Influence of reduced VAV flow settings on indoor thermal comfort in an office space

Abstract

The air temperature distribution in a space with reduced diffuser flow rates and heat loads was studied using simulation. Computational fluid dynamics (CFD) was used to analyze the room air distribution from a side wall diffuser at the design flow rate, and the results were validated with experimental data. CFD was used to predict occupant discomfort under a range of reduced diffuser flow rates. It was found for diffuser flow rates above 30% of the design flow rate that the temperature influence from the jet was minimal. At these flow rates, there was nearly a uniform temperature distribution in the occupied zone. The predicted maximum value of percentage of dissatisfied occupants within the space began to increase for diffuser flow rates below 30% of the design flow rate. The percent dissatisfaction at 1 m room height was greater than 25% for the lowest diffuser flow rate tested (15% of the design flow rate) directly under the diffuser, which was the highest of the test cases, but was 5% or less throughout more than 90% of the room. In contrast, at the higher flow rates, the percent dissatisfied index was 5% or less in only 60%–80% of the room due to increased velocity. Evidence of dumping was already found at the traditional minimum flow rate setting of 30% of design, and so there would be little harm in reducing the minimum flow rate further. Reducing the flow rate below 30% of design just moved the location of the dumping closer to the diffuser. For very low diffuser flow rates (below 30% of the design flow rate), it is recommended that desks be placed away from the supply diffuser to avoid discomfort. Overall, the simulation results indicate that uniform temperatures are maintained in the room at flow rates as low as 15% of design except immediately under the diffuser. This suggests that the VAV minimum flow rates can be set below 30% of design flow as long as the diffuser is at least 1 m from an occupant’s position.