With the combination of two fans and a heat exchanger in one single component there is the possibility to design a compact and highly efficient ventilation system especially for use in building modernization. One crossflow fan generates both airflows (outdoor/supply and extract/exhaust air) and simultaneously acts as counterflow heat exchanger. The space between the fan blades is filled with elements which operate as regenerative heat exchanger. The modified laboratory prototype of the so called Counterflow Heat Recovery Fan was optimized for the use as single/double room unit. The modelling and operation modes of the modified concept as well as simulation results of the fluid mechanical behaviour are presented. Based on the numerical optimization the first laboratory prototype of the single/double room unit was manufactured. The measurement results are compared with the simulation and the further research focus is discussed.