I have slowly been building my knowledge of RMH. I have noticed that many posts deal with rebuilding them, but I am also aware that this site is full of tinkerers. So, my question may be an impossible one to answer. Once a RMH is in place, how often do they need to be rebuilt, repaired, or whatever term reflects an activity greater than replacing a stove pipe on a conventional wood stove?
Good decisions come from experience. Experience comes from making bad decisions. Mark Twain
Once you build your J tube properly, it will last 7 years plus. Mine did before I replaced it with a batch box. I know of several others from Donkeys that have gone that long as well.
I did rebuild mine 3 times before I got it right...
As these are DYI stoves chances are you will build something off or not sturdy enough or you want to make it better
Lots of reasons to do a rebuild/changes.
I know the whole process seems daunting before you build one. To read about all the rebuilds is quite disheartening!
Have no fear... when your working with clay the ONLY "bad" part is the dust you create. Beyond that rebuilds are super easy. I literally have done them in the dead of winter1 Fast fast fast. Bricks knock apart easy, clay mortar rehydrates in just moments. If the mass is dry and warm you can rebuild a core... cover everything in sopping wet cob and fire that dragon up! It will dry out in a few hours and be 100% in no time.
Batchboxes on the other hand seem to need more care and maintenance. The secondary air tube needs to be built as a quick change or with an RA330 stub on it.
The box itself can be fragile so you must load wood carefully. Unless you buy a Pisla or a Dragon tech door you will be having door issues. Roof's are a big issue that can need repair.
Both my batchboxes will get rebuilds this summer.
Metal box stoves (never heard of a weed stove... lol) will warp, if they have firebricks they will crack , They start out strong but show their weaknesses in a few years.
They also send most of your heat directly out the chimney and they cost sacks full of money.... hmmmm rebuilding sound's not so bad anymore...
My J-tube has an air-cooled steel sleeve with P-channel for the feed, and is in fine shape after five years... I expect it to last another decade if I don't decide to change it to a batch box (would be awkward in my heater). In that time, I expect a minor repair to the hot edge of the P-channel, and maybe some minor patching of the cast refractory feed base walls. The perlite/clay heat riser may need replacing someday, an easy job if it does. The rest of the masonry and cob bell I expect to last centuries if the house lasts.
Rocket Mass Heater Plans: Annex 6" L-shaped Bench by Ernie and Erica