In heating, ventilation and air-conditioning systems, HVAC, recuperators are commonly used to re-use waste heat from exhaust air normally expelled to atmosphere. Devices typically comprises a series of parallel plates of aluminium, plastic, stainless steel, or synthetic fiber, alternate pairs of which are enclosed on two sides to form twin sets of ducts at right angles to each other, and which contain the supply and extract air streams. In this manner heat from the exhaust air stream is transferred through the separating plates, and into the supply air stream. Manufacturers claim gross efficiencies of up to 80% depending upon the specification of the unit.