This is a guest post by Cassie Langstraat.
When I first heard about rocket mass heaters I was kind of skeptical. I read that they could eliminate something like 80-90% of the wood of a conventional stove and I thought, “Bah! That seems like a pretty unreasonable claim to me.” So I put on my investigative pants and got pretty obsessed.
One of the first articles that I came across was Paul Wheaton's article on rocket mass heaters. He does a good job of breaking down the technology in a way that anyone can understand. So, I am going to relay some of his information as well as some other details I scrounged up! I want to tell you about several of the reasons why rocket mass heating is so important. Let’s jump in.
#1 – Rocket mass heaters are way different from conventional wood stoves.
They're more like super-efficient and super-comfortable masonry heaters, except that people are building them for a LOT less money. How do they do achieve this super efficiency you ask? Well, it has to do with the innards of the rocket mass heater. First, the sticks are put into the wood feed vertically. They are standing straight up. Then, only the bottoms of the sticks burn, and the fire burns sideways!
How does the fire burn sideways? Well, because the heat riser in the barrel is so insanely hot, it creates a convective current that pulls the fire sideways into it. Next, the thermosiphon draft pulls the smoke from the fire up into the riser and into the barrel. The sharp turn mixes that smoke with fire so it burns up the smoke! Thus, only leaving a tiny tiny bit of CO2! It’s nearly purely steam. This process is illustrated in the photo above, from Paul Wheaton's article.
#2 – Heating with a rocket mass heater is way cheaper and cleaner than using natural gas.
This one surprised me, because natural gas is supposed to burn really clean. I didn't know wood COULD burn that clean. It can, if you get the fire hot enough; wood can burn completely down to steam and CO2 just like natural gas does, with a little bit of mineral ash left over.
But if you consider the environmental footprint of extracting natural gas (like frakking), and refining it, and the miles of pipes or delivery tanks that have to be in place for you to use it, then harvesting local dead wood starts to look pretty clean by comparison.
A big part of these rocket mass heaters being so efficient is the actual “mass” part of them. There are several types of mass that have been experimented with, but the most common mass is made out of cob. (Cob is a natural building material made from sand, clay, water, some kind of fibrous or organic material (straw) and earth.)
When the exhaust runs through the cob mass, the mass absorbs the extremely high heat. Then it slowly releases the heat over the course of a few days. This enables the rocket mass heaters to continue heating a space for quite a while after the fire has stopped. This drastically reduces the amount of wood needed to sustain these bad boys. Some people have heated their home for years at a time using only dead yard debris and scrap wood, eliminating over $150/month in natural gas bills.
#3 – They are super comfy to sit on, like a full body heating pad.
Did you ever crouch over the heating vent in your nightie or pajamas, trying to get warm enough to get dressed? Good way to be late to school wasn't it? Moving hot air around is not an efficient way to heat things – air doesn't hold a lot of heat, plus it causes evaporation which makes you colder. Forced-air heating is pretty inefficient, the only reason we do it is that it's relatively easy to control with thermostats and vents, and not as dangerous as steam heat.
These rocket mass heaters are more efficient like steam heat, but way less dangerous – the surface temperature is just warm to the touch. They are usually sculpted into heated seats or beds, so if you need a warm-up boost you just curl up and tuck your toes under the cushions.
Over-worked friends just kinda melt onto the bench when they come over for dinner.
#4 – Rocket mass heaters can be completely DIY.
These rocket mass heaters can most definitely be built by you and yourself alone. Ernie and Erica have taught tons and tons of workshops on how to build them. Respected teachers all over the world have, too! This is not to say that is is 100% simple or fool proof. You definitely need to do your research and especially need to figure out which kind of rocket mass heater is best for your situation. However, with research and careful planning, you can definitely make one by yourself and manage to not spend very much money.
Building Process for Cob Style Rocket Mass Heaters
#5 – You keep far more heat inside your house with a rocket mass heater than with a conventional wood stove.
A conventional wood stove uses a lot of the heat to push the smoke out of the house. With a rocket mass heater, the heat gets absorbed into the mass, and extracts as much heat as possible before the exhaust leaves the house.
