This paper is based on findings resulting from ASHRAE Research Project RP-1449. ABSTRACT Conventional air conditioners have traditionally been deemed adequate for controlling space humidity levels in residential applications. However, as homes in humid climates have become more energy efficient, there is evidence that relative humidity levels in homes have been increasing (Rudd and Henderson 2007). This implies that sensible heat gains to the building have been reduced more than moisture loads, leaving a mix of latent and sensible loads that is poorly matched to the sensible heat ratio of conventional air-conditioning systems. Furthermore, requirements to provide mechanical ventilation per ASHRAE Standard 62.2 have affected space humiditylevels in a typical home.

A detailed simulation model was developed in TRNSYS to accurately predict energy use and space humidity levels for the wide variety of space-conditioning systems. The analysis considered various building efficiency levels, mechanical ventilation options, equipment performance and configuration options, and climates. The hours when humidity is a concern (e.g. over 60% rh) generally occur in the winter and swing seasons. When mechanical ventilation is added to an older standard efficiency house, the number of high humidity hours tends to increase slightly. As the home efficiency level continues to increase to current practice and to slightly better than current codes, the number of high humidity hours generally decreases. However, for very high efficiency houses, where the space conditioning air distribution ducts are moved from the attic to inside the conditioned space, the high humidity hours tend to increase again. Air conditioner enhancements that lower the airflow and allow overcooling at high humidity times generally cut the number of annual hours over 60% rh by half when the activation setpoint was 50% rh. However, precise humidity control (to maintain a setpoint) requires a separate dehumidifier or an air-conditioner system with full condensing reheat to provide dehumidification without sensible cooling. The operating cost for full humidity control is typically 10%–30% greater than a conventional cooling system without explicit humidity control, depending on the dehumidification system and the relative humidity control setpoint.