Cross-flow heat exchanger exchanges thermal energy from Exhaust air stream of conditioned space to fresh air stream. In flat type heat exchangers two airstreams cross by each other one of these airstreams is exhaust from conditioned air and other air stream is fresh outside air being supplied to ventilated space. A cross-flow heat exchanger is made of thin metal panels, normally aluminium. The thermal energy is exchanged via the panels. A traditional cross-flow heat exchanger has a square cross-section. It has a thermal efficiency of