A ventilation unit for providing supply air, preferably outside air (fresh air), to an apartment or parts thereof and for removing return air, preferably exhaust air (used air), from said apartment or parts thereof, said ventilation unit comprising: a supply air outlet for establishing supply air flow communication with said apartment; a return air inlet for establishing return air flow communication with said apartment; an exhaust air outlet for establishing exhaust air flow communication with the atmosphere; an outside air inlet for establishing outside air flow communication with the atmosphere; a heat exchanger having first air flow passages and second air flow passages for transferring heat energy from return air entering said first air flow passages and exhaust air leaving said first air flow passages on the one hand, to outside air entering said second air flow passages and supply air leaving said second air flow passages on the other hand; a first ventilator at a first location within the ventilation unit, for transporting air through a first air flow path starting at said return air inlet, passing through said first air flow passages in the heat exchanger and ending at said exhaust air outlet; a second ventilator at a second location within the ventilation unit, for transporting air through a second air flow path starting at said outside air inlet, passing through said second air flow passages in the heat exchanger and ending at said supply air outlet, wherein said first air flow path and said second air flow path comprise internal air flow ducts having rectangular cross sections, characterized in that a) both said first ventilator and said second ventilator are located downstream of the heat exchanger in their respective air flow paths (AFP1, AFP2); b) the air flow cross section of said return air inlet and the air flow cross section of said outside air inlet are greater than the air flow cross section of said supply air outlet and the air flow cross section of said exhaust air outlet; and c) at least a major portion of the entire length of each of the internal air flow ducts of the ventilation unit is of substantially rectangular flow cross section. a supply air outlet for establishing supply air flow communication with said apartment; a return air inlet for establishing return air flow communication with said apartment; an exhaust air outlet for establishing exhaust air flow communication with the atmosphere; an outside air inlet for establishing outside air flow communication with the atmosphere; a heat exchanger having first air flow passages and second air flow passages for transferring heat energy from return air entering said first air flow passages and exhaust air leaving said first air flow passages on the one hand, to outside air entering said second air flow passages and supply air leaving said second air flow passages on the other hand; a first ventilator at a first location within the ventilation unit, for transporting air through a first air flow path starting at said return air inlet, passing through said first air flow passages in the heat exchanger and ending at said exhaust air outlet; a second ventilator at a second location within the ventilation unit, for transporting air through a second air flow path starting at said outside air inlet, passing through said second air flow passages in the heat exchanger and ending at said supply air outlet, wherein said first air flow path and said second air flow path comprise internal air flow ducts having rectangular cross sections, characterized in that a) both said first ventilator and said second ventilator are located downstream of the heat exchanger in their respective air flow paths (AFP1, AFP2); b) the air flow cross section of said return air inlet and the air flow cross section of said outside air inlet are greater than the air flow cross section of said supply air outlet and the air flow cross section of said exhaust air outlet; and c) at least a major portion of the entire length of each of the internal air flow ducts of the ventilation unit is of substantially rectangular flow cross section. a) both said first ventilator and said second ventilator are located downstream of the heat exchanger in their respective air flow paths (AFP1, AFP2); b) the air flow cross section of said return air inlet and the air flow cross section of said outside air inlet are greater than the air flow cross section of said supply air outlet and the air flow cross section of said exhaust air outlet; and c) at least a major portion of the entire length of each of the internal air flow ducts of the ventilation unit is of substantially rectangular flow cross section.