Living with the Rocket Stove

There was a crisis in Northern New Mexico this winter. The natural gas was turned off.  The gas from Texas just stopped flowing!
People were left in the cold for a week.  This in turn taxed the electrical grid. Since those without gas were using electric heaters. Placing the electrical grid in danger of outage too.  A single pipe controlled the comfort and possibly the survival of thousands of households and businesses.
This scenario got me thinking about how fragile this system really is.  I barely even noticed as I threw another stick in the rocket stove.

I created the Solunit Rocket Stove mass heater about 3 years ago.  This is the first winter that I’ve fully depended on it. I was a little nervous, because it can be very cold here in the winter and this was to be my primary heat source.

All energy is fuel

Ultimately, it would be nice to not need a heater at all, right?
It is possible with passive solar design and orientation; using the sun’s energy as fuel.  One of the benefits of the high desert is the available sun light. Some 75% of potential solar energy is available to us every year.  The Earthships have done a good job of capturing this free energy with double south facing greenhouses and massive amounts of thermal storage.
Starting off with a building envelope that will maintain a comfortable temp is key to reducing the cost and use of fuel.  Passive Solar heating is by far the best bang-for-your-buck and is always the place to start when designing burnt-fuel use out of your house. Best part is passive solar design works anywhere with any type of building construction.

First let’s start with the free energy available to us from the sun.  Design a house to collect the energy for our needs.  Then let’s consider other  fuel-burning options.  Perhaps we could look at the Rocket Stove Mass Heater with the notion that it’s a good transition on our way to burn-less heating?
The Rocket Stove seems to do a pretty good job at
1. Burning the wood fuel efficiently and
2. Extracting and storing the heat before it leaves the building.

Let’s face it, though, not all buildings are constructed with optimal envelopes, and the sun doesn’t always shine.

The Scene

The high desert is a cold climate with short growing seasons and intense temperature swings.  Winter weather averages have been anywhere from -20º to 15º F at night, with day time temps at 10º to 45º F.
The Solunit is a 400 sq foot dwelling located at 7000′.  Walls are constructed of mostly strawbale with earthen plaster inside and out.   It’s solar heated, but not completely. Performance will improve as I construct the south facing green house etc.
The coldest I’ve seen the building this winter is 45ºF, after spending a month away. It doesn’t freeze even without auxiliary heat. The average temp inside the unheated building is around 50-55º in the winter.  That’s not exactly comfortable, but it’s certainly is a good starting point. This means I only need to bring the temp. up 15-20º to be in the ‘human comfort zone.’

Thermal bridges.  The weakest point in my building envelope is the stem wall. I lose a lot of heat through the conduction of my earthen floor and stem wall to the outside.  More insulation around the perimeter of the stem wall and more insulation beneath would have helped retain heat.
I was  smitten with the idea of running pex tubing in my earthen floor for active hydronic solar heating, but I couldn’t afford it at the time.  One would be able to further reduce fuel use with active solar radiant heat. Bringing the warmth of the sun to  parts of the house, that do not receive direct sunlight, via tubes of solar heated liquid. Oh fun, I still want to do that!

 

Burning Wood

Wood is a local renewable resource. Burning wood can pollute the atmosphere, like most fuel-burnt heating, but my hope is it comes out neutral in the end.  Trees sequester carbon through-out their life time. When I burn wood I release that into the environment all-at-once.  The same tree if left to rot will eventually release the carbon too.

Last fall I collected 3 chords of Pinion and Juniper from the local forest. It’s great because people leave all kinds of wood behind that is perfect for the rocket. Most people are looking for gigantic slices of wood and leave a mess of good sized branches behind in the forest. It feels good cleaning up the forest tinder while gathering wood to heat my home. I only gather ‘dead and down’ trees.
For 20 dollars one can get a permit for firewood from the BLM.  Another 40 dollars in bar oil and gas mix for the chainsaw and your off!
I love the forest.  It’s a lot of work gathering wood, but there’s no place I’d rather be.   Providing future light and warmth is gratifying.  I was ever so thankful to have a nice stash of wood on that cold ( -20º) winter night!
There’s a added benefit to wood as fuel…  It’s local.  I’d much rather gather or buy wood from a local source than fund the energy monopolies and their tubes.

