Thank you for the response. I was anticipating the response that you gave, but thought I would ask anyhow. Many residential furnace heat exchangers use the AL29-4C material, but not many (if any) furnace manufacturers make condensing furnaces in China.
I was thinking about different materials, but the 29-4C has become a selling point over the years... so marketing a furnace using a different alloy for the secondary heat exchanger may not be effective.
Also, forcasting is a big deal when using this material. It only gets made about twice per year... or so I am told.
Anyhow, thanks again.