Air Renewal Effectiveness of Decentralized Ventilation Devices with Heat Recovery

Central ventilation systems with heat recovery have shown their limits especially within the context of building energy retrofit. The difficulties to install these systems in existing buildings, to find available space for devices, air ducts, silencers and fire dampers and to independently control the air flow in each room according to the real ventilation needs have led to an increasing market for decentralized ventilation devices. A frequent criticism of decentralized devices is the poor ventilation effectiveness due to a high short-circuiting risk. Therefore, the University of Applied Sciences of Offenburg and the Fraunhofer Institute for Solar Energy Systems are evaluating together the inside air quality reached with decentralized ventilation systems. The tests described in this paper were performed in a test lab representing a single room were a decentralized ventilation system with heat recovery, heating and cooling functions was implemented. In a first part of the evaluation process, tests are realized to characterize the air distribution provided by the decentralized device by smoke visualization. The results are showing interesting differences between isothermal, heating and cooling cases. The characterization of air distribution in the volume of the room is not enough to evaluate the capacity of a ventilation system to provide a good inside air quality. Therefore, a second series of tests was performed with CO2 as a tracer gas, enabling a more precise quantification of the ventilation effectiveness. The results show the CO2 concentrations over time in zones with different air renewal levels for each of the 3 modes.