March 2011: Bath Fans

Bath Fans

The addition of bath fans should always be considered as part of an energy retrofit. Even if there is no evidence of a moisture issue or the bathroom does not have a shower or bath, they can provide potential exhaust ventilation should the building fall below 70% of the Building Airflow Standard. Sometimes it may be as easy as placing them on a programmable timer that replaces the standard wall switch. Bath fans are rated in CFMs and should be sized not only with respect to the size of the bathroom but planned duct run and corresponding length. The longer the duct, the more restrictions, what side of the house it exits on, the type of closure unit and the pitch of the duct all will affect the actual CFM exhaust capability. Fans can be located on walls when ceiling applications prove to be difficult.

Fans are also rated in Sones for their sound rating with 1 being the quietest and 3 usually being the loudest. If there is going to be a light and a fan within the same housing, it is always better to have them on a separate switches – especially during the winter when one may NOT want to exhaust moisture. All fans, whether existing or new, should be ducted to the exterior of the building with a continuous insulated flex duct that has excellent inner and outer sleeve connections to the exhaust port and the closure body. The plastic exhaust port should be placed with the flap closed and well sealed to the housing so as not to leak or be pulled off. Sometimes the extra holes in the casing of the fan can be metal tapped for improved air flow and the body of the fan should always be sealed to the adjoining surface.

One should always think about where they intend to exhaust the duct before the fan is set in the ceiling.  There should be no restrictions for the first 3-4 linear feet of duct run. One may need to use and adjustable elbow or 3” to 4” expander to make a smooth transition. There should be one dedicated duct (no teeing or joining) for each fan unless there is a centralized unit with individual passive grates located at various source points located around the house. If it is problematic to get the fan completely ducted to the exterior getting the termination point as close to an existing vent and on the leeward side of the building would be preferable. There should be good exterior closure unit; ones that have a floating shuttle (these are about 14” in total length and just the body can be used for through the roof applications) seem to work best at minimizing the exchange of conditioned and unconditioned air. Existing fans should always be check to insure the do terminate outside of the building shell and are in good working order. All fans could be checked with a pressure pan or flow hood for actual CFM capacity should the need arise for determining mechanical ventilation capacity.


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