"Rocket Mass Heaters" - BYOP

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mhambi

Burning Hunk
Sep 10, 2010
128
UT
I've seen a few mentions of this here... but never a thread dedicated to it. So here we go.

The Concept: http://hubpages.com/hub/Rocket-Mass-Heaters-Efficient-Home-Heating-With-Wood (Poor man's masonry heater? maryjane-inspired house heating bong?)


Here's the deal. I like 'projects'. I like to do it myself. I'm just weird enough, and dumb enough to consider something like this. Anyone else? :)


I've seen lots of these that 'look' absolutely horrific. I've seen ones I wouldn't want to touch with a 10-foot pole, safety wise. But I've also seen a few that were nicely executed. They currently seem to be the domain of ecohippiesâ„¢, so follow the links w/ care... (byop- bring your own patchouli) :lol:

This one is wicked nice!
http://www.flickr.com/photos/onyone/sets/72157622886719373/

I think this one is very nice as well
https://picasaweb.google.com/eawisner/CaliforniaRocketStove2#

I've got a +/- 500sq ft area I want to heat that would be perfect for something like this! Am I nuts?!
 
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The thing is, your insurance company. If it is not UL listed, they might not want to insure.
 
JimboM said:
The thing is, your insurance company. If it is not UL listed, they might not want to insure.


Let's assume for the sake of this thread that I'm able to get permission/permits from the insurance co/county.


Good point to bring up though!
 
I've been following these stoves for several years. They are an interesting concept and can work well. Read up on them if you want to try one out. There have been some failures too, so it would be good to learn from these mistakes instead of repeating them.
 
I don't buy into the efficiency claims at all. The same heat with less than one cord compared to 3-5 cord in most stoves? C'mon, get real. :roll: With some current cat stoves achieving near 90% burn efficiency and having very low flue temps, most of the available chemical energy in the wood is already being released into the room as heat.

The big accomplishment would be to reclaim the latent heat of condensation from all the water vapor that is formed by the combustion process itself. All current designs fail to gather that heat source (which, BTW, is about twice the heat lost in evaporating the water in wood that contains 20% water by weight), so there is no such thing as 100% total efficiency. Both the combustion water and the water evaporated from the wood retain their latent heat until the water finally condenses outside the flue and gives that heat up in the great wide open where you can't use it. In total, about 830 BTUs are always lost for every pound of wood at 20% MC (wet-basis). That's about 13 million BTUs out of a four-cord load of primo hardwood, or about half a cord of wood that gets wasted even in the most efficient units out there.
 
Efficiency is gained not from condensation but from exhaust losses. It's still not going to be magic, but the idea is like a masonry heater- burn fast, hot, wide open, and very efficiently so there's no junk to condense as creosote- and you can really extract a lot more heat from the exhaust gas.
 
True, the efficiency here comes from capturing waste heat heading up the flue that we normally need to prevent creosote accumulation. The principle is similar to a kacheloven or Russian fireplace design.
 
Ya, I really do get how it works. I just think the claims being made are ridiculous:

Less fuel: less than 1 cord of wood compared to 3-5 cords for most woodstoves
OK... now that's a magic stove.

Complete combustion: almost no smoke, no creosote, no pollution, and no wasted fuel
Just like an EPA secondary combustion or cat stove, no?

Captures More Heat: heat-sink traps 90% of the heat indoors, instead of sending it up the chimney
An EPA cat stove has about the same stack loss, the new hybrids may be even less.

Quick radiant heat: during the burn, the stove's metal barrel radiates like a woodstove
No it doesn't, no way

Lasting Comfort: after the fire is out, the masonry mass gives gentle warmth for 12 to 48 hours
So does my masonry chimney, so what?

Heating People, Not Air: Cuddling up to the thermal bench warms people directly, instead of losing heat to air or turning in front of the fire like a rotisserie chicken.
Well, that's a convenient way to heat yourself all winter.

Convenience and Safety: a 2-4 hour daily fire gives all-night warmth; no need to bank a smoldering, smoky fire that could harm you while you sleep.
I don't ever bank a smoldering, smoky fire.

Low cost and carbon-neutral: a small suburban lot can supply its own fuel from yard prunings, eliminating the cost and transportation of firewood, and ensuring sustainable heat.
Yard prunings... right. That'll heat my whole place for the winter, yup. 100,000 BTUs needed to heat my place is a lot of yard prunings

No waste: the fuel is burned completely, leaving white ash rather than charcoal and creosote.
Since when is charcoal considered a waste? And I don't make creosote, do you?

Natural and Recycled Materials: Rocket Mass Heaters can be built from recycled and repurposed materials, local subsoils, and non-toxic amendments like sand and perlite.
Well, I can't claim that one. Except, my stove is reclaimed instead of being sent to the scrap yard.

Nothing new here AFAIC.
 
I stayed in a little cob cottage with one of these that was built by the author of this book about rocket mass heaters. I can attest to the fact that while it was about 45F at night, the inside of the cottage was really cozy and I only burned about a 5 gallon bucket full of fir over 2 nights. The cottage had single pane windows and plenty of visible airgaps. The cottage was also made of cob - a mixture of straw, clay, and sand. This material has a huge thermal mass which stores the warmth of the day and the warmth of the rocket mass heater and releases it slowly through the night.

