I'm soon to be moving into my 8X6 polycarbonate greenhouse for a while (it's complicated) and desperately need an efficient way to heat it, I will be building a large pile of leaves against two of the external walls held with chicken wire to aid in insulation and partial heating via microbial action of the decomposing leaves. All Iv'e got access to in terms of building materials is: extremely high clay content soil, Sand and some bricks. If I have to get some refractory cement, stove pipe and insulation for the panels to protect them from the heat then I will do. So with these materials could I make a crude stove/mass heater to heat my greenhouse while I sleep?
You certainly could. In such a small space, you probably don't want a barrel of any kind; a masonry or cob bell would serve just fine. If you don't plan to be using this for the long term, you can build it entirely out of cob (with some scrap wood for forms for the combustion core). There was one built recently using bricks, that you could replicate in mostly cob. tiny house rocket mass heater: the cyclone batch style
That seems to me like a fairly large and time consuming build to generate heat on a temporary basis. RMH's for greenhouses are usually built differently from those for less humid places like houses. I wonder if there are smaller ways to generate some heat for your situation?
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Thank you both for the ideas, I will definitely use the terracotta pot method for the short term to keep the greenhouse above freezing, I already make my own oil lamps from glass jars and 60/40 mix of white spirits to sunflower oil. I like the idea of just using cob/bricks for the mini rocket mass heater, would clay soil hold up used as the core/heat riser or would it just fall to pieces do you think? as long as the exhaust is cool enough I could just use flexible flue pipe out the window on the greenhouse while the mass is heating up, then remove it once the cob is up to temperature. That would be simpler than a permanent stove pipe out the roof.
You could use the flowerpot idea for something to warm your hands by, but it's not magic. It won't heat a whole space, just a small area right next to it... two candlepower is still two candlepower, the pot just keeps more of it near the source instead of dispersing to the whole room where it would be undetectable.
As you say your soil is very clayey, it should make strong and durable cob. The heat of the fire will actually turn the inner parts into pottery, which will last pretty much forever unless you get a serious inferno to melt the inner face (not likely).
You do need a decent chimney for an RMH to work well; flexible duct is too rough for good flow. A few feet of steel duct or stovepipe will work well, and with the low exhaust temperatures can be safe going through the polycarbonate. I would give it some metal flashing just to be safe. The 4" batch box in the link above would only need some 4" duct for a chimney. I posted some pictures of a cob rocket cooking stove I built; you can see how simple it can be. https://permies.com/t/52509/Clay-Rocket-core-Bell-RMH#428887
Iv'e set up a little flowerpot heater and you'd be surprised the difference in temperature it has made I think it would be better with a metal core of washers plus the inside painted black but it'll do for the moment. With it being such a small space it really holds heat well. The link is brilliant thank you, i've got some old pallet wood I could use to emulate what they've created. I'll definitely give just the clay soil a try first to see how it holds up.