Installing Rocket Heaters Safely

Rocket heaters work efficiently, in large part, because they maintain very high temperatures 1,200°-2,000°F in two sequential combustion chambers. This helps insure that all the volatiles and most of the particles are consumed.  The first and hottest combustion chamber is also usually located close to the ground. This means it is critical that the burn chamber be extremely well insulated to protect the substrate from intense heat.

Our tests showed that the area under a 6″ burn tunnel with 2″ of perlite insulation can still be over 800°F. This temperature range is much hotter than the feet of cast iron stoves which are near the floor. Consequently, it is important to mitigate the heat from a rocket heater in a different way than traditional stoves, which only require a hearth pad.

Installing a Rocket Mass Heater

If you have your burn tunnel surrounded by a lot of clay cob like a traditional rocket mass heater, this mass helps absorb the heat. However, be sure that there is enough mass to accommodate your firing cycles and intensity. This issue is probably at the root of some buildings being burned down from rocket heaters.

Installing on Concrete

Concrete can work like the mass in rocket heaters by absorbing and conducting heat away from the site, throughout the slab. Depending on the size of the concrete area under the burn tunnel and what type of concrete it is, degradation may still occur. For example, if the concrete area under the burn tunnel is not very big, at a certain point, the heat begins to accumulate and raise in temperature.

One solution is to mount your burn tunnel container on a platform that allows an air gap between the burn tunnel and the flooring. Heat, which might otherwise accumulate to high levels,  is taken away via air convection.

Another option is to place a layer of 1″ refractory board on the cement, this will lower temperatures from above 800°F to below 300°F.

Installing on Wood

Wood requires more protection than concrete. Over time wood’s combustion temperature can be lowered due to prolonged exposure to infra-red radiant heat. So while wood may start at a spontaneous combustion temperature of over 400°F, this number can be halved due to prolonged exposure.

This danger applies equally to wood that is below an otherwise non-combustible top layer, such as tile and backer board. The non-combustible layers can accumulate dangerous levels of heat which are conducted to the plywood underneath. Over time, the spontaneous combustion temperature of the wood is gradually lowered and the heat accumulation from the tile layer can cause the plywood to ignite.

An air gap above a non-combustible surface can work if there is sufficient additional insulation below the burn tunnel container. The size of your burn tunnel should dictate how thick the insulation will need to be.  You should test your burn tunnel prior to installing to insure your wood is being exposed to temperatures no more than 125°F.

Standard Hearth Pads

Standard hearth pads are typically constructed of a non-combustible top board over a layer of mineral insulation. They are designed to combat radiant heat from a cast iron stove many inches away. A rocket heater is much hotter, usually much closer to the floor, and has larger conductive surfaces in contact with your floor.

They can be used as a part of your floor protection system, but don’t rely on them exclusively. For example, a hearth pad on top of some bricks or other spacers to allow air flow under the burn tunnel might be sufficient, depending on the size of your burn tunnel.


We recommend installation of a permanent temperature sensor of some sort to be installed under the burn tunnel next to the surface of the floor to monitor the heat level there. Compared to not knowing that your floor is getting too hot, the cost is minimal.


Be safe! This post is not meant to be specific installation instructions, but as a starting point for your own testing and verification if you are thinking about building your own rocket heater.



Wood Stove Types – Cast Iron, Masonry Heaters, and Rocket Heaters

Wood burning stoves can be roughly divided  into 3 categories based on how much of the heat from burning wood is stored vs. immediately released into the surroundings. The 3 types are;

  • cast iron or steel stoves
  • masonry heaters/ovens
  • hybrid stoves or heaters such as the rocket mass heater

Depending on your application, each stove type has its pros and cons, Dragon Heaters can be configured as any one of these three types depending on your requirements.

Heat Storage

Some materials can gradually absorb heat and just as gradually release it. We have another blog which lists the various candidate materials for this purpose and how well each of them does. The antithesis of heat storage is something which gets hot fast, radiates its heat, and cools off fast. An example of this is steel or copper.

