The rocket pros must all be busy doing heater building workshops? So I'll take a stab at your question and share with you what I've learned to date.
According to the authors of
Rocket Mass Heaters [highly recommended, and I have the newly release 3rd edition hard copy; $20] from:
http://www.rocketstoves.com
...they replaced the original barrel on their household
RMH after about 12 years as part of routine maintenance. However, the barrel showed no signs of degradation.
True that super high temperatures in the core and central riser area will quickly degrade steel parts, so steel is to be avoided or to be considered temporary "forms" for the masonry. The high burn temperatures are dissipated rapidly by the time the exhaust hits the heat shedding barrel. "Un-throttled" (running wide open) a typical RMH is capable of cherry red temperatures at the barrel's top. A brick or two slid across the fuel input opening is often used as an effective handle on combustion or "throttle" together with choosing how much fuel /
wood is being burned.
Additionally, the barrel's operating temperature can be tweaked to put less heat at the barrel's top, more heat down the barrel's side, by setting a larger gap between the top of the heat riser and the barrel. A gap of right about 2" seems to be the typical recommendation for an 8" system built primarily for heating.
Erica and Ernie Wisner's new book "
Rocket Mass Heater Builder's Guide" is slated to be released very soon. They've built over 700 RMH. I'm looking forward to learning all I can from them before starting my RMH build.
HTH, Byron
