so made a rocket stove to act as a water heater. used 4 inch thin metal pipe AC duct., Worked well first day test run.  ran for about 3 hrs.Today was working the a little better due to a shorter up pipe witch lowered the water pipes. After about 2 hrs running 30 min at max fuel scrap wood chopped up to the size of large kindling. I was able 150 gallons a hour up about 40 degrees (from inlet to output). 

Then disaster.

The metal lining melted and the…