Field tests of centralized and decentralized ventilation units in residential buildings - Specific fan power, heat recovery efficiency, shortcuts and volume flow unbalances

The energy efficient operation of mechanical ventilation systems depends on various parameters. In Luxembourg, field tests with 20 centralized and 60 decentralized mechanical ventilation systems in single- and multi-family homes were conducted in order to measure the actual performance of those systems in occupied buildings. The considered parameters were: Main air flows, internal and external recirculation, sensitivity to differential pressure, specific fan power and heat recovery efficiency. In many cases, the performance of the ventilation units was lower than expected. The systems showed deviations between supply and exhaust flows of up to 60%. In particular, the air flow in decentralized units was strongly influenced by pressure differences between the inside and outside due to wind or stack effects which leads to a decreased heat recovery efficiency. The total mean recirculation ratio was 6.5%, with a standard deviation of 12.5% for centralized and 13 ± 6.2% for decentralized devices. As a consequence, the delivered flow of fresh air is smaller by the amount of recirculation. The specific fan power, the ratio between air flow and power consumption, was measured with 0.475 ± 0.37 Wh/m3 for centralized and 0.22 ± 0.023 Wh/m3 for decentralized systems. The lower value for the decentralized systems can be explained by lower pressure losses due to the lack of ductwork. The heat recovery efficiency was 0.65 ± 0.24 for centralized systems and 0.7 ± 0.17 for decentralized systems which is significantly lower than nominal values provided by the manufacturers. The results of this study show that the overall energy efficiency of ventilation devices installed in residential buildings under real working conditions are often lower than expected. These findings could possibly serve as indicators for future research & development at manufacturer and commissioning level.