Cast Iron / Steel Stoves

Prototype Metal Dragon Heater Stove

Prototype Metal Dragon Heater Stove

This first category will heat up its immediate space in a hurry. All the heat from the fire is radiated by the metal into the space. The advantages of this type of stove are:

  • quick to install
  • small footprint
  • moderately priced
  • easy to obtain
  • heats space quickly
  • relatively lightweight, so more easily transported

This is ideal for a space which is not continuously occupied and needs to heat up quickly. For example a workshop or perhaps a cabin not frequently used.

The disadvantages of this type of stoves are that they go cold quickly and, consequently, have to be fired frequently.

In the prototype of a Dragon Heater shown to the right, a steel barrel is heated by the exhaust from the fire. A second barrel can be added for even more radiated heat. When the gases cool off, they fall down and are removed from the building through the chimney pipe.

Masonry Heater

Picture courtesy The Masonry Heater Association

Picture courtesy The Masonry Heater Association

The opposite approach to a steel box is a traditional masonry heater. These are very common in Europe, and Russia. They are designed to collect all of the heat via a massive thermal store and have no provision for highly conductive materials which will release heat quickly. Instead, the exhaust is routed through bells or large flues made of heat  absorbing materials, until most of the heat has been absorbed, and it exits out the chimney. These designs are usually heavy, permanent, and occupy a large area within a building (even during the summer).

There are some masonry heater kits, which help reduce cost, but in most cases, a highly skilled designer and builder are required, making masonry heaters expensive ($20,000+). They take a long period of “firing” to warm up and a correspondingly long time to cool down.  Because the heat is stored in the masonry material, it is even and comfortable; no one has to get up in the night to light a fire to keep the room warm.

For more pictures of some modern masonry heaters, check out the Masonry Heater Association website:

Hybrid Heater

In between these two extremes is the Rocket Mass Heater as described by Ianto Evans and Leslie Jackson in their book, Rocket Mass Heaters.  Their design is inexpensive to build.  It consists of a combustion system of feed tube, burn tunnel and heat riser. The heat riser is covered with a steel drum which absorbs and re-radiates a lot of the heat from the fire. In that way, it is similar to a cast iron or steel stove. In our tests, about 2/3 rds of the heat is radiated into the space from the barrel or drum, and 1/3 is available for thermal storage.

After leaving the steel drum, the exhaust is routed through a horizontal flue buried in cob (clay) which is made into a bench similar to a masonry heater. The cob absorbs the heat and releases it slowly. Thus, the rocket mass heater, in theory, yields the same advantage of not having to get up in the middle of the night to light the fire as the masonry heater at a lot less expense.

Traditional rocket heaters are labor intensive to build, are tricky to build correctly, and are problematic aesthetically unless you have an adobe or rustic style house and have a large space requirement. By offering pre-engineered shippable rocket heater cores, we hope to solve many of these drawbacks.

Dragon Heaters can be built to operate as any one of these style heaters depending on which one best suits your application.

Burning Wood – Thermal Mass Material Selection

If you don’t want you’re stove to go cold immediately after the fire is out, you need to store some of the heat from combustion. A stove’s ability to capture and store excess heat for gradual release later, to a large extent, is governed by the materials used. There are several important physical properties that govern a material’s ability to absorb the intense excess heat from burning wood.

Heat Storage Capacity

In the table below, there are a number of candidate materials.  The most obvious characteristic to be considered is the ability to store as much heat as possible in a given amount of space.  This ability is called heat capacity, and the table below is sorted by heat capacity.

Heat capacity is calculated by multiplying the density of a material by its ability to store heat. The later is referred to as its specific heat.  As you see from the chart below, you need both high specific heat and high density to have high heat capacity. Lots of materials are dense but do not hold heat well, or hold heat well, but are not very dense.

