Why can't range-hood exhaust ducts be designed to recover heat?

摘要

Many mechanical ventilation systems are designed with a pipe-within-a-pipe system that allows heat to be exchanged between the outgoing and incoming air-streams. Range hoods can't do that for two reasons. First, the pipe diameters get big fast. If your range hood requires a 10-in.-dia. exhaust duct, then the duct-within-a- duct solution would require a 14-in. or 16-in. duct. It's hard to find that much room for ductwork in a house. Elbows compound the awkwardness.