According to building codes in the United Kingdom, following certain bathroom ventilation regulations are required to pass inspection. This is to prevent mould from forming, which can cause respiratory issues such as asthma as stated by the Center for Indoor Environments and Health at the University of Connecticut Health Center. Currently there are no building regulations in the United States, however there are published guidelines (see Resources).
Along with mould growth, the bathroom is a potential hazard with the combination of electricity and water in the same room, so it must be sectioned into four zones as required by the Approved Document F of the Building Regulations. Zone 0 is in the bath. Zone 1 extends up the bath or shower wall until it reaches 2.25m (6.25 feet) above the top of the bath. Zone 2 includes .6m (1.8 feet) above Zone 1. Zone 3 expands Zone 2 laterally for 2.4 meters (6.4 feet). The engineer will use these zones to determine what type of bathroom fan is allowed depending on the location of the fan and electrical wiring.
Sizes of Bathroom Fans
According to the Building Regulations, bathroom extractor fans are allowed in two sizes: 100mm (4-inch) and 6-inch (150mm). The 100mm fan works best in bathrooms smaller than three square meters and the 150mm fan for larger bathrooms. Certain situations will require the large fan such as if there are no windows in the bathroom, if it is north-facing or in a damp basement.
Air Extraction Rate
The air extraction rate measures the rate that an extraction fan removes the room's air. It is measured in cubic meters of air that is taken out in an hour or litres of air removed in one minute. The Building Regulations state that the extractor fan must extract a minimum of 15 l/s in a standard residential bathroom. All 100mm fans surpass this regulation. High-velocity fans blow air at 90m3/hr, and the 100mm fans blow at 85m3/hr.