Mechanical ventilation systems with heat recovery for low energy residential buildings have gained an increasing interest during the last few years. This article focuses on single room ventilation units which do not need any ducting within the dwelling. Therefore, they are potentially very suitable for retrofitting use. The performance criteria of these units are similar to those of central systems. A recuperative and a regenerative ventilation unit were investigated by means of experiments and numerical simulations with reference to ventilation efficiency, thermal comfort, heat recovery, electric energy input and acoustics. It was shown that using these units, rooms can be efficiently ventilated (typical air change efficiency 0.6) at a good level of thermal comfort. Temperature efficiencies of up to 0.78 were found at low levels of electric energy input. The main difficulty in the design of these small units is to reach the target values of indoor sound pressure levels (< 30 dB (A)) and sound reduction indices (> 20 dB rsp. > 30 dB). With reference to acoustic properties, further development of these units seem necessary. (C) 2000 Elsevier Science S.A. All rights reserved.