Burning Wood - Heat Storage Capacity

Click to Enlarge – Heat Storage Capacity of Materials for Wood Burning Stoves

Working Temperature

Heat Capacity is a rating per degree that the material’s temperature is raised.  The key to dense energy storage is to be able to raise the material the maximum amount possible with the fire’s exhaust.  For example, comparing water to red brick, we find that water has a much higher heat capacity. However, since water turns to vapor so quickly, it can’t absorb very many degrees of heat relative to a red brick. So while water is an excellent heat transport material, it is probably not as good of a choice as another material which can be raised 500°.

Concrete also has good heat capacity, but it has a working temperature limit of about 400°F. Overshooting this number will cause stove failure. Any time we utilize CMUs (made from concrete) in a design, we use a layer of fireclay bricks between the CMUs and the heat to buffer the higher temperatures.

Glass takes about 2700°F to melt it. On the other hand, aluminum melts at 1220°F.

Thermal salts and magnesium oxide must be contained; at temperature, they are liquid, complicating construction.

Thermal Conductivity

Thermal Conductivity is a measure of the material’s ability to absorb heat.  You can see from the chart that the high(ish) thermal conductivity of soapstone along with its high working temperature and heat capacity make it an attractive material for wood burning stove.  It is more expensive than fire brick, but looks much better.

Likewise clay is very cheap, but is many times less conductive than fire clay brick.

Form  Limitations

Cordierite is available as kiln “furniture”, such as round and rectangular shelves in addition to pizza stones. We were unable to find a way to cast a shape of our choice, so you would have to build your design to incorporate a shelf of a specific dimension.

Chart Numbers

Testing methods, samples used for testing, the type of material, all conspire to create a wide variety of specifications available for any given material. Also very few sources use the same unit of measure. So when comparing materials be sure to do the conversions. The numbers shown in the chart above are only intended to provide a ball park starting place.

Burning Wood – Insulation Material Choices

High Temperature insulation is an important component of an efficient wood burning stove or heater. Using insulation that is not rated for the application will result in pre-mature failure. Here we discuss various insulation options and why some are suitable for wood burning stoves or heating appliances and some are not.  All materials can be purchased off the shelf and do not require molding or casting.

Why insulate?

The Dragon Burner

As you can read in the “How Dragon Heaters Work”, efficiency is directly tied to keeping the combustion zones hot. The primary (burn tunnel) and secondary combustion chamber (the heat riser) must be well insulated to insure maximum combustion temperatures. The heat riser on the dragon burner is made of vermiculite board.  The burn tunnel, though it is cast from an moderate insulative refractory, it requires an additional 2” (at a minimum) loose insulation to insure maximum burn temperatures.

Other Applications

Kiln and furnace designs require extremely well insulated chambers to capture the heat and raise the inside temperature to 2,000°F and higher. Any time you want to contain heat rather than release it into the surrounding space, you will need insulation.

Factors to Consider

In order to choose the correct insulating material for your application, you need to consider:

  • Working and Melting temperatures
  • Thermal conductivity when hot!
  • Form factor and strength of material
  • Cost

Working and Melting Temperatures

Whatever insulation you choose must be able to withstand the temperatures at the given location. So it is important to know what the potential temperatures will be 1st. Using this information you can narrow or broaden your choices.


You will notice in the chart above, the working temperature may be different from the melting temperature. A working temperature is the temperature that a material can endure over an extended period without undergoing some other physical or chemical change. A material can loose viability without reaching the melting point. They can change structurally and permanently in some way when kept above their maximum working temperature. So it is important to go by the working temperatures and not the melting temperature.

Low Thermal Conductivity

This is the technical term for “how well does this transmit heat”. Metals have high thermal conductivity; fiberglass insulation is fairly low. Materials with low thermal conductivity prevent heat from being removed.  The lower the thermal conductivity, the less insulation material is needed.

A complicating factor is that thermal conductivity in most materials is decreased as the temperature goes higher. In other words, the insulating material becomes less effective at wood-burning stove temperatures versus room temperature.

So when evaluating a material for suitability, check the thermal conductivity of the material at the potential temperatures to which it will be subject. This information is not always available but you can see from the chart below, it can make a big difference.  For example, although at 25C both vermiculite and ceramic blanket have a similar number, at 600C the ceramic blanket is much more effective.

