Heat exchanger in plants for ventilating rooms or buildings

In a ventilating system for a building:

• first and second duct systems separated by partition walls and arranged in heat-exchange relationship such that heat may be transmitted from air flowing through one duct system, through the partition walls, to air flowing through the other duct system;
• reversible fan means associated with each duct system for passing air through the two duct systems countercurrently;
• a first outside air duct placing one end of the first duct system in communication with outside air and a first inside air duct placing the other end of the first duct system in communication with inside air;
• a second outside air duct placing one end of the second duct system in communication with outside air and a second inside air duct placing the other end of the second duct system in communication with inside air;
• and valve means for controlling air flow including first and second branch ducts arranged in crossing relationship, each branch duct having a first end connected to said first inside air duct and a second end connected to said second inside air duct, a non-return valve in each said first and second inside air ducts at locations between the connections of the branch ducts to said first and second inside air ducts, and a non-return valve in each said branch duct, the arrangement being such that in one mode of fan operation outside air flows through said first outside air duct, said first duct system, said first branch duct and into said second inside air duct while inside air flows through said first inside air duct, said second branch duct, said second duct system into said second outside air duct, and in another mode of fan operation outside air flows through said second outside air duct, said second duct system into said second inside air duct while inside air flows through said first inside air duct, said first duct system into said first outside air duct.