Decentralized, room-based ventilation systems have become increasingly popular in the Baltic countries. Such systems are easy to install and, according to technical information, ensure high heat energy recovery potential for new and renovated buildings. The specified heat recovery efficiency is used for building energy simulations and to calculate the necessary heating energy that is needed to warm up the supply air. However, this value is stated at non-existent pressure difference between indoor of the building and the outside. In real-case situations, there is always some pressure difference due to wind and stack effect. In this study, a ventilation device is tested in a laboratory environment at different simulated outside air temperatures and pressure differences. The simulations are conducted in a climatic chamber where the air temperature and pressure differences can be set. The temperature is adjusted using a cooling device but the pressure difference with an exterior fan device. Different combinations of simulated outside air temperatures and pressure differences were tested. The results suggest that the heat recovery efficiency is highly dependent on the pressure difference and it rapidly decreases with the rise in pressure difference. If the pressure difference is in the range of 10–20 Pa, the heat recovery efficiency will be only between 20 and 50 %, while the stated value in the technical data sheet is 85 %. Even at a pressure difference of 0 Pa, the average heat recovery efficiency is 73 %, and only for the first few seconds of the supply cycle, the efficiency reaches 85 %. This can influence the calculated building energy efficiency class, as well as lead to undersized heating system elements.