Ventilation Requirements in Hot Humid Climates

In 2003 ASHRAE approved the nation’s first residential ventilation standard, ASHRAE Standard 62.2. Meeting this standard in new construction requires the use of mechanical ventilation, which in turn can often significantly increase the latent load faced in new homes. As the thermal performance of houses improves, sensible loads have decreased and existing equipment may not be able to deal with the remaining latent load. Failure to take this load into account can result in poor indoor air quality and moisture-related problems. As part of work through the Building America program, LBNL has simulated the effects of mechanical ventilation systems that meet ASHRAE Standard 62.2 on ventilation, energy use and indoor humidity levels. 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. This paper looks in depth at the implications of these simulations in humid climates. INTRODUCTION A key step in designing a home’s HVAC system is determining the correct amount of ventilation and the optimal system with which to provide it. There is no shortage of guidance on how much ventilation to use. The standard of care for ventilation system design is probably the 62 series of ASHRAE standards (62.1-2004 for non-residential buildings and 62.2-2004 for residential buildings). The reader can find a variety of books and other publications with recommendations including from ASHRAE (http://www.ashrae.org) Ventilation is not an end in itself, but is part of the system intended to provide a desired level of indoor environmental quality. One of the key aspects of indoor environmental quality is controlling contaminants that can have adverse health impacts on the occupants. As a practical matter, however, an HVAC designer rarely knows the sources, their emission rates or appropriate dose-response relationships and therefore is usually provided guidance in terms of the things he can control, like the ventilation and its efficiency. This is why standards like 62 focus on ventilation. Another key step in design can be to consider the internal moisture balance of the space. In many climates the resulting indoor humidity is typically so far from any problem area that such consideration is not usually done, but in hot, humid climates failure to properly consider moisture balances and the attendant latent loads can lead to discomfort or moisture-related problems such as mold. In cold or dry climates, ventilation acts to remove internally generated moisture from plants, bathing, cooking and other normal human activities. Ventilation standards are not set with this as a criterion, but it is a benefit of ventilation. In extreme climates this can lead to unacceptably low moisture levels, especially in certain individuals with breathing problems and/or dry skin. In hot, humid climates, this benefit is reduced and for many hours of the year, ventilation can be a source of moisture rather than a removal mechanism. Other removal mechanism must then be considered—most of which will require energy to operate. Dehumidification occurs in the course of conventional air conditioning, but as new construction improves, the sensible load on air conditioning systems decreases and so, therefore, does that incidental dehumidification. Since the latent load is not decreasing, there is the potential of having more times where supplemental dehumidification may be necessary. This paper uses detailed simulation tools to explore the range of humidities that conventional systems would generate in high-performance new homes and the impact that different ventilation strategies may have. The implications of acceptance