Vallox 51 MV R model in the figure. In the L model, the parts are mirrored. 1 Extract air fan 2 Supply air fan 3 Fine filter for supply air 4 Heat recovery cell 5 Bypass damper of the HR cell 6 Coarse filter for supply air 7 Coarse filter for extract air 8 Post-heating resistor 9 Control panel 10 Internal humidity sensor Internal carbon dioxide sensor 11 Humidity sensor (optional) 12 Carbon dioxide sensor (optional) 13 VOC sensor (optional) 14 Ceiling bushing for electric wires (optional)