Ventilation Effectiveness of Alternating Façade-Integrated Ventilation Devices

Domestic ventilation units are usually dimensioned based on the required airflow rate. However, their ability to actually exchange the room air volume is generally unknown. The presented research aims at analysing the ventilation effectiveness of alternating façade-integrated ventilation units (decentralized ventilation) experimentally. To this purpose a tracer gas analysis of a lab test case is performed. In the presented research a total of 15 tests have are carried out in a climatic chamber representing a single room equipped with two alternating façade-integrated ventilation devices with regenerative heat recovery (ZAS-D). The tests include summer, winter and intermediate supply air conditions. Further investigations are dedicated to the effect of thermal convection due to human heat dissipation on the room airflow. The aim of the measurements is to evaluate the ventilation effectiveness under various conditions. The required analysis of the age of air is carried out by means of the concentration decay method which foresees enriching CO2 as a tracer gas in the test room and observing its subsequent decay behaviour due to air exchange. The global air exchange efficiency ea is expected to range between 0.5 for ideal mixing ventilation and the maximum 1.0 defined by ideal piston flow. The values derived from these measurements vary from ea = 0.34 to ea = 0.60 and thus indicate inadequate performance for most cases. The placement of both ventilation units in the same façade does not seem ideal, as there is a high risk of short-circuiting currents. Recording at 17 sensor positions also provides information on spacial distribution and dynamics of the air exchange. The local air quality index ei,ja equals to 1 for ideally mixed volumes. However, almost ideal mixing ventilation, as often described in scientific literature, cannot be confirmed in this series of measurements. It can be observed that thermal convection causes an increase in inhomogeneity. Temperature differences between supply air and room air lead to thermal upward and downward forces due to density differences. As a result, the upper room volume is better ventilated in summer daytime conditions whereas the lower room volume shows better air exchange in winter conditions. The placement of a dummy, simulating human heat dissipation, and placed half way between the ZAS-D equipped façade generates an additional vertical convection current. This increases the global air exchange efficiency to the typical value for displacement ventilation ea > 0.5. However, the improved ventilation efficiency of the measured system only affects the part of the room close to the façade equipped with the ventilation units. In contrast, the room volume further away from the ventilation units can be regarded as stagnating.