A study on the ventilation of a building by using solar energy, a heat recovery unit and electrical heater (if it is needed) is performed. The heat recovery unit is used to increase temperature of fresh air by using thermal energy of the return air. Then, the thermal energy stored in a sensible tank is employed to increase the temperature of air leaving the heat recovery unit. If the air temperature at the coil outlet is less than the supply temperature, electrical heater is operated. The study is performed by TRNSYS software for the period of 1st of November to 31st of March. The ventilation system operates from 17:00 to 24:00 where a lower ambient temperature exists compared during the day period. The obtained results show that a ventilation system with solar assisted and heat recovery unit can considerably reduce ventilation energy consumption. The use of solar assisted ventilation system with heat recovery unit reduces energy consumption by 80% if it is compared with conventional ventilation system in which only an electrical heater operates. Furthermore, it is found that the design of ventilation systems without energy of electrical heater is possible. The study s performed by using weather data of Izmir city in Turkey. The supply temperature of fresh air is assumed as 22°C.