By Viralkumar Gandhi and Kishor Khankari, Fluent Inc.; Kevin Blackwell and Rob Hammon, ConSol, Stockton, CA

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ConSol and the California Energy Commission (CEC) are working on a joint project to improve the energy efficiency and thermal comfort of residential homes. These qualities can enhance the profitability of homebuilders through reduced warranty and callbacks. ConSol approached Fluent Inc. to develop a CFD model that can be used as a predictive tool to test the effect of several design and operating parameters of the HVAC system of a typical single-story, three bedroom house in California. The main goal of this ongoing project is to test the effect of ventilation airflow rates, and the location, size, and type of ventilation registers on the distribution of air and temperature within the home during the winter and summer months.

Geometry of the three bedroom house studied

Thermostat cycles showing the cyclic duration of heating and cooling cycles

Fluent used Airpak software to develop a 3D CFD model to simulate the winter situation in which the home was heated by hot air supplied through a number of ventilation registers placed in the ceiling. The velocity and throw of each register was obtained from the manufacturer of the registers, and was modeled through the builtin functions for ventilation registers in Airpak. Convective and radiative heat losses from the walls, windows, and partitions of the home were modeled by considering the effective U values (representative of thermal conductance) for each component. The feedback control from a thermostat was also taken into consideration in the transient model by monitoring the air temperature at the thermostat location in the home, and accordingly setting the ventilation fan to ON or OFF positions to maintain the temperature within a 68°F to 72°F range.

The computational model accurately predicted the distribution of airflow and temperature during the winter season. The thermostat cycle predicts that the fan turns ON about every 14 minutes and remains ON for about 4 minutes. These predicted durations of the fan cycles helped quantify the heating duty on the home. The analysis also showed that in certain rooms, the high airflow rates and throws of the registers, which are specifically designed for the summer cooling conditions in California, cause localized hot zones near the ceiling. The prolonged OFF cycle of the fan helps reduce the thermal stratification and give rise to more uniform air temperatures, which translates into improved thermal comfort for the occupants.

Distribution of air temperature at the end of the heating cycle showing thermal stratification in the house

Pathlines colored by air temperature showing the airflow entering from the ventilation registers

ConSol and CEC are planning to share these computational results with builders and prepare new design guidelines for residential ventilation systems. These new guidelines, based on the computational analysis, will help builders reduce risks and costs by improving the quality, comfort, and energy efficiency of residential construction. ConSol and CEC find this predictive tool a more cost-effective and efficient means of achieving this goal than through physical testing. Indeed, measuring velocity and temperature at several thousand locations in a house, comparable to the output of a CFD analysis, would be not only expensive and labor intensive, but virtually impossible to do!

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