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70-110 exhaust fan for 40 sq ft bathroom

Posted by essdana (My Page) on
Wed, Jan 25, 12 at 20:42

what damage will occur if fan is more powerful than needed? Burned out motor?


Follow-Up Postings:

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RE: 70-110 exhaust fan for 40 sq ft bathroom

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


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RE: 70-110 exhaust fan for 40 sq ft bathroom

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.


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RE: 70-110 exhaust fan for 40 sq ft bathroom

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.


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RE: 70-110 exhaust fan for 40 sq ft bathroom

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


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RE: 70-110 exhaust fan for 40 sq ft bathroom

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


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RE: 70-110 exhaust fan for 40 sq ft bathroom

"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 !


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RE: 70-110 exhaust fan for 40 sq ft bathroom

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


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RE: 70-110 exhaust fan for 40 sq ft bathroom

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.


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RE: 70-110 exhaust fan for 40 sq ft bathroom

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.


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RE: 70-110 exhaust fan for 40 sq ft bathroom

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


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RE: 70-110 exhaust fan for 40 sq ft bathroom

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 !


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RE: 70-110 exhaust fan for 40 sq ft bathroom

Thanks,

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


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RE: 70-110 exhaust fan for 40 sq ft bathroom

antss;

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

I most assuredly CAN make that statement because for the last 35yrs before I retired I was both a Master Plumber & Journeyman HVAC tech, and my job was determining the correct size of exhaust fans for anything from a half bath in a Photomat booth in a parking lot to a High School or college locker room where they had upwards of 36 to 40 showers going simultaneously.

First off, the size of the current duct has absolutely nothing to do with sizing the fan, and attempting to match a fan to the duct in the manner you describe is nothing more than a vain attempt by a hacker to get something that might work. When you do it for a living you cannot rely upon maybe's, might work, or whatever other guestimate you might like to try.

The proper method is to first determine the size of the load by determining how many cubic feet are in the room and how many air exchanges you desire per hour. Once you know the total cubic feet of air you must move in an hour you simple divide that by 60 to determine how many cubic feet you must move per minute, thus the CFM rating.

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.

Simply attempting to put a larger fan on the existing duct not only will not solve the problem, as the volume of air increases in the duct, the friction will increase exponentially and the end result will be overloading the new fan motor and causing it to burn out prematurely.

Now in regards to energy cost. You are absolutely correct, there is precious little difference between the electrical cost to operate the differing fans commonly available for residential use, However, for every cubic foot of air you send out the duct and eqaul amount of air must infiltrate into the house from outside. Not only have you already paid the HVAC cost to condition the air your pushing out, you have to again condition the air coming in, so any excess air flow is a big energy consideration.


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RE: 70-110 exhaust fan for 40 sq ft bathroom

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.


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RE: 70-110 exhaust fan for 40 sq ft bathroom

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.


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RE: 70-110 exhaust fan for 40 sq ft bathroom

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!


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