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bathroom fan for a small bathroom

11 years ago

Hi,

I have a small bathroom (7' x 7') with a shower/tub combo. I have a Nutone QT9093 fan (110 CFM). According to a fan size calculator, I should only need 50 CFM for a room this small. When I take a shower, the mirror still fogs up. With this fan, should I be able to take a shower and not have the mirror fog up? The fan vents to the roof and the roof cap has a sharp turn to keep the rain out.

http://www.ventingdirect.com/broan-634m-steel-roof-cap-for-6-round-ducting-with-backdraft-damper/p417814

Could the sharp turn be blocking airflow? My other option is to redirect the vent so it goes out the wall. Do you think this would solve the issue?

Thanks

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Comments (3)

lazypup
11 years ago
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.
lazypup
11 years ago
In addition to what I previously posted, check the draft damper in your fan to see if it is sticking shut. Quite often they get gummed up with moisture and dust and should be cleaned periodically.

The elbow on the top of your exhaust vent does not cause a significant reduction in flow, especially when you consider that the fan you have is already grossly oversized for the room.
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mtsullivan
Original Author
11 years ago
Thanks for the info lazypup. I had removed the damper as a test to make sure it wasn't blocking flow and still had the problem. It's not an intake air issue. The bathroom has an outside-facing window with a 1.5' x 3' opening. The mirror still fogs up if I take a shower with the window fully open. When the fan is on I can hold some toilet paper to the fan and it will stick, so I know the fan in pulling air out. And I can feel airflow if I get up on the roof. The fan has a heater. If I turn the fan and heater blower on at the same time, the mirror does not fog up. I'm sure circulating the air is helping.

This is frustrating because the fan should have plenty of power to pull steam out of the bathroom. Is there any way I can measure how much air is being moved?

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