Mechanical ventilation in new residential construction will continue to be a hot topic and much will be learned as new equipment and strategies are employed over time. Currently, the most commonly used approach to meet the Minimum Ventilation Requirement (MVR) in the code is a bathroom exhaust fan centrally located to provide whole house ventilation.
One of the challenges in whole house ventilation with exhaust only is in providing balanced ventilation. Typically, the fan intended for whole house ventilation is installed a second-floor bathroom of a single-family home. However, the make-up air must come from outside the thermal envelope. And the path of fresh air entering a home varies significantly from one home to the next. Thus, while the MVR may be met for the whole house, it is questionable that it is being met in all rooms throughout.
There are many factors that can affect air flow drawn from a single exhaust fan. This might leave insufficient air exchange in critical areas such as bedrooms where research has shown adequate ventilation is important. For example, carbon dioxide (CO2) (from exhaling) that is elevated in poorly ventilated bedrooms can cause negative health effects such as reduced cognitive ability.
Short of installing heat recovery ventilation and energy recovery ventilation systems, a simple solution that many builders are using is to install two exhaust fans in single family homes that are located apart from each other. The most common approach, particularly with larger homes is to install one in the master bathroom and one in the auxiliary mainsecond floor bathroom. In smaller homes, installing a fan on the first-floor bathroom and one in the second-floor bathroom may be a good strategy.
There are a few benefits worth noting to two fans. The first is that it is more likely to provide adequate ventilation in bedrooms when the fans are in nearby bathrooms. Second, it should provide more balanced ventilation in general. Exhausting air from two points should create an air flow pattern that is more likely to split across a home as opposed to a single exhaust point that may leave certain areas insufficiently ventilated. Additionally, if there are two fans, each one can be set a lower flow rate making them run more quietly. There is only one minimum ventilation requirement number per home. Two fans running at 30 cubic feet per minute (CFM) is less audible than one running at 60 CFM. The strategy should achieve an overall quieter indoor environment.
In summary, while not a complete panacea, the use of two exhaust fans to meet MVR requirements in new homes may help to improve indoor health as well as comfort.
