I love the concept of RMH's, but I'm concerned that without real walls to hold the heat, it may not be enough raw heat to keep us comfortable. We are getting a 30' yurt because its quick and easy and cool looking, but the only insulation is "double bubble" foil on both sides with air bubbles inbetween and its a comparatively low R-value. From what I'm reading it sounds like you have to keep the stove going or it gets cold pretty fast. That's why the RMH would be great, but do you think it will do well in a yurt? Maybe we should have both? One for long lasting heat and one for big immediate heat. Keep in mind a yurt is 14' in the center, but I am going to have a PV system that powers a fan for circulation.
Also, has anyone here gone from using a woodstove FULL TIME in the winter to using a RMH FULL TIME? I'm concerned that having to constantly feed it will get old...I'm so used to stuffing a wood stove with oak logs and coming back in 4-6 hrs....thoughts?
I don't have any experience with a rocket mass heater, but I do have a few points to share that should be useful. If you are using that foil bubble insulation with the foil as your interior wall surface, then that should be extremely beneficial with the rocket mass heater. The idea is to warm the mass up so it radiates heat for long periods of time. Aluminum is claimed to be able to reflect over 97% of radiant heat, as it is the third most reflective element in the visible spectrum and the most reflective element of UV and IR light - IR being radiant heat. This reminds me of the setup I saw on Dual Survival where they used a reflective blanket behind them and clear plastic facing the fire so light could come in but the air was trapped and the radiant heat gets to make 2 passes. Having the radiant heat reflect back into the space with a large mass radiating heat for a long time should keep it quite cozy.
As far as feeding the fire, it depends on the size of the RMH. A larger diameter system will have an equally larger burn chamber. I have built small prototype stoves for cooking using soup cans which needed to be fed every 5 minutes or it would go out. Of course pulling up a chair and cooking and feeding the stove wasn't a big deal in that instance. I think the idea is to figure out a time of day where you are busy inside or near your yurt working, reading, unwinding or similar where you can refill as needed until it is sufficiently hot, and then do that as often as needed, which could be every day or 2. For a mass large enough to heat that space the diameter of the system would be far larger than my little prototype and thus would have an equally increased time before needing to be reloaded. You should be able to easily work it in to a routine where you do a certain daily task and walk by the RMH every so often to fill it up.
The barrel of the RMH is designed to give off radiant heat in the short term, long before the mass heats up. This should negate the need for a second heater, although if you already posses one and have room for it then it wouldn't hurt to keep it around. I'm guessing you would have different areas inside for living and sleeping. You could use a radiant shield to put near the barrel to reflect heat towards a particular area if you are away for more than a day where the mass has gotten cold. This could be as simple as a sheet of steel or aluminum, something non combustible. I could see someone reusing an old car hood. I imagine having the barrel positioned where you want immediate heat would be an important part for planning out a house, with the yurt being more configurable than a standard house this should be easy to figure out.
My stove did have some mass added to it which retained heat for quite a while. I also have a metal melting furnace that stays warm for hours after a melt, so I think a properly built RMH would stay warm for a very long time. Insulating the mass from the ground beneath it would be likely be critical to avoid losing heat to the earth. Having some sort of floor such as cedar planks would keep air, objects, and people from losing heat to the floor. Your location geographically as well as micro climate will probably dictate how successful this would work.
You can build a Rocket Mass Heater (or any other type of masonry heater) in any size you need.
You can scale up and down the amount of wood it will consume per hour. The amount of wood consumed equals the amount of heat produced. If you need more heat per hour, build a bigger heater and consumer more wood per hour.
And separately, you can scale up and down the mass of masonry you're heating. More masonry changes temperature slower. Less masonry changes temperature faster. "Changes temperature" in both directions, remember. When a large mass is hot, it stays hot a while. No matter how cold the yurt is. But the flip side, obviously, is that a large mass stays cold when it's cold. You've got to burn and burn and burn to get it up to temperature.
It's just a matter of tradeoffs.
