Not Your Old Bath Vent Fan

There has been a quiet revolution in bath ventilation fans over the years – literally.

    Broan-Nutone recessed light/fan

Although fans work much the same way as they have for more than 50 years by using a squirrel cage fan operated by a small electric motor to suck moist air from the bathroom and expel it outside – the best fans have gone from over 4 sones (a measure of sound comfort; one sone is equivalent to the sound a quiet-running refrigerator makes) to less than 0.3 sones.

In addition, some of today’s fans take care of themselves by coming on when there’s a rise in room humidity over time, and then shutting off when the humidity is reduced. Some don’t even look like fans but look like recessed lights!

One reason for these innovations is that bath ventilation is needed more than ever before because houses are tighter than before in response to rising energy bills. A typical morning shower is one of the biggest contributors to air in a home that is too humid. At the very least this humid air condenses on windows, but it can also permeate walls as water vapor and causes huge problems from mold and rot to ruined insulation. It’s also a proven health hazard.

The cure for these ills is fairly simple installation of a fan that will operate at the rate of 8 air changes per hour. You will want to place it over or near tub or shower, and if you have a WC, you’l want a smaller unit in there as well.

Calculating Required Fan Size

Fans are rated by cubic feet per minute (cfm) they can evacuate. The larger the room, the higher the fan’s cfm rating needs to be to achieve those 8 air changes per hour. If your bathroom is under 100 sq. ft. (measured wall-to-wall, not free floor area), then use can use a short cut – just allow one cfm for every square foot of bathroom. So if your bathroom is 5 ft. by 9 ft., you’l need a little under 50 cfm.

For bigger bathrooms, you need to use the volume of the room (width x length x ceiling height) to calculate the cfms required. Just multiply the room volume times 0.13 and round up to the nearest 10. So a 9 ft. by 16 ft. bathroom with a 9 ft. ceiling requires a 170 cfm fan (9 ft. x 16 ft. = 144 sq. ft. Then 144 x the 9 ft. high ceiling =1296 cu ft. Multiply that times 0.13 and you get 168.48 cfm).

Choosing a Level of Quiet, Other Features

Most manufacturers will have several models that are produced at the cfm level you need. The quieter the fan, the higher the price, but that difference may not seem very important if you realize that this is an appliance you use every day in close proximity.

You also have a number of options in how the fan is configured. The traditional light/fan combination is still common. You can also get a light/fan/heater combination, but this is not rated for use in the shower or over a tub even when connected to a ground fault circuit interrupter (GFCI).

Some newer fans operate on humidity sensors so you never actually have to turn them on, and their auto shutoff switches can be timed for between 5 minutes and an hour. It’s recommended that a properly sized fan run about 20 minutes after the shower is turned off.

New bath fans have also become more stylish – not all use the timeless white plastic grill. Some just look like attractive light fixtures; others are designed to imitate recessed lights – in fact that’s just what they are only they also are ventilation fans. The Energy Star program can also be found in the world of bath fans those that earn the rating are particularly stingy with electricity.

Important Installation Details

The duct run between the fan and the outdoors has a lot to do with how well a fan does its job – and even how quiet it is. A short, straight run of smooth-wall, rigid ducting is ideal. The typical duct diameter is 4″ but fans can operate even more quietly when they have less resistance – so one company is promoting 6-in duct for some of their products. If a turn is required, keep the radius as large as possible, and make sure the terminal (exit) vent has back draft flaps to keep cold air from entering the duct.