Would you be wrapping the copper pipe around the bigger pipe? In what way did you plan on regulating your water temperature?
You may want a slightly larger diameter burn chamber so there's plenty of air space, not to mention save you time splitting logs.
Is this going to be naturally aspirated? As I mentioned in another thread, imparting heat from the exhaust gases into intake air heats the process exponentially. In addition, it burns the wood more efficiently.
I like the sand idea, but it's just as practical to use dirt. Fire brick (like the 4x13in variety)in at least one layer around the burn chamber is advisable, though.
Make sure the piping for the flew gas is resistant to corrosion so you can clean out the creosote, rosin, and other wood by-products with proper cleaners come summer. This is especially essential if you're burning green wood, pine, and other sappy woods.
-Andrew