I can heat the house with an arm-full of wood!?
It’s been a relatively mild winter this year.  It’s spring now and I still have more than half of my wood.  I guess I’m saying I heated my house with 1.5 cords of wood this winter.
I wasn’t home the whole time.  So, I can expect 2-3 chords to last from Fall to Spring: lavishly burning.

I’m a bit of a home-body, especially in the winter.  Just the kind of partner a rocket stove wants.  I  toss a little wood in every hour or two and keep it purring.  The rocket probably wouldn’t work as a primary heat source for someone that isn’t home very often.  I suppose that’s true of wood heat in general.

On average it’s a 4 hour ‘active’ burn. I say ‘active’ burn because the stove is actually ‘on’ for much longer while the coals burn down.  So really it’s more like a 8 hour burn-down period. The stove consumes roughly  3  ( 3″ x 2′ ) sticks a hour, with modest amounts of attention from the owner.   Admittedly – I like to play with fire.  The subtle rocket sound purrs and lets you know when it’s time for more wood.

I’m quite happy sitting on the stove toasting my buns from last nights burn. The mass bench can easily be at 90ºF  24 hours later. It’s a smooth easy radiant heat. Very human friendly.  It can be chilly in the house, but when I’m sitting on the stove’s heated bench…
I’m warm! My tea is warm! Life is good!

Seems the rocket has personality traits and moods depending on external pressures. Let’s just say that it can be a bit persnickety.  While burning the rocket stove this winter I noticed a few things about it’s behavior I’d like to share…

Creeping fire & Puffs of smoke

When the guts of the system collected enough soot and ash the stove lost some of it’s draw. The draw was not enough to suck all of the smoke into and out of the system and smoke entered the living space. No bueno!  Earlier I purchased a Carbon Monoxide & Smoke Detector to be on the safe side.  I really didn’t relish the idea of being the ‘guy who died from his experimental rocket stove.’  The alarms never went off, but I’d notice the smoke.
On occasion, while super windy, the pressures can force a puff backwards. Mostly this happens when only coals are burning.
When long sticks protrude from the stove’s feed tube, which is part of the idea of the ‘self feeding’ stove, fire sometimes wants to creep up vertically between the sticks.  When it creeps the smoke can enter the living space.  These situations can be resolved by putting the cap on the feed tube. If smoke does puff or fire does creep it’s caught in the cap and is sucked back down through the system exiting the building.

Priming & Pressures

About 5% of the time I’d have to prime the system before starting a fire. Sometimes the system is stubborn or the air in the system is actually flowing in reverse. I prime the system by going outside, opening up the “clean out T” and light a small bit of newspaper. This get’s the air flowing in the right direction.  Then it’s safe to start a fire inside.
From time to time there can be a smell of burnt wood in the house. It seems to happen when it’s cooler inside than outside.  Happens more in the spring. I cover the mouth of the stove with a couple of bricks to reduce the air flow and that takes care of the smell.

Warning. If you don’t use your rocket someone will

It might be a good idea to plug the duct work when not in use.
While visiting the Moonunit, last fall,  I noticed the stove would not lite. It had been inactive for 3 years.  Upon further investigation I found a  mouse nest plugging the burn chamber. I pulled  out the nest and an asphyxiated mouse. God knows if there was more inside the guts of the stove. There must have been since I was unable to get it flowing. Of course I didn’t make it as easy to clean out the duct work on that stove, so there’s no way for me to run a pole or chimney sweep through it.  Duh! Lesson learned.

