Hi Benjamin,
Welcome to Permies!
Your question about a
RMH for a small greenhouse is going to take you on a journey. Just because you only need to heat a small space does not mean you can just have a tiny RMH. How easily the stove will burn is partly dictated by the diameter of the ducting the post combustion gasses must flow through. The smaller you make it the less forgiving the whole thing will be.
So, I understand you asked about the size of the barrel. I think the 55 gallon size is easier to work with, but you probably don't have room for it. I had to dig about a food below the floor of my greenhouse to get a smaller barrel in below the (flamable material) ceiling and roof. It is not ideal. I'm planning to build an extension on the south wall, just so I can build a taller ceiling and accommodate the larger barrel.
The amount of turbulence in the combustion areas, the draw, have to generate
enough single direction "push" to get the gasses through the horizontal ducting and out... this equals larger cross section combustion chamber and ducting.
I think people have built stoves with diameters of combustion and risers as small as 4 inches, but if this is your first RMH, that might turn out to be very frustrating! I've rebuilt my RMH in my greenhouse 3 times, it burns ok, and I am glad to have it, but it is very temperamental.
Do you have any references to consult, like Evans and Jackson's book on RMHs or Paul Wheaton and the Wiseners DVD set, so that you have a clear sense of how these variables fit together?
There is going to be a RMH and general rocket stoves extravaganza starting next week at Wheaton Labs. It is a great opportunity to learn a lot from people with lots of
experience. Check and see if there are still spaces! You won't be sorry!
Paul Wheaton had a kickstarter last spring for a more complete reference for RMH building... it is going to be released later this fall (I think), and I'm sure there will be a way for you to purchase the DVD or the downloadable streaming version.
There are also threads here on the forums and you tubes to watch. Here is a great place to start
http://www.richsoil.com/rocket-stove-mass-heater.jsp
I don't doubt that there is a
rocket stove application that will help heat your greenhouse, but I think you have some studying to do.
Best luck. Have fun.
Thekla
