Bathroom fans vent to the outside of the building using a duct that leads the moist air and odors from the bathroom to the exterior. When properly installed, bathroom fans are designed to expel the air to the outside without allowing cold air to flow back into the building. There are a few reasons why cold air doesn't come in during winter:
One-way airflow: Bathroom exhaust fans are equipped with a one-way damper or flap at the outlet. This damper opens when the fan is running, allowing air to flow out, and closes when the fan is off. The damper prevents outside air, including cold air, from entering the duct and ultimately the bathroom when the fan is not in use.
Vent termination design: The vent or exhaust outlet on the exterior of the building is usually designed to prevent backdrafts. It may have a hood or cap that helps direct the airflow away from the building's exterior walls. This design discourages cold air from being drawn back into the duct.
Positive pressure indoors: Typically, the inside of buildings is maintained at a slightly higher pressure compared to the outside. This positive pressure helps to prevent outside air from being drawn into the building through small openings, including the bathroom fan vent, when the fan is not operating.
Proper installation: A correctly installed bathroom exhaust system ensures a tight seal around the vent duct and the exterior wall. A well-sealed duct prevents cold air from infiltrating the bathroom from outside.
It's worth noting that in some cases, poor installation or maintenance issues can lead to problems like air leaks, backdrafts, or cold air entering the bathroom. Regularly inspecting and maintaining the bathroom fan, exhaust ducts, and vent terminations can help prevent such issues and ensure the fan operates efficiently and effectively. If you notice any problems with your bathroom fan during winter or at any other time, it's a good idea to have it checked by a qualified professional.