Your next question should be, "Somebody help me SIZE a RMH for my yurt." :)
We live in a 30' yurt (from Pacific Yurts) in the Kootenays in BC, Canada. For the first two winters we used a Pacific Energy wood stove. The down sides were that it was harder to regulate the temperature (it got too hot at times, and heated inconsistently throughout the yurt w/ hot and cold pockets) and once the fire went out, it got cold very rapidly. The up sides were that it heated the place quickly and it was nice to watch the fire through the window on a cold winter's night.
Last September we installed a Rocket Mass Heater and moved the wood stove out into the storage shed. Unfortunately, we have had some design issues (if you decide to build one, make sure you either research it really thoroughly or consult w/ people like Erica and Ernie Wisner, who really understand RMH's). Regardless, we still really love our RMH for different reasons than we do the wood stove. We love the rustic look of the RMH (we built cob stairs in ours leading up to a loft we constructed) and that it lets off a nice, steady heat over a longer period of time. It is great for rising bread or making yogurt or kombucha. It is wonderful to get up on a cold morning and "cuddle up" to the RMH... our cats love it too for that reason. But we found that in a yurt we have to feed it pretty constantly to keep things warm enough, and this also presents a challenge when we went out for the day/evening and had to come home to a very cold place. Also, while our wood consumption is perhaps less than w/ the regular wood stove, it is not as much less as we had hoped for (it might be different in an insulated structure). But perhaps the greatest drawback is that, with the RMH alone, it takes a very long time to heat the place up (3 or 4 hours) on a cold morning, so we want to either stay in bed or stay huddled on the RMH... forget facing the world! So midway through the winter we ended up buying a lovely kerosene heater to compliment the RMH and heat the place up quickly. The two work quite nicely in tandem, but kerosene is definitely a little more costly to run.
Just today, in preparation for the coming winter, we moved our old wood stove back in, and we are looking forward to having it up and running again. As the temps have been starting to drop, we have not yet been running the RMH due to the "design issues" previously mentioned (the Wisners have since given us some remediation suggestions which we will attempt to implement before winter hits). We hope, however, to use the two technologies together, and I suspect that, the combination will be the best that we can achieve in a yurt.
My best guess is that for someone looking for the most comfortable and efficient way to heat a yurt (assuming you can afford it), either a very slow burning Blaze King Princess (though I cannot claim to have ever used one), or the combination of a wood stove and a RMH, is the way to go.
It sounds like the RMH you built was not sized quite large enough for your load. It may be useful data to know what size your system is, and how much/what kind of mass/ducting etc. That with your climate could help other people size RMH systems better for their yurts.
Since a standard RMH includes a large barrel surface for instant heat while burning, a larger sized RMH would probably give the best results for you, with only one hot object taking up space instead of two. Obviously that is not likely to be practical in your case.
"Keep in mind a yurt is 14' in the center, but I am going to have a PV system that powers a fan for circulation."
Circulating hot air from the ceiling to the floor is a worthy idea, but if you put a fan in open air, it will serve as more cooling than heating, as moving air has a cooling effect. What you would want is a fan that draws hot air from the ceiling and delivers it directly to the floor level through a pipe or duct. I have seen a simple commercial product that does this, and if you are handy, you can easily build one yourself.
Here's a photo of our RMH, which is still a work in progress - one of the things that is great about these systems is they are relatively easy to alter after the fact. We still need to build up some more mass around the base of the barrel (E & E suggested about 1/3 of the way up), do finish coats, etc. But it is quite large, as you can see. While it does take up space, it serves as not only as a seat, but also stairs and wood storage (out of view on the other side). Under the loft we built in quite a bit of storage for clothing and kitchen items, along with a small bathroom (sure made us aware how important it is to have solid wall to be able to construct shelving, etc). So we don't mind the footprint, as it is so functional.
As for our winter climate... it can get as cold as 20 below during the really cold snaps, but - 10 is more typical.
Our ducting is 6". The mass is rock and cob. We do have an overhead ceiling fan that can push and pull air, as well as the smaller stove fan, but the pipe system you mention sounds like a better setup, which I am curious to know more about.
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