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wrdanner 06-08-2015 05:03 PM

Rocket Mass Heater in a yurt
Has anyone here used a Rocket Mass Heater (Stove) inside their yurt? I'm looking to build a 30' yurt in the near future and it looks like it will require a serious heat source, bigger than I expected. I would think a Rocket Mass Heater would help by retaining heat in the dirt/other dense material after burning.

We're building our yurt attached to a shed/garage (which has the bathroom and laundry) so we're also considering traditional HVAC using gas heat but based on the things I'm seeing on the forum it sounds like it would require a large unit and would use lots of gas.

We're building in southern North Carolina, on the coast.

Jafo 06-08-2015 05:52 PM

Re: Rocket Mass Heater in a yurt
I have no experience with them, but I do understand they are very heavy. You will want to make sure your foundation can support it. They are also labor intensive.

I seem to recall Steve at Surely Yurts had some experience with them, he may chime in.

Bob Rowlands 06-09-2015 09:03 AM

Re: Rocket Mass Heater in a yurt
I'm unfamiliar with rocket mass heater. If there's dirt or rammed earth in them I'm guessing that's for affordability. Going off on a $$$ tangent here, Fine Homebuilding hardbacks from the 1980s have several good articles about masonry fireplaces. All require a substantial concrete foundation. A void in the yurts floor framing would be required.

A traditional gas fired forced air system could heat or cool your yurt pronto, since a yurt of 30' is 706 sq. ft. There's no flywheel effect like with a masonry heater. Fast to heat or cool at your fingertip. Less costly. Every heating outfit knows them inside out. Any wood yurt foundation will support the furnace and supply ducts.

As far as fuel economy, that would totally depend on how well insulated and sealed your yurt is. No yurt approaches a conventional 2x4 framed house for holding heat or cool. But the heated/cooled cubic footage of a 30' yurt is half that of even a 1400 sq ft house, so to me it would be a wash.

Good luck.

hierony 06-09-2015 03:26 PM

Re: Rocket Mass Heater in a yurt
I haven't gotten there yet, but I plan on making a sort of masonry heater for my portable 20' yurt. First I need to finish my insulation and layering and get my platform moved and seal everything up though. I have a cheap adjustable 25k btu/hr propane heater in the mean time to test how much heating power I actually need--yurts can work _very_ differently from traditional homes in terms of heating, throwing off estimates. My cost estimate so far is ~$2k for a masonry heater with design by professionals/construction by myself, for what it's worth.

There's a good number of ways to build masonry heaters (varies by country/region of origin, air movement principles)--the RMH are tempting for their DIY nature and typical lack of engineering. But if you keep a sharp ear on what people say during builds, they require a good bit of tuning to get working just right & efficiently. Partnering with someone knowledgeable about them & air flow dynamics, and hopefully local, would probably be a good idea.

Like Jafo says, make sure your foundation/platform can take it--with a basic heat lose estimate, you can fairly easily estimate the required thermal mass based on frequency and wood mass of burns. Standard wood decks should hold 40 psf live load--a 5' square could hold 1000 lbs by that standard. A ballpark weight is probably 1000-5000 lbs for a small/medium homemade heater.

A note on insulation: 2x4's have an r-value of ~4, windows from 1-4. Insulation gets cut out for electrical boxes, pipes, etc. Plus nails (metal is a good conductor) and imperfect installation. So you're actual total r-value of the whole wall assembly is not that of the insulation (although it can be close). A yurt doesn't have this problem--it uses a layer of insulation with no interruptions from studs or nails or framing. Mind you, the materials are also usually thinner (except when the mongolians use 5 layers of 1" thick felt during winter...). Yurts also have negligible thermal mass compared to most other construction techniques.

Hopefully Steve will have some experience/info to add.

What are you making your platform out of? Are you building or buying your yurt? How do you plan to insulate it?

Hope that helps a bit.

Bob Rowlands 06-09-2015 07:54 PM

Re: Rocket Mass Heater in a yurt
Last Friday I hung the entry door on the next custom home I'm going to trim. The insulators were in there. Nowdays EVERY gap to the outside of the house gets a bead of foam. I mean every gap. At ganged studs, every seam between the studs. Plate lines between double plate and top plate. Around windows doors etc. Add R19 insulation (R13 in a 2x4 wall), R38 in attic, tyvek, sheetrock, texture and paint, and you have an R package and 'tightness' factor that exceeds a trad yurt by quite a margin. No yurt has 3.5" fiberglass batts on it, that I know of. As for many layers of felt, I'm unfamiliar with that. Just sayin.

Zelig 06-11-2015 03:45 PM

Re: Rocket Mass Heater in a yurt
[QUOTE=hierony;5409] (except when the mongolians use 5 layers of 1" thick felt during winter...). QUOTE]

Can you elaborate on where you heard how much felt the Mongolians use? We've got two 1/2" layers of felt for our yet-to-be-erected yurt and you're making me nervous. Mind you, I was somewhat nervous to begin with.

hierony 06-11-2015 06:34 PM

Re: Rocket Mass Heater in a yurt

I did a quick internet search, and everything I found was vague in terms of felt thickness. I have however read that some people's stay-overs in mongolia have been in uninsulated yurts in which one could find many direct holes to the cold outsides :( Who has what quality/quantity of insulation is understandably variable.

I need to double check, but I believe that comes from Paul King's book, which is in a stack of books somewhere... A quick summary of what I remember off hand: the mongolians, being nomadic shepherds, had lots of sheep and thus wool to make lots of felt. Not being farmers, they didn't have cotton for canvas/liners. They'd use the felt, layering up during the winter to hold in more heat/protect against the winds. I think he said the felt was 1" thick and they'd use 5 or more layers during the coldest times, with some being older felts. But I suspect the 1" thickness may be slightly exaggerated/inconsistent. With a cotton cover, canvas, maybe some tyvek, and your two layers of 1/2" felt you should be well ahead of them.

One thing to consider: you can seal up your yurt a bit better than old school traditional yurts. They have a crown ring in part to allow smoke from an open fire to escape--you get to use stove pipe and not have a giant opening in your roof. Also, your fuel (wood?) is up to two times more energy dense than dried camel/pony/sheep dung...

Bob Rowlands 06-11-2015 07:20 PM

Re: Rocket Mass Heater in a yurt
Also, your fuel (wood?) is up to two times more energy dense than dried camel/pony/sheep dung...

lol Now I really have heard it all! :D

thebitmaster 06-12-2015 12:05 AM

Re: Rocket Mass Heater in a yurt
The most modern RMH info is coming through Paul Wheaton's Permaculture Podcast, and you can get to info on them here:

rocket stove mass heater

The most advanced ones have almost no particulate exhaust and are getting upwards of 20x as much efficiency as a wood stove. They are also super fussy. There are simpler ones that are still the most efficient way to heat a home. There are also several episodes of the podcast where they geek out on the details for, literally, hours. I'm almost disappointed that I live in Texas, where having one would be useless. :)

Zelig 06-12-2015 03:13 PM

Re: Rocket Mass Heater in a yurt

Originally Posted by hierony (Post 5423)
I need to double check, but I believe that comes from Paul King's book.

I have Becky Kemery's book and all I found in there was reference to "sometimes several layers" of felt. No mention of thickness or number of layers.

I do believe that my insulated chimney, lexan toono covering and house wrap will give us some advantage over old school yurts as you mentioned, but I'm wondering what the btu rating of dried dog turds might be. Perhaps birch would be a better bet.

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