#6 – People are living comfortably in a rocket mass heated tipi through Montana winters.
Like I said, up at Paul Wheaton’s land there is a ton of experimentation going on. One of the coolest rocket mass heater innovations he has is the one that is inside of the tipi. It is a traditional cob style rocket mass heater that heats a completely uninsulated canvas tipi. Each of the people that have lived in it have reported utmost comfortability. This is huge because Montana gets down to -25 degrees Fahrenheit in the winter. I actually visited Paul’s land in December and got to hang out in the tipi and it was completely warm and toasty in there!
Warm and comfortable in a canvas tipi in the middle of winter.
#7 – Rocket mass heaters use old technology with a new twist.
This taps into what I was saying earlier about the heat getting absorbed into the mass. What it does is mix modern science with knowledge from hundreds of years ago. Burn the smoke and capture heat from the exhaust while focusing on utilizing the more efficient forms of heat: radiant and conductive heat are favored over convective heat.
The new twist is using modern materials (barrels, ducting) to make the building process cheaper and easier, and also making the heaters more effective at getting the heat where people want it: right where they can sit on it and tuck their cold toes under the cushions.
- Convection is heat transfer via moving air or water, which is how most of our standard heating systems work.
- Conduction happens when two surfaces touch each other – such as your body sitting on the warm stove exhaust.
- Radiant heat transfer happens when warmth radiates from a hot surface to strike another surface. It doesn't require air movement. Examples include warmth radiating from a fire or from the sun.
#8 – There are many different kinds of rocket mass heaters.
The most popular design for a rocket mass heater is a 6” Annex Variation but there are also so many different designs. Here are a few:
- Daybed Bypass Variation
- Bonny Convection Bench Variation
- Batchbox Variation
- Cob Style
- Pebble Style
- Wood box Style Shippable Cores
- Small Molded Shippable Cores
- Rocket Mass Heater that doubles as a BBQ, cooker, and smoker
- Rocket Stove with a griddle, oven, and hot water
- Batch Box Variation with heat directed into the floor
Bonny Convection Style Rocket Mass Heater. Image courtesy of Village Video DVD, How to Build Rocket Mass Heaters with Ernie and Erica
As you can see, these rocket mass heaters provide a multitude of opportunities for heating your home in a clean, efficient, cheap way. They also give us the massive opportunity to do something and build something that can save the environment we live in!
But it can be challenging to build a rocket mass heater without seeing it in front of you. This is why Paul Wheaton and Ernie and Erica Wisner are creating a new DVD set that shows the ins and outs of building a bunch of different types.
The DVDs will cover cob style rocket mass heaters, pebble style rocket mass heaters, shippable cores, and even the Rocket Mass Heater Innovators Event Paul hosted out at his land. As more and more people learn how simple, efficient, and eco-friendly the rocket mass heaters are, the faster the world will begin moving – scratch that – sprinting in the right direction towards saving this beautiful earth we inhabit.
To get a quick look at a rocket mass heater and “meet” Paul, Ernie and Erica, you can watch the video below:
You may also enjoy:
- Solar Water Heating Basics
- Passive Solar Heating Basics
- 5 Things You Need to Know Before You Buy a Wood Burning Stove
Cassie Langstraat is a writer who grew up under the big blue skies of Montana. She recently got her B.A. in English from The University of Montana and has headed out west to try out California life. She is a very passionate person who has a fire in her soul for all things regarding literature, permaculture, and feminism. When she isn’t vehemently writing or poking around in her garden, she enjoys reading heady philosophical novels, going fishing in the dark, and eating cheeses with names she can’t pronounce.
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Mark Stephenson says
I’m quite familiar with rocket stoves and variations on that theme because I’ve built several. I just wanted to let you know how much I appreciate the this clearly written piece on the subject. Well done.
Mark
Laurie Neverman says
Glad you approve.
Jerry Garber says
in a RM heater: How is burn rate controlled? By the amount of wood burning at the time?
modern wood stoves like Vermont Casting add secondary air which completes combustion without
increasing burn rate. On RM how is this handled?
