ERSR is a heat recovery unit containing a rotary exchanger (that can be supplied in a hygroscopic material), offering high efficiency and low pressure drops. The unit is fitted with plug fans with reversed blades (on both suction and delivery) (that respect ERP2015) and a directly coupled motor, controlled by an EC device for the smaller sizes and an inverter for the larger ones. It has F7 filters on both delivery and return, and a differential pressure switch for checking the filters are not clogged. In the intermediate seasons, the outdoor temperature may be more pleasant than the indoor one, so ERSR models have a “free-cooling” function (i.e. the use of fresh outdoor air to cool the rooms, at zero cost because the recovery unit is deactivated).