
owner had run flex....all 45 feet of it. The bathroom fan was run in 3 inch
duct, and the motor was long time dead....took about 45 minutes to remove
the old fan, and box, pull the old duct out, put a new section in and put a
4 inch hole in the roof, put a termination cap on it, and seal the roof.
Quick, and cheap insurance that the fan will last, and do the job its bought
and designed for.
No is the wrong answer.
When you reduce airflow, particularly on the cheaper fans like the one you
have...you reduce the load on the motor...now that sounds great right?
What you get, is a fan that speeds up....oh...sounds better right?
Oh..and believe it or not, I understand the cold. We work in it daily.
Tomorrow they are calling for up to 15 inchs of new snow, and I plan on
being right there in the middle of it.
To anyone that has a clue about any aspect of HVAC and how and why motors
fail most often, it makes tons of sense, particularly since you are trying
to get 110CFM down a pipe not rated at that...at least not with the pressure
that the bathroom fan will produce.
You do know that for 100CFM, 6 inch is the better choice? Some of the
Panasonics we install have been known to have a 8 inch duct off them,
depending on the length of run.
Bends, or 90 degree turns? If they are 90 degree elbows, you need to figure
in an extra 10 feet to the total run of duct....and that adds to slow down
your air flow, and add static pressure.
Nope. See, you can only run flex duct to 14 feet. Period. After that, it has
to be metal ridgid duct. Add in the 10 feet you may have if those are 90s,
and you can have only 4 feet of flex.
You are forgetting something....something called TEL, or total effective
a 90 adds 5 feet to the TEL, and a 45 2.6.
Also, if you are in the US, metal duct, is all that is now allowed for
bathroom fans, be it flex, or ridgid.
Ok...when you cut the airflow to the impeller, the load is removed from the
motor. The motor will then speed up. This does two things, particularly on
those cheap motors in those fans. It overrevs them. This will speed up the
wear factor, and if the fan is allowed to run at a higher than rated RPM,
and it will, since you are at a no load speed, or close to it, the motor
will overheat faster.
Yes...this will happen, and if you dont believe it, close the duct off..run
the motor, and then remove the obstruction...the motor will slow down as
load is increased.
Now, if you leave it as you say, and then go to a 4inch later, when you add
full load back to the unit, it will probably fail in short order.
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