Temperature and humidity sensors are located on outside air inlet 26 and on the inside air inlet 24. A controlling unit (not shown) controls volume of air supplied and exhausted by respective fans 5 and 3 based upon the temperature and humidity of the respective sensors. The two flows exchange heat via heat exchanger 1. Respective dampers are provided to bypass air supply and air exhaust so as to bypass the heat exchanger 1 under specified temperatures and humidity values.
