LB-07-034 -- Experimental Evaluation of a Downsized Residential Air Distribution System: Comfort and Ventilation Effectiveness PDF

New challenges surrounding air-distribution effectiveness have emerged from downsizing of the HVAC systems in energy-efficient residential buildings. As a result of downsizing, the system design flow rates are significantly reduced— potentially leading to poor air mixing and undesirable spatial temperature patterns and, consequently, adversely affecting the comfort level and ventilation effectiveness. Unless these challenges are successfully met, the system downsizing trend, as well as the efforts to implement related energy-saving measures, will be undermined.

This paper examines the performance of a prevalent airdistribution configuration incorporating a high sidewall supply air diffuser and a near-floor, wall-mounted return air grille in a test room during a period of recovery from setback. To address the aforementioned issues of interest, tracer-gas tests were conducted along with spatial temperature measurements in the room. In the experiments, two levels of supply airflow rate were considered, low and high, of which the latter is comparable to the design range commonly applied in standard- practice residential buildings. The results indicated that, at the lower flow rate and velocity, both ventilation effectiveness and comfort were compromised because of stratification.

The constraints with the trace-gas instrumentation led to inconclusive results with respect to air mixing and ventilation effectiveness. To overcome this limitation, a frame of reference was introduced for a more accurate assessment of the experimental system performance. The analytical solutions for an ideal case in which the indoor air was perfectly mixed at any given instant provided the reference points for comparison.

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