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two questions about a fan in my small 5 x 7 bathroom

13 years ago

Hi,

My house was built in the early 1900s and has a second floor bathroom that is consequently pretty small (5 x 7). I'm renovating it but am not increasing the square footage.

I'm looking at the Panasonic WhisperCeiling series having read good things about the brand here. Two questions:

1. Is 50 CFMs adequate for a small space like this, or would 80 CFMs be preferable? This is a small space but it's my main bathroom and I shower every day in it.

2. I'm confused about fan switches! :) The current fan is on an old timer, but is the trend now to wire it directly to the light switch to have it come on all the time? Or are there new-fangled ways to have a timer function?

Thanks!

Adam

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Comments (4)

steve_o
13 years ago
R.E. Williams (where I bought mine) has a fan-size calculator on their site; assuming you have 8' ceilings in your bathroom, you need to move 37 cubic feet per minute at a minimum. So 50 cfms is more than adequate to the task.

When my fan was installed, the electrician added a switch for the fan independent of the bathroom lights. It works fine. There are times, however, when I'd like to run the fan (like after a shower) without wasting energy by having the lights on. I'll admit that I've had bigger issues to think about it :-) so I haven't pursued the matter much, but I would love to just add a timer to the fan. Neither the lights nor the fan need to be coupled like that, IMHO.
oldhousegal
13 years ago
I have the bath fan in my bathroom wired to it's own timer. That way it's not on when the lights are- only when you want it on. I love the timer- I never forget that it's on, since it goes off on it's own. I won't have another fan without a timer in any of my bathrooms, now that I've had this one.
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A bathroom exhaust fan should produce 4 air exchanges per hour. Your bathroom is 7 x 7 which is 49sq.ft Assuming an 8' ceiling the total volume would be 49 x 8 = 392cu.ft We want four complete air exchanges per hour so that would be 392cu.ft x 4 = 1568cu.ft per hour. There are 60 minutes in an hour so we divide 1568 by 60min and it equals 26.13cfm. You currently have a 50cfm fan, which is twice the size you really require yet your having problems with the mirror fogging up. I suspect your problem is not the fan. Nature abhors a vacuum therefore a fan cannot push air out of a room unless there is provision for an equal amount of air to enter the room. You can test the theory by running the shower and leaving the bathroom door slightly ajar to allow more air to enter the room. Generally they make a slightly higer gap under the bottom of a bathroom door to allow more space for makeup air to enter the room but in some circumstances that does not provide enough makeup air. The solution then is to install a duct through the wall to an adjacent room and put a grille on either side. That will allow the additional air.
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cat_mom
13 years ago
Our fan is on a separate switch, but not on a timer. Occasionally we forget to turn it off, but not too often.
lazypup
13 years ago
A bathroom exhaust fan should be sized to provide a complete exchange of air in 15minutes (4 air exchanges per hour).

To compute the fan size you begin by computing the total volume of the room, length x width x height.

You stated your bathroom is 5x7 and assuming an 8ft ceiling we get 5x7= 35sq.ft x 8ft height equals a total volume of 280cu.ft.

We want the fan to exchange the air four times per hour so we multiply the room volume by 4 to determine the total volume of air to be moved per hour. 280cu.ft x 4 = 1120cu.ft/hr.

Now divide the total CFM/hr by 60min/hr to determine the required CFM of the fan.

1120Cu.ft per hour / 60min per hour = 18.66cfm.

The purpose of the fan is to insure we remove any residual water vapor in the room after you finish your shower. For optimal performance you should have a fan switch that has a 15 minute delay on break. With this type of switch the fan will start as soon as you turn the switch on, but when your leaving the room you turn the switch off and it will continue to run the fan for another 15 minutes causing one more air exchange before turning the fan off, thus it will provide a maximum reduction of moisture in the air.

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