Bathroom Exhause Fan Question(s)

[MisterEd]

First of all, thanks for this forum.

Iwant to install a new QUIET fan in our (relatively small 44 SQ FT) master bathroom in a 60yr old home. B/R has a tub/shower. Above the bathroom is an opened attic with insulation on the floor. I currently have an old, noisy, useless ceiling fan installed that has 4" flex (not-insulated) ductwork running from the fan unit to the side wall of the house..

I am looking to installing an inline fan. They seem to be the quietest around. I am looking at the PANASONIC unit which is rated at 120CFM which I know SHOULD be plenty http://www2.panasonic.com/webapp/wcs...odel=FV-10NLF1).

I was also considering the larger 240CFM unit http://www2.panasonic.com/webapp/wcs...odel=FV-20NLF1). All of the reviews I have found rate these fans very highly.

Now it would SEEM that the 120 would be plenty big and the 240 would be way too big which leads to my question.

Is there any problem with using a too large unit (other than it costs more and uses more power)? Will it be too strong so as to not work properly like getting a central A/C unitthat is too big?

Part of the reason I am considering the larger (240cfm) unit is because I will be using 4" or 6" insulated flexible duct (depending on which size unit I choose). The run isn't too long (maybe 25 feet total) but there are a number of unavoidable twists and turns to get to the inline fan from the bathroom ceiling and from the inline fan to the outside wall. There is at least 1 unavoidable close to 180 degree turn and 2 90 degree turns. From everything I have been reading the flex duct itself and the turns greatly increase the static pressure and reduce the efficiency of the fan requiring a more powerful fan.

So what do you'all think? With this situation should I go with the 120CFM or the 240CFM. Cost and power use not withstanding and taking into consideration I have no other option in mounting location of the fan unit so the direction of the ducting can't be changed.

Thanks in advance, I await your expert opinions!

[cacher_chick]

The regular Panasonic fans are super quiet. When I turn mine on there is no motor noise- all that can be heard is a bit of air movement. One thing to think about is that you are paying to heat your house, and the vent fan is also sucking the heat out of your house. There is really nothing to be gained be over-sizing.

I would work hard to avoid the routing that you are suggesting. The ideal vent routing is straight up and out through the roof. If you have a standard shingle roof, it would be a quick and easy job to install and flash a vent outlet on the roof.

[jimbo]

A fairly conservative design for lav exhaust calls for 8 air changes per hour. For 44 sw feet that means about 45 Cubic Feet per minute. 240 cfm is WAY overkill. You would probably have a serious breeze in the room. I like to go above the basic design, and I would go with the 120 cfm, but not the 240

[kreemoweet]

What could be more unpleasant than stepping out of a hot shower into a sizeable cold draft caused by an
overly large exhaust fan? Please tell why, if the fan inlet is into attic space, the exhaust cannot go directly
(more or less) up and through the roof, which is the usual and sensible thing to do?

[jadnashua]

Through the roof works in some areas (and may be the only reasonable choice), but in areas with potential high snow load, it can create problems.

I'm going to add another choice for you - I've had one for years and find it really excellent, quiet, and with other benefits as well. Look at www.solatube.com. These are tubular skylights which are available with an in-line fan and an auxiliary light kit for use in a bathroom. On a bright day, it provides around the equivalent of a 300W lightbulb of essentially free light. The light comes in when it might not make it through the window, if you have a window (I don't). I rarely turn the light on except at night or before the sun comes up. Still reach for the light switch after years...

In NJ, you'd probably want the extension to raise it up a little off the roof. Mine still gets covered by snow occasionally, but still lets light in. It emits an eerie glow in the winter at night from the reflected street lights, too if there's snow on the roof.

[Hardt]

I installed the Panasonic FV-08VQ5 WhisperCeiling 80 CFM fan in both my bathrooms a few years ago and I think I made the right choice. My bathrooms are twice the volume as yours and it does the job perfectly. I did some research before I bought them and the guideline was that the fan should do 8 complete air changes in one hour. I hope my math is correct: 728 cu ft bathroom / 80 cu ft/min exhaust fan = 9 min. for complete air change ; 60 min / 9 min = 7 changes / hr. This fan is 0.3 sones and the FV-20NLF1 is at 1.0 sones. I can just hear my fan and I think if I installed the fan you are considering, I just might find it bothersome. FWIW. BTW, the FV-08VQ5 is about $103 and the FV-20NLF1 is at $141.