Rocket Stove Teaser

I’m finally getting to work on the rocket stove video series. Here’s a picture of my refractory mix being tested:

Homemade Refractory Testing
Homemade Refractory Testing

I’ll upload short clips of the build to youtube, but if you want full details, it will be provided on DVD. This outdoor rocket stove furnace will be much sleeker than my first experimental build and should cost fairly little to construct.

If you haven’t participated in my poll on DVD interest, please do so:

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My furnace design is the hydronic or boiler type, rather than the mass type of rocket stove commonly seen. If you haven’t done so, read this article on the different types of rocket stove heating systems. It will help clear up a lot of the confusion on the matter.

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Upcoming Rocket Stove Furnace Video Series!

I’ve gotten hundreds of questions about my rocket stove heating system. Since a picture is worth a hundred words, videos must be priceless. I’m going to build a new outdoor rocket stove hydronic furnace and video every bit of it. This will be an improved model, both in efficiency, function, price, and appearance.

All these videos will be uploaded in abbreviated form to be viewed free of charge on my youtube channel. In these videos, I’ll cover the materials needed, how to make refractory cement, how to size and cast the stove, hooking up the water system, and getting everything running.

I may also offer a DVD for a reasonable price for those of you without high speed internet or who would like the series in offline format if I get enough interest in the following poll:

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I’m planning to begin uploading the series in November, so be sure to swing by my youtube channel from time to time or better yet, subscribe here.

Also, subscribe to my RSS feed to keep up to date with the other alternative energy experiments I conduct. I hope your visits to my blog and youtube channel are inspirational and get you closer to getting off the power grid or providing power for yourself during emergencies.

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Demystifying the rocket stove

There seems to be a lot of legend associated with rocket stoves. I often hear of a rocket stove as being something you can put a tiny amount of wood in and magically heat your home. While a properly built one is capable of high efficiency, one has to keep in mind the limited nature of the fuel being fed to it. Let’s consider a good hardwood that most of us have access to – oak. On average, across species, seasoned oak will have at least 6200 BTU per pound. So if our rocket stove burns that amount of wood with 100% efficiency in an hour and transfers that heat energy to the air with 100% efficiency, our best output is 6200 BTU/hr. The real number is going to be lower. Let’s figure efficiency off the fire and exhaust temperatures. Supposing the hottest combustion temperature is 2000 degrees and your measured exhaust temperature is 200 degrees. This is going to give you a 90% efficiency, which gives us a real world output of 5580 BTU/hr.

To put this into perspective, that is just a bit more heat output than your run of the mill space heater. Can you heat your entire home off a space heater? Contrary to what certain manufacturers claim, the answer is no. You can maintain a 45 degree temperature difference in a 1152 cubic foot room (the volume of a average sized bedroom) with that kind of heat output. To heat your average 1200 sq. ft. home, you’ll need something between 30,000 and 40,000 BTU to maintain good heat in the winter.

What does that 45 degree temperature difference mean? Let’s say the average overall temperature is 25 degrees outdoors. With a 45 degree difference, you’ll have a nice comfortable 70 degrees inside. You’ll need between 5 and 7 pounds of wood an hour to do so, even with a 90% efficient rocket stove. At this rate, you’ll go through a cord of wood in a month. That’s with a 90% efficient stove and a well insulated house.

If you want to heat your indoor air, regardless of the type of stove, you are going to have to burn a lot of wood. So where do people get the idea that rocket stoves are miserly on wood consumption? The answer is in the rocket mass heater. Instead of heating air, they focus on heating your personal space. People who use them dress warmer and maintain a much lower temperature differential. With a blanket over you, sitting on a bench that doubles as the thermal battery, it doesn’t take much energy to keep warm at all. Kind of how a heating blanket can keep you warmer with 130 watts than a 1500 watt space heater can.

For a great read on heating your personal space vs. air, check out this great link:

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Clearing the air on rocket stove heaters

It seems that people confuse the term rocket mass heater with any type of rocket stove heating system. While all rocket stove heaters have a rocket stove at their core, how they handle the heat output is where they all differ. I believe some clarification is needed starting with this popular term:

Rocket Mass Heaters

Rocket mass heaters use a rocket stove to efficiently burn wood. The hot exhaust is then tunneled through a maze like pipe that goes through a thermal mass. As it does so, the heat is transferred from the exhaust to the thermal mass, where it exits the outside relatively cool. So what is this thermal mass? Think of it as a battery for heat instead of electricity. It is made of materials such as rock, brick, and cob that absorb massive amounts of heat and then slowly release that heat energy to the surroundings. Many people have turned this thermal mass into something decorative and functional by styling benches and furniture out of it.

