Energy analysis for balanced ventilation units from field studies

Balanced ventilation units are well known to provide a sufficient amount of fresh air in residential buildings in a controlled way, without relying on ever-changing naturally driven forces. During colder periods, heat recovery ensures a reduction of the ventilation heating load. Outside the colder periods, recovery is reduced or shut off automatically, providing mechanical ventilative cooling. During warmer periods, the recovery is used again to provide a comfortably cool supply of fresh air. The combination of a balanced ventilation unit with ground heat exchange, provides precooling in the hot season, so that the ventilative cooling is taking place for the entire hot season, and not only during cool summer nights. 
Field studies are used in this research, to explain the automatic control between recovery and no recovery. Six projects in Western to Central Europe are monitored with balanced ventilation units, and two of these units are combined with ground heat exchange. For an entire year, the relevant parameters are recorded in each project, and hourly average values are analysed. The heat recovered, the ventilative cooling, and the cold recovered have been evaluated and given as a function of outdoor air temperature. The values are summed into annual heating recovered, annual ventilative cooling and annual cold recovered. The sums are compared to the electrical consumption in the respective period.  
The yearly averaged ventilation rates range from 95 to 250 m3/h. From the results it is clear that recovery is dependent on flow rate and temperature difference outdoor-indoor. The values for annual heat recovered range from 1850 to 4570 kWh. When ventilative cooling takes place, the amount of cooling with respect to the indoor temperature is also a function of flow rate and temperature difference outdoor-indoor. Without precooling by ground heat exchange, the annual ventilative cooling with respect to indoor temperature range from 393 kWh to 657 kWh. When precooling is used, it is shown that the annual ventilative cooling is increased to 1480 kWh because the period for ventilative cooling is extended. The annual cold recovered by the units range from 27 kWh to 61 kWh . 
The annual electric consumption of the fans has been evaluated using measured values of fan rotational speed and air flow rate, and an average fan efficiency for the relevant fans in the units. The energy benefit can be expressed in terms of the seasonal performance factor SPF, which is the energy gain (heat recovered, ventilative cooling and cold recovered) divided by the electrical consumption of the fans in the respective period. In the heat recovery period, the SPF is evaluated to range between 16 and 23 with a positive outlier of 47 in a project with low average air flow rate. In the ventilative cooling season, the SPF is evaluated to range between 3.7 and 4.9, and a value as high as 9.8 for the project with low average air flow rate. In the cold recovery season the SPF ranges from 2.0 to a value of 4.5 as maximum.