Abstract: Exchanger means for exchanging a first fluid with a second fluid, in varying proportions and employs a first duct, carrying the first fluid, a second duct carrying the second fluid, and a transfer chamber connected to both ducts, through which some, or all of the second fluid is able to be transferred back into the first duct, and variable control means in the form of first and second damper blade means in the chamber which can be swung together, thus dividing the chamber and preventing transfer, and which may be swung apart to procure varying proportions of such transfer. The chamber also has inlet means for inlet of the first fluid and outlet means for discharge of the second fluid.