Rocket mass heaters have the advantage of only needing an active fire for a couple hours at a time throughout the day. By burning a hot fire for a couple of hours, the thermal mass is recharged and can pump heat slowly into the room for several more hours to come. Once the mass has cooled, it is time to start another fire and the process continues. To balance the slow steady heat release of the mass, often rocket mass heaters will incorporate a radiant system to provide quick room warm ups when the fire is going. This is often in the form of a metal barrel placed over the heat riser. This provides a quick heat transfer to the room and a cooktop to boot. This radiant system brings me to the next type of rocket stove heater.

Radiant Rocket Stove Heaters

Most radiant type of rocket stove heaters are small and are used to heat garages and workshops quickly. As mentioned in the previous section, sometimes they form a hybrid with the mass type heating systems to provide quick heat. Typically, these tend to be a bit inefficient because they are often built without insulation. The lack of insulation leads to more combustion byproducts. By taking time to add insulation, their efficiency can be improved and the exhaust made cleaner. Heat from the exhaust is radiated to the surroundings by exposed metal surfaces from the stove and pipe, much as is the case in traditional wood stoves. Between often times lacking insulation and a lot of heat escaping through the exhaust, the radiant rocket stove heaters are of dubious advantage over traditional wood stoves with a small hot fire.

Forced Air Rocket Stove Heaters

By building a small insulated shed around a rocket stove with the hot exhaust passing through a couple of barrels, one can build a outdoor forced air furnace. A blower transfers the cool indoor air around the barrels, heating the air, and transferring it back indoors. With a well made rocket stove, efficiency can be better than commercially made units. The main drawback to this system is that air is a poor heat transfer medium and those blowers consume a decent amount of power to do so. The main advantage is simplicity.

Rocket Stove Boilers

By placing a coil of pipe above the heat riser in a rocket stove and pumping water through it, a rocket stove boiler can be made. From there, the hot water can be pumped into a in-floor radiant system, radiators or baseboards, or a heat exchanger in an existing furnace. It is for this reason that I recommend rocket stove boilers for most people with existing homes. The main drawbacks in a boiler system are complexity and cost. Much of this is offset if you are using one to replace an existing boiler and using the remaining system.  The advantages are the fact that they are easily integrated into existing homes, can heat greenhouses, barns, pools, etc., and can provide domestic hot water. It is the type I chose to use myself for these reasons.


So we see from all this that not all rocket stove heaters are the mass type. Which one is best for you depends on a lot of factors. I hope this makes you able to make a better decision when choosing one for your needs.

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Two successes – jam free feeding and burning green wood

Over a month and a half into my new feeding system and no jamming has occurred with the wood bundles. This seems by far the easiest way to keep a rocket stove fed without giving it constant attention. I hope this breakthrough can help those who use rocket stoves to heat their homes do so more conveniently. Today, I’ve been experimenting with burning relatively green wood. It has only had a month to season. I started the fire with seasoned wood, and cut the green wood to 1.5″ thick pieces. This is then bundled and fed in the stove. Seasoned pieces this small when fed to an established bed of coals would cause a near instant boil over. Since some of the energy is wasted in drying the green wood, this is not an issue.

So what does this do to the exhaust? Normally we associate green wood with lots of smoke. Not here:

Burning green wood with no smoke.
Burning green wood with no smoke.

So yet again we see a benefit of using rocket stoves over traditional wood burning methods. The heat reclaimer blows warm air around the wood on its way to be fed into the fire, helping to season the wood on its way down. Once the fire is going, it can season its own wood supply.

A lot of neat chemical reactions occur when moisture is present. The water turns to steam. When the steam comes into contact with glowing charcoal (mostly carbon), it forms CO and H2 (often referred to as water gas), both of which are combustible. As long as the coals are glowing red, enough temperature is present to keep this reaction going. The thick insulation of the rocket stove helps maintain these temperatures, making the effective combustion of green wood possible. There’s a few other possible reactions here, but they mostly involve the production of CO and H2, just in different proportions.

I’ve had green wood burning all day, maintaining my hot water supply at 130 degrees and a 45 degree temperature difference in the house (its in the upper 30s outside, so its pretty hot inside). Do I advocate burning green wood? Not necessarily, but it does come in handy when your experiments all burn wood and your supply of seasoned wood disappears at an alarming rate 😉

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How the rocket stove furnace is connected to plumbing

I get a lot of questions on how my rocket stove heating system is connected to plumbing. First, it is now an open system, meaning that it is connected to a fresh water supply. There is a check valve that prevents the hot water from backflowing into the cold side. A pump keeps the water circulating through the system at all times.

