I would like to use 1" type M copper tubing to make a fireplace insert that fits over my cast iron fireplace grate. I will use a small fan to force room temp air through the tubing to be heated and back into the room to heat the room. The system would be sealed against CO. The melting point of copper is well above what should be experienced in the firebox. I'll have an alloy cuff between the pipe and the fan to cut heat transfer so the fan won't get too hot from the copper pipe.
Before I do this, I want to make sure I'm not missing something obvious (or subtle).