new bath fan installed - now what?

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 appropriately.
b
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Hamilton Audio wrote:

I would try to seal it up. How depends on how much space there is.

Yea, you are only getting about 56% of the vent area that you would with a 4". That means you are dumbing down that nice new vent, making less than 100 cfm and increasing the noise.

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Joseph E. Meehan

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Hamilton Audio wrote:

snip///
snip/////
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
Frank
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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 sound is....
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.
b

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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. PJ
On Tue, 24 Feb 2004 01:51:21 GMT, "Hamilton Audio"

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wrote:

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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 airflow? maybe...
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....
b
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Code is: 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 idea....period.

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? Wrong.
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 length. 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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