Rocket stove plumbing diagram
Rocket stove plumbing diagram

The water comes from the hot side of the hot water tank and enters the cold side of the rocket stove. The home hot water system is located on this side of the system between the hot water tank and rocket stove. Once in the rocket stove,  the water is heated from the hot exhaust of the rocket stove as it snakes through 110 feet of copper tube. The water exits the rocket stove at temperatures between 150 and 200 degrees.

This hot water is pumped through a heat exchanger where air from a blower takes the heat from the water and transfers it to the air. On the cold side of the heat exchanger is a thermal switch. The blower kicks on when the water reaches 150 degrees and shuts it off when the water drops to 130. This ensures that my hot water is at least 130 degrees. When the water exits the exchanger, it is fed through the circulating pump. The pump is installed on the cold side to lengthen its lifespan.

The water then goes to the hot water tank for storage. Any time the hot water is used in the house, the pressure drops on the hot side of the system. The pressure on the cold side is then higher, which forces the check valve open, admitting fresh cold water to the system.

For safety, there is a temperature and pressure valve installed both at the rocket stove and the hot water tank. This prevents unwanted steam generation. If the temperature of the water reaches 210 degrees, the valve pops open and cold water is added to the system automatically until the temperature drops to a safe level.

If you have any questions or comments on how this all works, feel free to leave a comment below. Yes, I am missing an expansion tank, and it will be added in the near future :)

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How my rocket stove boiler works

I get a lot of questions pertaining to how my rocket stove furnace actually works. Hidden beneath a layer of concrete and 4″ of cast refractory are the inner workings. Unfortunately, I didn’t have time to take pictures as I built it, so I’ve taken the time to make a simple diagram of its internals. Take a look:

We’ll begin with the right hand side at the feed tube. It is made of three lengths of 2′ long 8″ black stovepipe. The joints are held together and sealed with red RTV silicone. It sits about a foot off the bottom of the firebox, making a total of 7′ of wood capacity. Combustion air is fed through the top around the fuel. It is preheated from waste heat collected from where the feed tube sucks heat from the firebox.

The firebox is cast around a 8″ cylinder and is 4″ thick on all sides. At the ash cleanout, I made a door out of refractory with a rebar handle. It, too, is 4″ thick. The heavy insulation retains all the combustion heat possible to ensure a complete burn of all wood, tars, and wood gases.

The heat riser is 2′ long horizontally, takes a 90 degree turn and continues up for another 2′. The purpose of the heat riser is to combine the wood gases and tars with the remaining combustion air to eliminate all smoke. The end result is a mixture of hot gases consisting mainly of carbon dioxide and steam. The flame stops before the end of the heat riser if the fire is maintained properly.

The hot gases then rise another 3′ through a chimney with 2″ thick insulation. Within this insulated chimney is 50′ of 1/2″ copper pipe at the bottom and 60′ of 3/8″ copper pipe at the top. The pipe is coiled and placed snakelike in the chimney to pull as much heat from the exhaust gases as possible. The water is pumped in from the cold side of the system at the top and exits at the bottom.

The exhaust takes another 90 degree turn and exits about the same temperature as a dryer vent would.

Many people have voiced concern over creosote production. In a normal wood stove and pipe situation their concern would be legitimate. Because of the heavy insulation, the creosote has been burned in the firebox and heat riser before the exhaust ever comes in contact with the copper coils.

If you have further questions, feel free to leave a comment below.

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Heating water with my rocket stove furnace

When the weather is hot, you normally wouldn’t run your heating system. This is the problem I have ran into with my rocket stove furnace. I’d still like to cash in on some cheap water heat, so I decided to experiment with heating water without boiling it and not increasing the temperature of the house. By turning the blower off and keeping the fire low, I’ve been able to do just that. Check out this video:

As you can see, btu output of the furnace is determined largely by the size and amount of wood put into the system.

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Longer burn times

To keep from having to continually maintaining the fire, I have built a gravity fed system. There are a couple of things that I had to do to make the system work. First, enough wood had to be put in to last at least 6 hours before needing a refill. Second, the wood had to be placed in such a way that it would not jam in the feed tube. I ran into a bit of trouble with the feed tube sucking heat from the fire. To counteract, I made a heat reclaimer that preheats the fuel and combustion air from this waste heat to improve efficiency.

Here’s a video on the feed system and heat reclaimer:

Here’s a video on how I prepare the wood for jam-free feeding:

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My Rocket Stove Heating System

If you are new to this site or my youtube channel, you may not have seen some of my older information on my rocket stove heating system. It is a hydronic type of heating system, where the heat from the rocket stove heats water that is circulated through copper coils in the chimney. That water is then pumped to a heat exchanger and hot water tank for later use. Here are my older videos on the system:

Producing the heat:

Turning it into house heat:

This should get you up to speed on the basics of the system. It has come along quite a ways since these videos.

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