Laurie Neverman says
With our masonry stove (which is similar), there is no controlling the burn. The goal is to burn as hot and fast as possible, and then close down the chimney and fresh air feed, trapping the heat in the masonry. The masonry then slowly emits the heat over time. Ours stays warm for over 24 hours, but we burn twice per day in cold weather to boost the output as high as possible. It’s not recommended that we burn any more than this, as there is only so much thermal capacity in the masonry.
Jerry G says
I’ve built serveral waste oil burners and know how quickly a steel unit burns out
and I’ve checked the max temp at 1050 max using my infrared thermometer.
and had to replace it yearly.
I know that 50 gal barrel going to burn thru quickly as well. But it appears none
has used the designed long emough to get a feed back on what problems lay ahead.
Laurie Neverman says
I’m with you. I know plenty of people use 50 gallon drums for burn barrels, and eventually those fall apart, and conditions in a burn barrel are a lot cooler than a rocket stove. I offer space for alternatives here on the site, because I know a lot of people want less expensive DIY options. In our home, we went with a refractory cement masonry heater kit with brick facade, and there has still been settling and decreased burn efficiency over the years. (Our chimney guy has been working to find and seal all air leaks each year when he comes out.)
Jerry G says
I’m seeing pictures of RM with barrel matted in with COB
And I’ve noted few recent posts. Makes me wonder if there’re not
many interested in RM because it’s not insurable or meeting building regulations.
I got a plumber who heats his shop with a 12in dia infra red tube running along 2.5 ft under
the roof level but above all else under it.
I like that idea as an alternative and forget the mass feeding continually with pellets.
but using the burn chamber.
Laurie Neverman says
A lot of alternative building techniques have a tough time meeting code/getting insurance. Of course, many willing to build that way are also inclined not to have insurance/be off grid/unregulated.
The IR tube transfers heat well to the rest of the building? It doesn’t just give a hot spot near the ceiling?
Juri says
What about carbon monoxide? Is that an issue in a well insulated home? I guess you safe a lot of fuel, but its still an open fire in your home.
Has anyone experience with this issue?
Rudy says
There is no carbon monoxide able to get in your home with a properly built mass rocket heater. The exhaust is forced out of the home by design. For technical details watch some of the youtube videos. Thx
Laurie Neverman says
As Rudy notes, the combustion gases should remain contained within the unit until they exit the dwelling. Here’s a demonstration:
tom curry says
I have one in my home. You must remember that carbon monoxide is flammable to when heated in these things so it gets burned up
Pat says
Is there any noise generated by a RMH? I’ve never actually seen one in operation, and heard that the strong draft is what gives the “rocket” in the name…
Laurie Neverman says
I don’t have a standard rocket mass heater, but I do have a masonry stove, which is similar, and it’s no noisier than a normal fire.
Dan Keith says
The sound is hard to describe. It’s not exactly noisy, it’s just the sound or air being pulled into the pipe, and the burning wood crackling, which in the case of my BBQ grill is a six inch pipe. You could almost compare the sound to a woodstove door left wide open while the fire is burning inside it, but as the fire area of the “J” is so much smaller, the sound is less than that. Also less noisy than say, an open fireplace, again because the fire is much smaller.
Laurie Neverman says
Thanks, Dan.
Judy L Irby says
Hi Laurie,
Do you know any one can
build me a rocket mass heater from start to finish.
I am so excited about the
concept come with the
bench. I live in Colorado.
Thank you!
[WORDPRESS HASHCASH] The poster sent us ‘0 which is not a hashcash value.
Laurie Neverman says
I’m a novice in the rocket mass community, but hopefully some others who are watching this comment thread may have some suggestions.
Eric Olson says
Did you ever build your RMH? I live in Black Forest and am curious. Thx!
Dan Keith says
Great article. I haven’t made a heater yet, but have used the “J” to make a great BBQ grill. It’s a “J” made of black sheet iron stovepipe with a BBQ grill at the top of the J supported by a stand, and it’s inside a woodshed that is covered by a roof but is mostly open to the outside air. I use kindling sized wood and dried plant stalks as fuel and grill every Sunday for me and my GF. The unit works great. Getting it to draft is tricky sometimes until the “rocket effect” starts, then it’s great from then on.
