Rocket Mass Heater in a yurt

[wrdanner]

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]

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]

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]

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.

[Zelig]

[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.

[wrdanner]

Originally Posted by hierony

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

Everything is open for consideration at this point. The yurt will be on the coast of North Carolina so I'm trying to figure out how to deal with the humidity and hurricanes. I'm considering building the yurt myself. Having no air underneath the yurt would be the most sound way to approach the hurricane issue but that also makes it more difficult should we require HVAC to deal with the hot summers. There will be some shade where we're building but I just don't know how uncomfortable the yurt will get without it. There will be a garage attached to the yurt and the kitchen/bathroom are in there. The thought was to have HVAC for the kitchen/bathroom and then if we found it was necessary, run ducting under the yurt. This requires the yurt to be built off the ground though.

The bedroom will be in the yurt so it needs to be fairly comfortable in a region where the overnight lows can stay in the low 70's with high humidity. It's possible we could go with a window unit in a glass window in the bedroom but it's all speculation at the moment. I've been trying to learn as much as I can to figure out what's the best way to approach it.

If the yurt was built on the ground, a rocket mass heater would give the yurt thermal mass. We would actually consider putting two of them in, on each side. I've looked at them on permies.com and the idea seems really cool, plus you get a seat or something else out of the surface if you do it right. This would alleviate my concern of constantly having to keep a fire running full tilt, unless we chose to add traditional insulation to the walls and then cover the studding to help with heat loss. The added mass will also help with hurricanes. In a hurricane, I'm less concerned with the yurt coming apart than the whole structure trying to blow away. I have no experience with this other than people's reports of them holding up well because the shape of the yurt causes the wind to push it down into the ground. I think the biggest issue here would be a lateral, sheering movement. Obviously the yurt would have to be well anchored no matter what we do but adding mass would help. Of course all that mass radiating heat in the winter also means mass that has to be cooled in the summer, unless the rocket mass heater was portable. I've seen some cool designs for portable ones as well.

[Bob Rowlands]

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.

[hierony]

Zelig,

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...

[Zelig]

Originally Posted by hierony

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.

[Marshall Eppley]

There is a lot of videos on YouTube on bio mass rocket stoves kinda wondering how it would work out. Portability would go right out the window though.