Where Vent fans are needed:
Proper ventilation in bathrooms is a necessity to control moisture as well as odors. When mold or mildew forms on the bath walls or ceiling it is likely caused by poor ventilation.
In the past, bathroom ventilators were not required if a bathroom had a window. However, the building code now requires a bath fan for any room with a toilet, shower or tub. If the bathroom has a separate compartment for the toilet then 2 fans would be required. A separate area with vanity sink does not require a vent fan.
Ducting into the attic used to be acceptable. That practice has been found to cause considerable moisture damage to the attic insulation and ceilings as the water vapor condenses in the attic. Just because you have a vent fan in the ceiling of your bath does not mean it is exhausted to the exterior. Today's code requires all exhaust fans to terminate outside the building. This can be through the overhang, roof or wall but there must be a direct connection to the exterior. Just venting into the overhang or up to a roof vent is not acceptable.
Remember that the more air to be exhausted the more make up air is required otherwise the fan will fail to operate properly. The most common approach for intake air is to provide about a 3/4” gap below the bathroom door for air to enter as the fan operates.
Even after you install the proper fan and duct all is lost if the fan is not operated long enough to exhaust the moisture from the room. It is recommended that the fan continue to run for 20 minutes after the shower is turned off. A timer might be considered in place of the wall switch. Automatic Humidity Fans are also available that will turn the fan on and off based on the moisture level in the room.
Selecting a Vent Fan:
Following are some criteria for selecting a bath fan.
Size- For rooms with 8' high ceilings the proper size vent fan should be rated at cubic feet per minute (CFM) equal to the square area of the bathroom times 1.1. For example, a 5' x 9' bathroom is 45 square feet and would require a fan rated at 49.5 CFM. A taller ceiling will require a proportionately larger fan.
Type- Axial style fans (propeller style) are found on less expensive ventilators. Squirrel cage blowers offer superior performance and are more likely to move the rated amount of air once installed.
Noise- The level of noise is measured in sones. The lower the sones number the quieter the fan. A quiet fan will be less than 1.5 sones. (A typical refrigerator operates at about 1.05 sones) All of the quiet operating fans will be squirrel cage design. Axial fans will have noise levels of as much as 6 sones. That is 6 times the noise level of the 1.0 sone fan.
Ducting- Typical duct size will be 4'' diameter but larger ultra quiet fans will often require a 6'' duct size. Be aware of this if you are replacing a fan and plan to connect to the existing ductwork. Length of the duct is also important. Usually 30' is the maximum length for the duct and you need to deduct 5' for every elbow used. Smooth wall duct will provide better air movement than corrugated duct. Insulating the duct in the attic can help reduce condensation in the duct. If possible, slope the duct downward toward the exterior so any condensation will not run back into the fan.
Energy Star- Bath fan units with the Energy Star logo will save as much as 65% on energy consumption. The ultra quiet Broan QTXE80FLT is rated at 80 cfm @ .3 sones with fluorescent bulb and earns the Energy Star Label.
Appearance- Today's bath fans can look more like a decorative light fixture than the typical fan- light combination. The illustrations show a Broan Model 754 Decorative fan-light (80cfm @ 2.5 sones) and a Broan Model 744 fan-light that appears as a recessed can light. (70 cfm @ 1.5 sones)
Humidistat Control- Some fans have humidity sensors that control the fan. When the fan senses high humidity it turns the fan on and turns off when the humidity level is normal.
Wall Mount- There are a few selections of through the wall fans but they tend to be noisier and less desirable because they allow more cold air infiltration to the room since there is no insulation in that area.
Remote Fan- This blower is mounted in the attic and the duct runs back to the room and connects to a ceiling grill. This style fan can be beneficial when 2 or more nearby locations must be vented.
Heater- Vent fans often incorporate a heating capacity such as infrared bulbs or resistance electric heat. Installing a separate heat bulb or resistance heating unit will provide better options on appearance and function for the fan selection.