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

tThe energy efficient operation of mechanical ventilation systems depends on various parameters. InLuxembourg, field tests with 20 centralized and 60 decentralized mechanical ventilation systems insingle- and multi-family homes were conducted in order to measure the actual performance of thosesystems in occupied buildings. The considered parameters were: Main air flows, internal and externalrecirculation, sensitivity to differential pressure, specific fan power and heat recovery efficiency. In manycases, the performance of the ventilation units was lower than expected. The systems showed deviationsbetween supply and exhaust flows of up to 60%. In particular, the air flow in decentralized units wasstrongly influenced by pressure differences between the inside and outside due to wind or stack effectswhich leads to a decreased heat recovery efficiency. The total mean recirculation ratio was 6.5%, with astandard 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, theratio between air flow and power consumption, was measured with 0.475 ± 0.37 Wh/m3for centralizedand 0.22 ± 0.023 Wh/m3for decentralized systems. The lower value for the decentralized systems canbe explained by lower pressure losses due to the lack of ductwork. The heat recovery efficiency was0.65 ± 0.24 for centralized systems and 0.7 ± 0.17 for decentralized systems which is significantly lowerthan nominal values provided by the manufacturers. The results of this study show that the overall energyefficiency of ventilation devices installed in residential buildings under real working conditions are oftenlower than expected. These findings could possibly serve as indicators for future research & developmentat manufacturer and commissioning level.