Bending tube for spiral ??

[pef]

Hi,

I have a question, how to make spiral with steel tube ??

It's to build a special wood/coal furnace like this one :



Water flows through the hose and is then heated water is sent to radiators in other parts of the house.

My trouble is to make the heat recovery.

Someone have an idea ??


( sorry for my english, I speak French... )