Laurie Neverman says
We have a similar effect in our Tempcast masonry stove. The draw isn’t terribly strong until it gets up to temp.
Jerry says
I’ve seen RMS idea applied to greenhouses.
Why not move that heavy COB mass to the basement and take the heavy load off the floor?
You could raise the heated mass to 160-200F reducing the mass. And draw off that mass as needed.
Use water instead. set up like a hot water boiler system?
because. 1cu/ft concrete 1 degF stores 23 btu wear as 1 cu/fi water weights 62 lbls/cu ft giving 2.7 times more
per cu ft.
500 gal insulated water tank x 8.34 x 90deg could allow 15000btu/hr for a whole day.
OR
use (2) 275 gal fuel oil tanks.
Plus use a power vent system if needed. Still the RM heating part to get a clean burn.
I think the most important part is to burn all the fuel completely before any heat is passed to a heat exchanger.
Jerry says
Moreover
Chimneys depend on temp difference from top to bottom for a draw.
I prefer a stainless steel ceramic wool wrapped. min mass to warm up.
I don’t know min temp needed for a good draw but I’m searching.
one problem is getting a chinney draw with a cold mass. Some have used a temp bypass
in their flue. i think it could be setup with a bimetal coil to automaticly control min flue temp.
Using (2) 275 tanks could make it a gravity heat flow. But the tanks would have to be modified
with heat tubes welded in to heat the water.
Scott Bonner says
I have been looking at RMH for some time. We recently moved into a house with an brick fireplace that has what I am guessing is a mass heater on the side. At one point the fireplace had a gas insert installed. I am wondering if this fireplace could be converted to a RMH. Can anyone lead me in the right direction?
Laurie Neverman says
I’d tried inquiring with a local heating contractor to identify what’s going on with your unit, or searching for rocket mass groups in your area. Offhand, that doesn’t sound like an RMH setup, but it’s impossible to say without being able to look the whole thing over in person.
jason says
i love the idea of building a rmh in the cabin i’m starting but i don’t have room to build a bench like most of these show do you have any sugestions?
Rowan says
You could use it for radiant heat flooring.
Kelly says
There are variants that use a masonry (brick) “bell” which is basically a hollow area in which the hot air stratifies with its exhaust at the bottom of the bell. These can be built vertically and have a footprint of approx. 2.5 x 3.5 foot and about 4-5 foot tall. It’s like standing the bench on end. Care should be taken in the area of initial exhaust area using firebrick. Internal routing of the hot air inside the bell would first go through the firebrick/refractory cement area and be cooled enough so that common red clay brick (not cement brick) will be used for the outer walls of the bell.
There are batch style RMH burn boxes that take less fueling and wood prep compared to a J or L type
Cathy says
I am investigating turning a small horse lorry into a tiny house, and wondering if it would be possible to incorporate a small version of a Rocket Stove fur cooking and heating such a small area, without using the cob?
Richard says
The trouble with what you propose is that to cook on a rocket stove without the oil barrel is that one normally would cook over the heat riser and you would end up with a smokey truck. If you had the barrel over the riser you could cook on the top of it and you would need to vent the exhaust from the base and take a stack out of the side [or roof] of the truck. You would be losing most of your heat that way as the point and effect of the mass is to retain the heat for a long time. If you just exhaust straight out most of the heat will escape with the draw up the flue.
Kelly says
Matt Walker has a RMH that he uses to cook on, He is one of the pioneers in the field, see link below to his “tiny masonry cook stove and heater”
http://walkerstoves.com/tiny-masonry-cook-stove-plans.html
George says
I love the rocket mass heater (RMH) concept. I hope to build one someday for my own home. However….
In the section #1, you erroneously state that RMH’s somehow mange to produce less CO2 and more stream which, according to the chemistry of combustion, is impossible.
RMH’s more efficiently and completely burn the wood fuel AND capture more of the heat than conventional wood stoves. But the carbon in the wood CAN’T turn into water vapor (H20). It has to turn into CO2 or CO.
