Dismantling a Gas Furnace

Gas furnace's could be a few types but since you mention ducts it must be hot air system. The reality is depends on size = weight, so lets assume you have a 250 pound unit, as is worth $ 25 bucks if you take it apart completely will Be another $ 7 go $ 8 - Taken apart there is short steel - copper - Mix steel- AL . Now some systems are small 125 pounds so the gain is less . The motor you want the shell so look for 4 long screw bolts that hold the 2 Aluminum flat plates (at opposite ends) to housing of motor Use a tool (socket set) or grind off, then pull shell out , next is a relay switch for gas pipe - copper is wound in a transformer like fashion on switch box - use hammer to get at it . For me that's all that s worth pulling the heat exchange is short steel (in some yards) but remember the furnace might have some non metal so once you expose it you might be told to remove insulation
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One last thing the CU on those motors is not so easy to remove, I would advise you to get a 4 1/2 grinder they have become a very low cost item from $29 and up, cut the shell vertical on motor housing peel away now deal with the copper wound on steel - meat ball !

P.S.
The term meat ball is for a copper wound on steel