Frequently Asked Questions: Exhaust Fans.
How do I calculate the size
fan for my bathroom?
- For an 8 foot ceiling, take the square footage of the room, multiply
it by 1.1.
- For any ceiling over 8 feet, take the height of the ceiling and multiply
by .1375. Take this figure and multiply by the square footage of the
room. This will equal the recommended CFM's. (Example- 10' x 12' room
with 10' ceilings. 10' x .1375 =1.37 x 120 square feet = 164 CFM's.)
- Both calculations will give you the minimum recommended CFM.
What is CFM?
Cubic Feet Per Minute: Unit of measure for expressing volume (quantity)
of airflow. For an exhaust fan it would be the quantity of air exhausted
- the same quantity of "make up air" must be brought into the
room to replace what was exhausted.
What is a Sone?
A Sone is an internationally recognized unit of loudness. The Sone Value
you see is the laboratory decibel readings translated into a number corresponding
to the way people sense loudness. Sones are linear like inches. Double
the sone is double the loudness. One sone is equivalent to the sound of
a quiet refrigerator in a quiet kitchen.
What is Static Pressure?
Most fans are rated at 0.1" S.P. (static pressure), which is the equivelant of 10'
of recommended type ductwork, and one elbow. Longer ductruns, not using the recommended
type ductwork, using more than one elbow, among other things will all increase S.P.,
thereby decreasing your CFM (ultimately, the performance of your fan).
Can CFM be converted to L/S (litres per second)?
Yes - 1 CFM = 0.4719 L/S.
Can L/S be converted to CFM?
Yes - 1 L/S = 2.111 CFM.
Can I install a fan and fan/light
over the bathtub or in a shower?
Products that have been Listed by Canadian Standard Association (CSA)
or Canadian Underwriters Laboratories Inc. (cUL) for use over a tub or
shower may be used in this application. These products will be marked
as "acceptable or suitable for use over a bathtub or shower when
installed in a GFCI (Ground Fault Circuit Interrupter) protected branch
circuit. Click the product Specification Sheet on our Website to confirm.
Can I install a heater over the
bathtub or in a shower?
No. Broan recommends against that. Heaters are not certified for installation
over a bathtub or in a shower for this type of installation.
Can I cover the exhaust fans
with insulation in the attic?
Any fan unit can be covered with insulation, up to R40.
Can I cover a heater
with insulation in the attic?
As long as it is a heater/fan combination, it can be covered with insulation
up to R40. Our heat lamp only unit is NOT IC rated!
Where do I install an exhaust fan
in the ceiling?
Usually an exhaust fan should be installed in, or closest to, the shower
enclosure. Provide inlet for "make up" air by an undercut door
and/or air diffuser. A forced air diffuser is also an inlet. Imagine a
line between the inlet and the fan - position both to "sweep"
What would be some reasons for an
exhaust fan not removing the moist air from the area?
- Too small a fan for the size room being exhausted
- Incorrect size of duct used to exhaust room
- Obstruction in the duct
- Roof or wall cap not opening correctly
- Incorrect position of fan in room
- Insufficient make up air into the room being exhausted
What accounts for "make up
air" into a room?
Make up air is the air needed to replace air being exhausted by the fan.
Inadequate make up air will diminish performance of the product. For a
bathroom installation, make up air is generally provided by a gap at the
bottom of the bathroom door. A 1/2" gap between the bottom of the
door and the floor or carpet is the minimum acceptable gap measurement.
Large CFM fans may require additional make up air.
Can I install the exhaust fan in
Certain models are suitable for wall installation; please refer to the product specification sheet to determine suitability. Mount the fan as high as possible on the wall. When wall thickness is an issue, use "thinner" fans. You can contact Technical Support for other fans that may be used in wall applications. Any fan with a light should not be installed in a wall.
Can I use a 3-inch duct pipe on
a bath fan with a 4-inch duct connector?
No. It will reduce the performance of the fan and create excess noise.
Can I use 4-inch duct on
a fan with a 3-inch connector?
Yes. A 3 to 4 inch adapter will be required to make this transition.
What type of duct is recommended,
galvanized metal or flexible plastic?
It is recommended, where possible, to use rigid galvanized duct. It has
less resistance to air flow and allows the fan to operate much more efficiently.
If flexible plastic is used, insure the duct is stretched out and as straight
Can I use a utility fan in a bathroom?
Utility fans are not recommended for bathrooms. Broan offers a complete
line of exhaust fans to meet any style or specification.
When and why would a roof or wall
cap be used?
A roof or wall cap should always be used. It will provide:
- Termination for the end of the duct run in the wall or on the roof
- Protection from the elements (water, wind) coming inside
- Protection from animals invading the opening
Can I exhaust my fan into my attic
instead of out the roof or wall?
No. You should never exhaust air into spaces within walls, ceilings,
attics, crawl spaces or garages. The humidity may damage the structure
What type of exhaust fan is recommended
for hot tub area?
A high CFM rated device is normally recommended for this type of application.
Can all exhaust fans be put on
a variable speed control switch?
This depends on the product model, as some motors are not designed for
use with speed controls, but most can.
Why is there water dripping from
my exhaust fan/range hood?
There could be two reasons for this issue:
Condensation can form in the duct pipe when it is exposed to a cold
attic environment. The recommended solution is to wrap the duct pipe
with insulation. The duct pipe should also have a damper at the discharge
end of the pipe.
- During very cold weather, frost can form on the vent's roof cap. As
warm air rises inside the duct, some of the frost may melt and drip
back down the duct pipe. In certain very cold climates, some minor leaks
and drips may be impossible to stop.
My exhaust fan in the bathroom
is old and noisy. Can I upgrade to a new, quiet fan?
Unfortunately, all bath fans have unique housings. Sometimes, some housings
may accept more than one model; however, chances are high that you will need to
replace the housing, and in some cases, the ductwork.