Hi MM,
I have just read your post in relation to your boiler. Some years ago I built a boiler with the same concept as Ivan mentioned ie 4in tube inside a 6in tube.
Worked great until one day the electric supply board ESB called and asked me what was I using the 4in stainless steel pipe for? which was the chimney from the boiler situated directly below the ESB cables, that run across my back garden.
They asked me to take it down because of the danger, there only being 6/7 ft between the top of the pipe and the cables.
The reason I had the chimney so high was because I was burning waste veg oil and I didn't want the neighbours calling to the door because of the smell.
I had all the hot water requirments that I needed from that boiler.
I might relocate the boiler some time in the future.The boiler was a drip feed system. The only drawback I had, was that I had to be in attendance most of the time while it was burning.
If you would like to see the plan and pics, go to the yahoo vegoilburner group forum.
Click on photos and scroll down to 'Jim's boiler'.
http://groups.yahoo.com/group/vegoilburners/photos/album/1777202628/pic/listHope the above will be of some help to you.
Regards Jim.