Copper heat exchanger often called duct coils or furnace radiators, are used for installing into forced air furnace to provide heat from hot water. They are usually installed into the cold air side of the furnace. They work in much the same way as the central air conditioning unit. These copper heat exchanger can also be used as a stand alone heat source by simply attaching a fan or squirrel cage to one side of the unit.Copper for heat exchanger can be installed in either the hot air side or the cold air return of a furnace. Please note that if installed in the cold air return you will need to install a 3 way zone valve in order for the fan to operate properly.