Fireplace heat exchanger

[firemanrob]

I have seen some of the professionally built fireplace heat exchangers on the internet which look to be about 4 or 5 2" metal tubes shaped like a u. Some have a blower in them and some don't. The manufacturers charge between 200 and 400 for one. I think it looks simple enough to make one, but I'm wondering what type of tube or pipe to make the heat exchanger out of since it will be getting hot. My concern is certain types of metal might give off gasses or fumes when heated. Any Ideas?

[lwalper]

It's usually pretty thin-walled stuff -- looks like exhaust pipe to me.