A couple drawbacks of rocket mass heaters that haven't been mentioned in this thread are that the wood needs to be chopped into small pieces like kindling. Smaller than for a traditional woodstove. The burn times are very short on the order of 20-40 minutes before it needs reloading. Typically it is run for several hours in the evening and then the stored heat lasts through the night. Also they can be tricky to get started because since the flue is snaking around through the house it might not draft well. Starting it involves lighting some paper to get the draft started then slowly adding wood and getting a fire going. Also once its running, the draft is regulated by stuffing wood in the box. This blocks a lot of air and slows down the burn. Care can be taken in the orientation of the wood to get a good burn. So in general I agree that it did seem to be very efficient but it required a lot more attention than just throwing logs in a woodstove.

The barrel does radiate a lot of heat. It's exposed to the uncooled combustion gasses so maybe it doesn't radiate heat "like a woodstove" but it does radiate heat "like a wicked hot barrel".

I think these make a lot of sense for small cob homes because they are easily incorporated into the building materials which are noncombustable. But adding one to a traditional large american house might not be ideal. I think the main advantage is that cob homes and rocket mass heaters can be built for very little money...they just take time and your own labor. Is it more efficient than some $3000 pellet stove, maybe not, but that stove probably costs more than the whole rest of the cob house.

Also the "claims" they are making are probably comparing a rocket mass heater to an old crappy woodstove not a perfect modern one.
 
Yea I have watched a holf dozen youtube vids. I still don't buy into it? How can you heat a house in Canada on one cord of wood a winter? There is only so many BTU's in a cord of wood PERIOD. And then you take the fact that there never 100% efficient, if that were the case you would have to have cold air as exhaust and no steam either. I think even if I had correct insulation and a house half the size there is no way the heat load could be supplied by ythat amount of wood here in the south?
 
I am in the process of building a small greenhouse, plan on using a rocket heater to heat a mass of water in 55gal drums to keep the place warm in the evenings. We shall see how it goes. Hoping to be able to grow veggies all year in Michigan.

In effect I am using a rocket stove like a wood boiler to heat a storage tank. If running the heater for 2-3 hrs will heat 200 gallons of water it might keep the greenhouse warm for a couple of days, warm is above 55 ish. At night. I am going to build a small one and try heating a 55gallon drum of water and see how the temp drops over a day or two as a test. I think I have most of the materials to run. A test or two
 
I built a rocket stove in my detatched garage about a year ago. I wish I had pictures to show but I temporarily dismantles the unit for space reasons. I plan to reconstruct it this coming summer once my garage is finished. Mine did not consist of a thermal bench just for testing purposes. It also did not consist of the vertical feed tube aS this made little sense to me. Just a long horizontal burn tube for a REAL cigar burn. Worked awsome for burning pallets as I could put full four foot pieces in, which saved me from having to cut down. Tons of fun to play with. Even would lay a 4 foot cast iron pipe in the coal bed to provide additional secondary air downstream. Here were my two major observations from my setup.

Thermal efficiency was far superior to any traditional woodstove due to the fact that you can utilize as much heat as you can since it is self drafting and does not rely on heat rising to propel itself. The problem with this is severe condensation. I had water pouring out of my 8 foot horizontal stove pipe being used for exhaust. If you incorporated a thermal bench you had better keep that puppy running 24/7 in order to keep your exhaust temps above 240 or whatever water condenses at. Maybe you could have a drain at the bottom.

My other main concern was with the self drafting theory. The way I understand this is that the exhaust gasses are being "pushed"out of the system by the heat riser pulling in fresh combustion air. I'm not a rocket scientist but I'm under the impression that the exhaust is pressurized, presenting the opportunity for carbon monoxide to creep into the heated space. I only ran mine with sufficient ventilation. If and when I put humpty dumpty back together I will probably have 2 co detectors in the garage.
 
Dutch, the vertical feed allows the stove to downdraft, which does a fair job of self regulating air/fuel mix for a passive system. It is much more efficient with the vertical feed than most horizontal rocket stove configurations. Not saying yours isn't efficient, it's possible to achieve the same efficiency in a horizontal feed with additional air at just the right spot. Your intake pipe may do just that.

The systems typically run with negative pressure, so your concerns over excess CO shouldn't be an issue, any more than any other wood burning device. It is very wise, however, to use good CO monitors in any wood burning installation. I know that's not news to anyone here, just saying I agree you need monitors.
 
I've looked into these a bit, and I had the same reaction to the exaggerated claims, which really only come from a few individuals. There are also some credible engineers doing research on the design as a way of improving third-world cookstoves, and they don't claim any miracles, just better efficiency than open fires.

This forum is the best info source I've found: http://donkey32.proboards.com/ . There is one guy there with experience in masonry heaters, and he posts sketchup drawings of his designs as well as test data from his runs. They confirm some of the problems noted here -- you have to feed it continuously; fuel/air balance is tricky; there's condensation, backdrafting, etc. Still the efficiency is fairly high.
 
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