Risk Assessment of Heating, Ventilating, and Air-Conditioning Strategies in Low-Load Homes - Orlando, Fresno, Denver

TO5 2.1.3: HVAC Risk Assess. in Low-Load Homes

Modern and energy-efficient homes that conform to the U.S. Department of Energy's (DOE's) Zero Energy Ready Home (ZERH) requirements are being constructed at an increasing rate. ZERH requirements closely align with the International Energy Conservation Code (ICC 2012). One challenge that faces these homes in the marketplace, however, is the risk that traditional heating, ventilating, and air-conditioning (HVAC) systems will not provide adequate comfort. DOE's Building America Research team IBACOS found that low-load homes (ZERHs) have differing room-to-room load densities and highly variable load densities throughout the day and year based on solar gains and internal gains (Poerschke and Stecher 2014). Current engineering guidelines for air-based space-conditioning systems use a methodology that was developed more than 50 years ago, which was based on the concept that buildings are dominated by externally driven shell loads (Straub 1956). Significant advances in thermal enclosure performance mean that peak loads that are still externally dominated can be strongly influenced by internal gains, hourly loads in different rooms, and thermal lag-all of which require rethinking the traditional space-conditioning system. Homeowners want to live in durable, comfortable, and efficient homes. Such homes are achievable with appropriate care in the design and construction phases. This project provides valuable insight into design techniques for maximizing comfort in an efficient home. Homebuilders need solutions that will ensure occupant comfort and achieve the high level of energy efficiency demanded by modern standards.
To identify cases in which comfort standards may be compromised and to identify strategies to mitigate this concern, IBACOS used detailed TRNSYS (TESS 2015) models (Version 17) to simulate three different house geometries in three climate zones. Each house was simulated with a traditional HVAC system, a high-velocity HVAC system, and a multihead mini-split system. As appropriate for each respective system, different control strategies were evaluated for their effectiveness in maintaining thermal uniformity in the house.
These simulations enabled IBACOS to identify which strategies work best for particular climate zones or house geometries and to propose solutions that homebuilders can implement to reduce the risk of occupant discomfort in individual rooms.
The results of this study indicate that the control strategy and ideal system type vary with house type and climate. Single-story homes with centralized open layouts can be adequately conditioned with a single thermostat in all the simulated climate zones. Two-story homes with a high window-to-wall ratio on the southern and western exposures need multiple thermostats to provide adequate comfort. In these cases, continuous fan operation does not significantly improve comfort; a discretely zoned approach is necessary to prevent undercooling of some zones.