In the "Stockholm Project", different blocks of multifamily buildings have been extensively monitored for about three years. Temperatures, airflows and electricity use have been registrated each hour. As an additional base to this examination, ten fan units in the buildings have been intensively studied. The results show that the specific use of power for transportation of ventilati n air varies between approximately 1 and 4 kW per m³ and second. The results from the measurements indicate a notably low level of installation efficiency. The total efficiency of the ten units varies between 15 and 57 percent and has a mean value of 32 percent. The loads of the units have been comparatively low and consequently they have had an influence on the power factor. The operative conditions related to the loads also tends to have a disadvantageous influence on the efficiency. The regained energy from heat recovery plants in two of the installations, compared to the total electric energy for operating the fans in the system, gives an approximate relation of 2 to 1. Regained energy compared to transformed energy specifically related to drops of pressure in the heat exchanger gives the relation 6 to 1. Because of heat losses from ducts and inability to utilize the extracted energy from the exhaust air, the mean value for the useful recovered heat has been considerably below the efficiency of the plant. The cause of the variaton in specific use of power for the examined units, mainly depends on the total pressure drops and the dimensioning of the fan motors. The power demand for the electric motors has in general been overestimated. 90 percent of the examined motors have an output power exceeding the requirement of power for the fan. The loads vary between 38 and 97 percent, with an average of about 64 percent. One of the main reasons why the power of the motors exceeds the normal requirement, is the design of regulating extra needs of airflow on limited occasions, such as cooking hours. During normal operation the air flow is reduced by a damper. This gives an extra pressure drop for the main part of the running hours, which causes essential losses of energy. The combination of high pressure drops and low efficiency in ventilation plants create unnecessary use of electric energy. With a careful desigq of the system, well insulated ducts and an efficient regulation of air flow, the possibilities to improve the mode of application of the recovery system and decrease the use of electricity seem to be substantial.