Cooking and Warm Tea

I’m well hydrated in the winter because my tea is always warm.   One of these days I’ll have to make a rocket stove cook book! Stew, chili, slow cooked veggies, and warm water for dishes. The rocket makes some awesome nachos too.  The average temp on the top of the barrel is 250-300ºF while actively burning.  I’ve never got it hotter than around 500º. One can place a box on the top of the barrel to retain the heat, making a nice little oven.

The Barrel

On one hand I like the look of cob sculpted much more than a 55 gallon metal drum.
When it’s cold and I want heat the mud takes a while to heat up around the barrel. On the other hand, once it’s all warmed up the metal barrel can be very hot! With the mud surround it won’t melt your skin off if you touch it. You can be a little more intimate with the mud-covered barrel, snuggling up to it for intense heat.
Easy access?
If the drum were not set in so much mud I might be able to remove it and inspect the innards. While cleaning the oven I really wanted to take the barrel off and get in there to have a look and clean it. Not the case when it’s submerged in mud.

Maintenance

I’ve been cleaning out the burn chamber on Sundays. It’s best if I let it settle down on Saturday, so I’m not pulling out hot coals. There is  around a gallon of ash every week.
It’s been recommended to place the stove by the door or somewhere where it’s not in the middle of your living space. It can get messy; bringing in wood… Dust, bark and filth.
Reaching down into the burn chamber to remove the ash can be lame.  There is room for design improvements here. I’m sure someone has already figured this out?

I’m so glad I didn’t snake the horizontal  flue ( bench flue ) with contorted bends and turns. Yeah, maybe I’m not extracting every last BTU out of the stove before the heat leaves the building, but when it came time to clean it out… it was simple! It’s a straight shot.
It was dirty in there ( I hadn’t cleaned it in 2 years ). Pushing a chimney sweep through the system was revealing. One thing I noticed is where I used long screws to fasten the flue piping there was major build up.  So if you build one use rivets or short metal screws, if you fasten the piping.  I was really surprised to see how much soot collected on the screws, then more soot collected on the soot. Pretty soon the stove wasn’t performing very well.
Do your self a favor… If you build a rocket stove make the clean outs accessible, consider the collection of ash and soot as a reality.  I see a lot of rocket stoves being made as if the duct work will never collect with soot. It will! And you’ll need to get in there to remove it.

Rocket Size

The 8″ system works fine, but there have been times when I would have liked a hotter stove. I think if your living in a cold climate it might be worth constructing a 10″ system for those times when you really want intense heat.  Once everything warms up it’s fine, but if you been away from home for a week, that’s the time when you’d like a little more kick! More heat, now.  I ended up using propane heat a few times while the system charged up.  The mass can charge up with heat or cold. So if you have a ‘mass cooler’, after a month of inactivity, there’s a slow curve to get it charged again. Brrr.
Of course once it’s charged with heat it’s the same story. There’s a slow curve before the heat dissipates completely.
Don’t build a 4″ system for home heating. You’ll probably be able to get a way with a 6″ or 8″ system in moderate climates.

 

Off-grid

Rocket stoves are not connected to a distributed power supply like natural gas.  I didn’t realize the importance of this, until the recent grid failure.  I can understand the convenience of propane, oil or natural gas for home heat.  Though, it seems a little volatile. Cost can’t be controlled. Or rather, I can’t control the cost myself, leaving me at the mercy of energy providers.  The grid can just ‘turn off’ one day… and what-to-do then?  Not a situation I want to be in when it’s 0º outside.

I wish I had hard stats for you, in terms of temperatures, all charted out in a nice colorful graph, but I don’t.
I am comfortable.  That’s my gauge for now.  All-in-all I’m pretty happy with the rocket stove because it has met my needs and enriched my life.  It’s a different kind of heating experience: you touch it, it embraces you with warmth.   It feels like it charges your bones and warms your blood.

