Humidity Implications for Meeting Residential Ventilation Requirements

In 2003 ASHRAE approved the nation's first residential ventilation standard, ASHRAE Standard 62.2. Because meeting this standard can significantly change the ventilation rate in residences there is a concern about how these ventilation rate changes may impact humidity. This paper examines the effects of providing ASHRAE 62.2 levels of ventilation on humidity in residences that are typical of new construction (based on International Energy Conservation Code requirements). Four different systems were simulated in six climates of varying outdoor humidity characteristics (Charlotte, Houston, Kansas City, Seattle, Minneapolis and Phoenix). In order to capture moisture related HVAC system operation, such as the lack of dehumidification from typical air conditioning systems at the beginning of each cycle, we developed a simulation tool that operates on a minute-by-minute basis and utilizes a dynamic model of air conditioner performance. The simulations also include the effects of internal generation. Typical of most residences, the dehumidification in the houses is provided by the operation of cooling equipment that is controlled by temperature, rather than humidity. The results show that although 62.2 compliant ventilation systems increase average indoor humidity in hot humid climates, the number of high humidity events is unchanged. In less humid climates 62.2 compliant ventilation systems do not significantly affect the indoor humidity. Other factors such as occupant density, climate and air conditioner operation are more significant factors in determining indoor humidity.