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# Long description

This diagram shows a layer cake of ductwork. The two sets of alternate layers carry fresh air into the building and stale air out from it. Heat can pass through the walls of the ducts into the adjacent air stream, although the streams themselves are separate. At the top left of the diagram three blue arrows labelled ‘cool fresh air’ show air entering the first air path through the heat exchanger. At the bottom right three yellow arrows show this air stream emerging and are labelled ‘warm fresh air’. At the top right three red arrows labelled ‘warm stale air’ show air entering the second air path through the heat exchanger. At the bottom left of the diagram three orange arrows labelled ‘cool stale air’ show this air leaving.