 

 

 

13 Responses

  • Sophie says:

    Thanks for this great information on cob rocket stoves. Have you heard of solar cookers? That’s another way to cook your food off the grid when it’s sunny (including cold, sunny weather). I’m trying to figure out whether I can make a solar oven, or even a parabolic oven, out of cob. Solar cookers are needed mostly by poor people in developing countries who have to go and fetch wood everyday, and who don’t have money or access to fancy materials. It would be awesome if you could try making either a cob solar oven or cob parabolic solar cooker (lined with aluminum foil) and post your stories so that poor people around the world could find out how to make solar cookers for really cheap. You can find lots of start-up plans at: http://solarcooking.org/plans/. I’m going to Haiti soon and would love to know how to make them for cheap, but I don’t have the means to experiment myself right now. Any suggestions? Thanks!

  • ryan jones says:

    My wife’s design was chosen for university of utah’s “design build Bluff” program. They build a home for a native american family on the reservation in bluff utah. her design incorporates earthen wofati style thermal mass on the back wall with straw bale on the front and a rocket stove for heating. Is it possbile to send you a pdf of the floor plan for your thoughts on the general design of the home and most importantly; size and layout of the rocket stove system. very important that it works well as this will be the only heating system in the house.

  • Are you getting much smoke coming back into the dwelling. I have tried something similar in England. We had a lot of smoke come back in. Our oven was not as contained as yours but it was well vented. It is possible that the smoke was blow back in from the outside venting. Any suggestions?

    • R.L.Wilson says:

      Put a combination of three t`s on top of stovepipe,will look like an H.Worked on my rocketstove mass heaterto eliminate blow back.

  • Joy says:

    Thank you so much! You put a ton of valuable info in these posts. I’m about to start an outdoor project with wood-fired cob pizza oven /rocket stove /mass bench heater combo. First time with this, so your input has helped immensely. I’m in s.e. AZ at 4,000 ft. The natural gas outage affected this area too, came along with record-low temps. Thousands of frozen well pumps, burst water pipes, freeze-killed trees, whew.

    Most valuable thing of all in your articles is that you came back and reported how your stove is working. I’ll be reading all of your site for sure.

  • Jeremiah says:

    Thanks for sharing all your pictures and experiences, it looks great. I’m just now investigating these rocket stoves while I finish up my latest wood gassifier project. Couple questions: wouldn’t multiple pipes provide better efficiency due to increased contact area? Did you see much or any loss in the draft after covering the barrel?

    • Yes multiple pipes could have extracted more heat, though I choose a simple and easy to clean option.

      You know – I didn’t measure or notice loss in draft after covering the barrel with mud. Wasn’t really looking!
      Do you think it would be a ‘slow-down’ area?

      • Jeremiah says:

        I’m not sure but I thought it might. I say this because the exposed barrel provides an area of rapid heat release. In theory this should cool the air inside the barrel more and help to cause a downdraft which would then pull air through the riser. I thought it might be contributing to your blowback but on the other hand it may be helping to keep the riser hotter which would be helping the system. Increased velocity will increase the burn temperature but also reduce the contact time it has to disperse the heat. I was just brainstorming about efficiency which is always a fine line. It’s something I will do some testing on.

        The covered barrel is very aesthetically pleasing (it looks phenomenal imo) and you have solved the blowback issue with the lid. I was just curious if you might have by chance noticed a difference in the pull by the sound or other means.

  • Nathan says:

    Thanks so much for sharing your experiences. I am flat out owner building in Central Victoria in Australia and I am excited by this stuff. My 10 acres is forested with very good burning Eucalyptus which is a serious fire hazard so I intend to use it in a lovely rocket mass heater/stove. I know exactly how you feel in a forest too.

  • Alyssa says:

    Hey Mic,
    Wow! What an informative piece. Good stuff to keep in mind for when I build one. Leaving for Taos tomorrow am. See you soon…

  • neon says:

    Excellent informations, thank you for sharing!
    After reading “The Year of Mud” blog, I was afraid the rocket stove is not a good idea.

    I am interested, how do you start the fire? Paper at the bottom and slip your hand through the stacked wood with the match-stick? No problems?