To state otherwise might suggest that a RMH has a smaller Carbon footprint due to less CO2 produced from the wood, which is incorrect. It might have a smaller Carbon footprint because it burns wood fuel more efficiently and captures heat more effectively resulting in less fuel used.
JohnA says
I’m a little late to this discussion but would like to point this out: you’re right but unless its since been edited, the article only claimed “a little CO2” not “less CO2.” Nor does it claim conversion of CO2 to steam.
Granted, not very precise but not the same as claiming what you describe. Also, the only fuels listed are renewable biomass therefore it doesn’t actually have a net carbon emission, basically no carbon footprint at all.
Jason Patterson says
“The sharp turn mixes that smoke with fire so it burns up the smoke! Thus, only leaving a tiny tiny bit of CO2! It’s nearly purely steam.”
I suspect that that is the line that George objected to. I think that because it’s the reason I came down here – to write a similar comment. The implication is that the smoke is burned up, consuming the CO2 and somehow converting it into steam. You don’t wind up with a tiny, tiny bit of CO2 – most of the mass of the exhaust gas is CO2 if you’re burning dry wood.
A later line is clearer, but this one is messy at best.
Jerry says
I also like the RMH concept. But let’s do some thinking here.
All mass heaters have warm the mass first before you get warmed.
the heat is delayed. So if you come home from work expecting a cosy nest
you’re going to wait till the mass gets warmed up. Unless someone has keep it up.
If you are thinking of putting a RMH in anything movable, expect cracks, because
the more ridge the mass will be more brittle.
Laurie Neverman says
We have a large masonry stove, and that retains heat for over 24 hours. Not sure about the heat retention on these types of units.
Jerry Garber says
When you say yours retains heat for 24 hrs.
several questions
How many cubic feet of COB ?
What outside temp?
Take a infrared thermometer and give me some reading.
The hottest and lowest temp of the mass.
Please
Laurie Neverman says
My masonry stove is a Tempcast kit with brick facade, no cob. Outside temp never gets so hot that I can’t comfortably touch it. I would estimate that hottest mass temp in the low 100s, and lowest mass temp is house temp because it’s in the center of the home.
Monnie says
If you burn less wood, you create less CO2. It’s the most efficient way to generate heat from a fuel source, so it will produce less CO2.
Ken says
Could this heat a second floor if the rmh was on the first floor? Or how big of an area could it heat?
Laurie Neverman says
Generally, most I’ve seen have been geared to single level/relatively small spaces. We have a traditional masonry stove in our basement, and very little heat reaches the main level from the stove.
Mat says
I would say a duct/hole over the riser/barrel would cause the heat to rise into upper level. But I’m a newbie 😛
Jerry Garber says
I see no reason why doing a ceiling duct to the second floor wouldn’t work.
When both my wife were growing you we sleep in the upstairs with
a ceiling duct above the only stove in the house before furnaces
Kristie says
Those are some great pics of mine that were posted in this article of the rmh being built in the wofati we lived in montana.
rick says
good information, great pics and video, I like the use of sheet metal and firebricks, thank you so much.
Gary Lindenberg says
Some great info on building the rocket mass heater, My question is by super cooling the exshaust gases yo’ll get condesation of water this can be a very powerful acid and how do you work with this also not in you video but others used galv. pipe for thermoal mass flue would this acid water not eat it thru it should flue be ceramic type
Jerry Garber says
Gary you bring up a good point cooling whether slow or fast condenses steam. So far I haven’t see any design with a water trap like power vent furnaces use but personally I would put one in.
Kelly says
The pipe used in the cob benches isnt really required. You could use brick and cement then just cob around it
donna porter says
dose the RMH have to be ( built on ground level ), we live in a raised house. so what is the weight capacity of a RMH itserlf
Laurie Neverman says
Rocket mass heaters, by their nature, tend to be very heavy. Load bearing capacity of the heater itself would vary, depending on materials used and how they are used. It is generally best to build them at ground level. An elevated build would need to be strongly supported.
