Not sure even Bent Glass would need WHE.
You're right that there is a lot of heat leaving a furnace, a WHE is one way to recuperate that heat and offset operating costs.
On the other hand,a pound of fuel/air mixture had to go into the furnace for every pound of exhaust gas coming out. Heating that incoming air to furnace temperatures takes a lot of energy. It seems to me that the more practical and elegant route is to use an air-to-air heat exchanger to transfer heat from the exhaust gas to the incoming air. Every degree of temperature rise before the air hits the furnace is that much fuel you don't have to burn. With no moving parts or wear, it seems much cheaper and simpler.
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