Bathroom ventilation is necessary for two reasons; exhaust moisture from the space and remove odours. There are challenges and precautions in both applications when deciding what type and size of fan to install.
Sizing the fan
The fan removes air from the room. This air, in turn, is replaced by air drawn in from adjacent rooms and supply air ducts. Too little exhaust is ineffective and too much exhaust is inefficient. The recommended air change for an average bathroom is 8 per hour. This means that the fan must be able to exhaust the volume of air in the space eight times within one hour. Most fans, however, are sized by the amount of air they will exhaust per minute (CFM).The formula is then quite simple:
Installing the fan
This is where everything goes wrong. There are a lot of points to consider when installing a fan correctly. First, we have to determine the proper position for installation. The fan needs to be placed close to the source of moisture. It also needs to be away from where air enters into the room such as the door, window or heat vents. This ensures that the air removed is not the air that just entered the room. Determine where the fan will be exhausted. (Do you mean right out of the house?) YES EXHAUSTED RIGHT OUT OF THE HOUSE, and “in the attic or crawl space” does not count. Dumping the moist air from a bathroom into these spaces is a recipe for disasters in rot and mold.
The exhaust point should be through the roof with a proper roof vent, out the soffit with the proper soffit vent or out an exterior wall with a proper wall vent. Regardless of what location, you need to ensure it is away from any window or area where air is being drawn back into the house. The ducting material used and the manner in which it is installed will dictate the overall effectiveness of the fan. Always screw and seal all of the joints and insulate the duct along its entire length. Try to have a continuous slope to the exhaust point and avoid creating valleys in your ducting. This will ensure that any moisture accumulated in the duct goes out the exhaust and not back into the house. Static pressure, created within the ductwork, will choke the fan and limit its performance. This resistance is augmented by the length of duct, number of turns and restrictions in the ducting, ridges on the inside of flexible ducting and restrictions at the exhaust point. Therefore, use only straight ducting material such as sheet metal ducts or PVC ducting instead of flexible duct material, and try to keep the number of turns and elbows to a minimum. Always keep the distance between the fan and the exhaust as short as possible.
Finally, the duct size should be at least 4” diameter. In cases of longer runs or more turns, use 6” diameter duct. Nonetheless, always consult the instructions supplied with the fan unit for appropriate sizing.
Other things to consider
Like all things, quality costs money. Good fans are quieter and better made, with ball bearings and better designs. The noise level and power consumption are two things to consider. The noise level in exhaust fans is specified in Sones. Good fans will have a noise level of 0.05 sones and consume around 20 Watts of electricity, while other units will run at levels of 3 to 5 sones and have an electrical consumption of more than triple.
Consider installing an in-line fan where the motor may be placed in the attic reducing the amount of noise inside the room. These fans are also a great way to introduce multiple exhaust points inside a bathroom. You can have a vent in front of the shower as well as one directly behind or above the toilet.
Timers are a great way to operate fans. You set the duration so that they work effectively in removing the moisture and then automatically shut off to conserve energy.
If removing odours is the only concern in a room, consider installing fan units that connect directly to the toilet and filter the odours at the source. These units, such as the one found at www.panfan.com, are very effective and eradicate the need to remove excessive amounts of heated or air conditioned air from inside the house.