Jerry Garber says
I don’t know the exact weight of cob but it’ll be close to the weight of concrete
118 lb/cuft
Kelly says
I think an average mass would be in the range of 3000lbs. That’s just the mass not the burn unit.
D Reid says
How do you get the fire/draft started? Lighting the vertical wood with a cold stove and flue will caused the fire/smoke to back into the house?
Jerry says
For draft getting fire started. Why not replace that costly chinney altogether with a powered draft fan like used on the latest
furnaces. Put it on thermostat control?
Mike Wilson says
I live in a residential neighborhood. Does anyone ever have legal problems building one of these for their home or shop? I am seriously considering one for my small shop, one reason is that the chimney regulations for a regular wood stove are really expensive to implement.
Laurie Neverman says
You’d have to check with your local building code. My guess is that they would require any sort of combustion device to follow the same guidelines.
Linda Robinson says
I live in a cabin built on concrete piers; no way to retrofit a rmh inside. Can anyone think of a way to build the heater outside along an exterior wall and bring heat inside?
Laurie Neverman says
The whole point of a rocket mass heater is to keep that hot mass inside the building, so putting it outside is counter-intuitive. If you could put supportive pilings at a similar depth to your current pilings, and put a load bearing concrete slab on top of them, you should be able to put a stove on top of that, but having it outside the building envelope – well, I probably wouldn’t bother with a RMH.
Linda says
Thanks.
Jerry Garber says
The only way I see is cut out a hole in the floor and install a slab in the ground to support the weight above it .
Seperated by standoff legs and insulated around those legs to minize heat flow from the weight above.
Instead of COB I would be switching to a 275 gal fuel oil tank sitting above floor level and filled with water instead.
The tank would have to be fitted with several heat ducts coiled inside.
Jerry says
Why not build an extra room into your cabin and locate RMH in there?
chris says
Thx for the nice article. I’m down with rocket stoves, totally, but I wouldn’t be happy having to look at something so ugly in my house every day. Do you know of any examples where they have been decorated or camouflaged to look nice? If so, I’d love to see some pics. As you mentioned, masonry stoves are expensive but they can also be built to be beautiful and blend in with a person’s chosen decor.
Laurie Neverman says
I’m not sure if the photos in the post weren’t loading for you, but I changed some settings and hopefully you can see them now. You can finish a rocket stove in a variety of ways, as shown in the photos.
crazysquirrel says
I wonder what the ratio of the burn tube vs the discharge tube is.
Also does the opening for the fuel/air supply need to be smaller or the same size as the discharge tube (aka chimney tube)?
Any thought about ducting outside air in for combustion instead of using already heated inside air?
PS do NOT allow bots or anyone else access to my name or email address please.
Laurie Neverman says
Dr. Mr. Squirrel,
I never sell or use use anyone’s email in any way without their explicit permission. (For instance, if they subscribe the newsletter or to comments.)
You’d need to contact the rocket stove peeps directly for their specific design preferences. Unfortunately, they’re long since given up responding to comments on the post.
Per my experience with our masonry stove, which has some common elements, we do use a dedicated fresh air feed. It has a standard 8 inch diameter pipe for air intake, which then travels down an interior wall and around the stove to wash the glass at the front of the stove. I think our chimney is also an 8 inch, because the stove is located in the basement so the chimney height is quite tall.
louisa says
I would like to build a double layered one, so bench along the bottom, then reading nook higher up. (I had seen a picture of one like this, I think made by a german company) would the bend in the exhaust pipe to get to the second level cause me problems do you think? Also could I then vent it up an existing chimney rather than a horixontal exhaust? Any tips would be greatly appreciated.
Laurie Neverman says
The longer the run, the more potential for your fire to not draw as effectively as it should up the chimney. Venting up an existing chimney shouldn’t be a problem if everything is sized correctly, but you should really work directly with someone who has experience building these to make you you get your calculations right.
JAKEM says
The efficient burn does not reduce the amount of CO2 produced (for a given amount of wood). If anything it increases the production of C02. This is not a bad thing however, as the longer chain carbon compounds (smoke) are worse for the environment, and C02 is prefferable.