The impact of wind pressure and stack effect on the performance of room ventilation units with heat recovery

Abstract

Room ventilation units (RVUs) with heat recovery represent an alternative ventilation solution for renovated residential buildings, allowing simple installation through the façade. However, wind pressure on the façade can have a substantial impact on fan performance and thus affect the airflows through the RVU. Airflow imbalances between supply and exhaust negatively affect total heat recovery and supply air temperatures. Moreover, RVUs installed in apartments on the ground floor of multi-story buildings with a natural ventilation shaft are highly affected by negative pressures due to stack effect during winter periods. The issues that characterize these units were examined in this study in relation to the fan type and fan operation, aiming to establish why the performance is varying for different units. The study simulated the performance of RVUs in a typical Danish test apartment under various wind pressure conditions. To investigate the combined impacts of wind and stack effect, the simulations assumed the test apartment being located on the ground floor of a 4-storey building with an 11-meter natural ventilation shaft. The authors performed dynamic annual simulations with different fan characteristic curves and fan operating points for centrifugal and axial fans. Simulations tested the additional impact of adding a humidity-controlled damper in the intake of the natural ventilation shaft to reduce pressure differences across the building envelope due to stack effect. Results showed that centrifugal fans were less sensitive to pressure differences than axial fans due to their steeper fan curves and generally higher operating pressures. When taking into account wind pressure and disregarding stack effect, the sensible heat recovery decreased by up to 8% in RVUs with centrifugal fans and up to 22% in RVUs with axial fans at the same airflow settings. After including stack effect, the decrease in sensible heat recovery was instead up to 20% for RVUs with centrifugal fans and up to 52% for RVUs with axial fans. The inclusion of a humidity-controlled damper reduced the pressure differences due to stack effect and halved the negative impact on sensible heat recovery on average. As a result, the RVUs were able to provide considerably higher supply air temperatures. The study showed that a humidity-controlled damper could reduce the negative impact of stack effect on the performance of an RVU.