No. Just extra heat loss as you push out more conditioned air than needed.

Also realize that you usually want the fan oversized a bit to compensate for airflow losses due to the duct; length of duct, turns, etc.

That fan is waaaaay to big,,

The room is 40sq.ft and assuming a ceiling height of 8ft that would be 320cu.ft.

A 70-110 fan would cause a complete air exchange in about 3 to 4 minutes. Not only would that be an excessively huge waste of energy as you push your conditioned air out the stack, that bathroom would be as drafty as if the shower was out in your yard.

At 320 cu.ft the correct size fan would be
320cuft x 4 air exchanges per hour = 1280cu.ft/hr

1280cuft/hr divided by 60min/hr = 21.33cfm

I don't think there's a fan that small. Do I even need a vent? Maybe I could get by with a dehumidifier like the one linked below combined with this odor removal: http://www.bedbathandbeyond.com/product.asp?sku=108974&;. Does the air need to be vented out; could it just circulate?

This is for a rental property so I need an easy and convenient solution that the tenants would actually use.

Here is a link that might be useful: Dehumidifier

"That fan is waaaaay to big,,"

lazypup - you cannot possible make that determination sitting in your chair at a computer screen!

As mongo already hinted you don't know what how long the duct work for the fan is, how many elbows it has in it, the size (3" would make a HUGE diff.), or what material it's made of. Smoothwall PVC is very slippery, insulated flexduct less so, and corrugated aluminum will cause a lot of friction even in a 100% straight line.

You'd better get some facts or make a site visit before you make recs. like this !

So is the only downside to a 70 exhaust fan is high energy costs? No draft, no burned-out motor?

Maybe put the fan on a short timer so it can't get left on? I just bought an 70 cfm for our 21 ft. powder room with a 14 foot duct run. Didn't really see anything smaller. We'll use a timer, too.

Deedles, what brand/model did you purchase? I bought Broan model 665RP, 4 sones, heater �light- exhaust fan. I wanted the Panasonic Whisperwarm but believe its 110 cfm is too overpowering. I'm concerned about the noise level of the Broan fan.

I just bought a Hunter light/fan combo. We have a similar one in our current teeny bathroom and it works fine.

Here is a link that might be useful: bathroom light/fan

the energy use difference between the two will not even be noticeable on your energy bill each month -

and in case you didn't get the drift of my last post, the smaller one may not even be powerful enough depending on your install !

Thanks,

I am going to stick with the 70 cfm and install a timer. Thanks everyone!

hey pup - I'm plenty versed in the building arena and am still in the game as opposed to siting on the bench armchair quarterbacking.

"I most assuredly CAN make that statement "

yes - but you'd be making an inaccurate one!

"Once you know the CFM you then determine the lenght of run from the fan to the point of discharge and use the friction head loss tables to determine the correct size of duct. "

Since you're so versed on this - how long is essdana's duct run? Did his/her builder installer use 3" or 4" ducting?

What's the static pressure difference between 3" and 4" duct?

What was it made from?
What's the flow loss when using corrugated aluminum duct vs. smoothwall PVC?

Do you need a slide rule for the math calculations ?

Are there any elbows/bends in the duct run?

How many ?

It's clear you know how to do this:
"Once you know the CFM you then determine the lenght of run from the fan to the point of discharge and use the friction head loss tables to determine the correct size of duct. "

But, since you don't know the sizing, material , length and pressure loss of essdana's duct work install - you can't ACCURATELY say that the 40 CFM fan in question will overcome the static pressure of his/her duct work.

Sure it works on paper with your slick tables - but the real world presents some curveballs. You certain learned this on your journey to Master status. And, you're not going to get to use anything but 4" duct for this fan anyway 3" if your builder /customer is cheap. So overcoming high static pressure with a clever oversized duct plan isn't really an option.

Another thing -

This fan we are talking about - 40CFM, is that a max. rating, average? At what static pressure was that measurement taken or is that maximum free air movement? If it was rated at free air , the second you add duct work (any kind) that rating is going to drop on a pretty steep curve as 40 CFM fans tend not to be high quality and accurately rated from the get go.

So - simply going rule of thumb based on your "experience" using a few tables can get you into trouble in a real bathroom.

That's my point. Your 40 CFM "idea" sized fan is great in your air conditioned office. What if the maker fudged' their rating and the thing only moves 30cfm when a duct is attached? What if two 90 deg bend are added into the duct run? How much restriction does the roof cap add to the system?

How many room air changes is that 40cfm fan really going to produce at essdana's house?

You can't answer that- you can only answer what the theoretical maximum would be based on your beloved tables and some educated guesses based on your experience. They are still guesses, and are no better than my guesses - so settle down boy.

All I know is, I've been in so many bathrooms with useless noise machines that were "properly rated" for the installation. Useless for moving stinky or moist air out of the room in a reasonable amount of time.

Years ago, my father, who loved to take long showers, was sick of the steamy bathrooms and had his contractor put in a fan rated for a commercial shower room with a dozen showers or something. This is in a VERY small bathroom, probably 40 sf. His contractor said it was WAY too much. But, it WORKED. You get out of the shower, there's no steam fogging the mirrors.

My husband replaced the useless noise box in our laundry room with a high-cfm Panasonic. The ~60sf laundry room is also a passageway between the house and the garage down the stairs. He often does projects in the garage with stinky chemicals. The Panasonic pulls out the fumes coming up the stairs; none reach the living area of the house. After a roof repair over the garage, the repairmen failed to replace the insulation, so the garage gets super hot in the summer. The heat used to rise into our living area on the 2nd floor. After installing the Panasonic fan, the temperature in our house dropped 6-8 degrees on a hot summer day.

So I am inclined to ignore what the formulas say to get, other than to use that standard as a base number to double or triple. Motors will not burn out; that's not even a consideration.

Andersons - your happiness is not important. Pup only wants people to save money and adhere to charts and guidelines.

As long as the fan that is put in is "properly" sized and done so by a master air specialist - the fact that it doesn't do what you want it to do or is so loud that it drives you nuts (and you don't use it) is inconsequential. It's proper after all!