I'm in planning stage of my RMH stove. My question: The duct work that is routed inside the mass, is there a limit on that length? What I want to do is run the duct work along the inside of the exterior wall. This distance is ~ 25' (duct work is double that amount. Leaving burn pot, out, and then returning). Is there a certain cut off where the distance is too much for it not to work correctly? Reason for my desired length is to maximize the mass heater area.
You need the book (Rocket Mass Heaters - by Ianto Evans and Leslie Jackson) for the basic correct sizing information and design considerations. Ernie and Erica Wisner have a forthcoming book, the Rocket Mass Heater Builder's Guide, that will give even more specific details and dimensions proven to work.
This forum and the donkey32.proboards.com forum have lots of good information if you know how to search. Be very wary of youtube videos - the bad ones seem to outnumber the good ones, and very few will come back to explain how their radical new idea for steel combustion chambers failed in a few months.
You will have to excuse people on forums when they are asked a question that gets asked 1000 times before. I just typed in ducting length in the search and found some related posts. Read Erica Wiesner's post in this thread it address's this issue well.
"Duct is a word for pipes, normally ducting is used for air while 'stovepipe' is for hot smoke. 'Pipe' is mostly used for water pipe, but also for stovepipe or duct.
Because our exhaust (smoke) is not hot and not very smoky, we often use galvanized air ducting for the bench instead of stovepipe.
Either one will work, and stovepipe will impress people more (and probably last longer, though neither has failed yet except in constant damp conditions).
We recommend stovepipe on the loose-fill "portable" design because the loose fill does not guarantee a seal if the pipe does fail.
Elbows cause the exhaust to slow down, and too many can make it so the exhaust does not move through correctly. Therefore we start with a length of 40 in a straight line. With 1 elbow fully bent to 90 degrees, subtract 5 feet: 35 feet for an L-shaped bench with one bend. 30 feet for a U-turn, and so on.
And the same problem for me to understand what to do with vertical chimney. Earnie is talking some seconds later. But I do not understand what he wanted to explain.
15 feet vertical chimney... you can overcome??...10 feet of drag/draft??
If you have a tall, warm vertical chimney, you can go a bit longer. We add maybe 5 to 10 feet of horizontal length for each 15 feet of vertical chimney, depending on how much hotter it is than outside air. We usually work to about 100 F (40 C) at the exit if we can get it.
For one example:
We have a bench with 19 feet of pipe and 5 bends, plus a 15-foot chimney. The bench surface temperature is about 90 F (35 C) just after firing in the evening, down to 75 F (23 C) when we start the fire the next evening.
The chimney stays about 115 F (45 C) on the metal surface, and our weather in winter is usually 0 to 30 F (-15 to 0 C), occasionally colder. Works well for us, and I think it would still work with another 10 feet of pipe.
For another example: A bench with 32 feet of horizontal pipe, 2 1/2 bends, and a 20-foot chimney, operated 8 hours per day in a climate with winters between 30-50 F (0-10 C). Works well for its owner, but can be a little slower to start than ours if the bench is cold. " Erica Wiesner
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