Many kiln references will suggest 5-8 time more vermiculite, for example than if a ceramic blanket is used.

Thermal Conductivity of High Temperature Insulations

Thermal Conductivity of High Temperature Insulations


Form Factor and Strength

Some materials on this chart are loose and must be contained in some way; for example, exfoliated vermiculite or perlite. Some come in a particular shape and can only be cut (vermiculite board, ceramic fiber paper, and calcium silicate board). Others can be molded by the user and dried or cured in place (clay slip with perlite added). Each of these form factors may have a place in your design.

A word about Perlite

Many rocket mass heater designs include a recommendation of clay slip with perlite. Clay can tolerate the temperatures created by an efficient wood-burning stove. However, it has high thermal conductivity (low insulation value). Adding the perlite (which can also tolerate high temperatures) makes the end result insulative.  The clay dries and keeps the perlite fixed in a particular shape.

Types of Perlite

There are two types of perlite, massonry and horticultural. What is the difference between the two? Masonry perlite coated with silicone, which keeps water from getting trapped inside the perlite. Horticultural perlite is used precisely for the purpose of storing extra water; it does not have the silicone coating.

Consequently, masonry perlite is recommended for applications that will be exposed to water, such as when mixing with clay slip or it is used outdoors. Water trapped inside the perlite, when heated could theoretically cause steam and ruptures.  Having said that, people who have more hands on with this issue than myself at the Donkey32proboards indicate they have not had issues using the horticultural perlite. So I guess I will just leave it there.

While it possible to treat vermiculite to resist water intake, it is not as commonly found as perlite treated the same way.

Wood Ash

While it cheap and available it’s insulating properties derive primarily from the trapped air. But since there is no structure to maintain the trapped air it tends to settle and loose effectiveness.  If you want to mix it with clay slip,  I think the sawdust burned out of the slip would perform better, but I could not find in thermal conductivity test data to support any ash options. Perlite and Vermiculite are relatively inexpensive and offer better micro structures for insulation, it does not seem worth going with a sub par option.

Fiberglass and Rock wool

…have binders in them that limit their “working” temperatures. I could find no exact numbers showing this since. Rock wool does serious off -gassing at 400F. (not to be done inside) Check the MDS for the material you are using.  Fiberglass should just be eliminated for consideration almost everywhere. I have seen it used around burn tunnels and heat risers, and as you can see from the chart its thermal numbers are way too low.

Rock wool, even though it has a high melting point, is never placed in extreme heat locations, only as a 2ndary insulator behind fire bricks. I suspect this is due to deterioration from the binders used in its manufacture but I don’t know for sure.

All the numbers shown are from various manufactures data sheets. Check out the data sheets for any product you are considering. The numbers can vary from the charts on this blog a lot. Also be careful to compare apples with apples. Some vendors report thermal conductivity using btus and other Watts. It matters, they are not the same!


Wood Heat Storage – Flues vs. Bells

Users often want to capture excess heat from burning wood and then have it gradually released later, overnight for example, when no one is tending the fire. One of the simplest ways to do this is by heating a significant amount of thermal mass (water, clay or “cob”, brick or stone), from the exhaust after combustion. There are a number of schemes for this and how much heat you can store is dependent on both the materials used and how the heat from the exhaust is transferred. I am going to leave the materials issue for a different article.

When using solid forms of thermal mass such as clay, brick or stone, there are two basic approaches to the passive capture and storage of heat from wood burning exhaust. One approach uses flues, another chambers or bells.


The most common approach for heat capture is to use flues. Using flues, the hot exhaust from combustion is given a circuitous route through some form of thermal mass (clay, stone, or brick). The tricky part is that the path must be long enough to allow sufficient time for the hot gases to transfer their heat to the surrounding mass, but not so long it loses too much heat and velocity, causing the stove to stall. Many masonry heater designs rely on this approach, as do most “rocket mass heaters”.

In the case of masonry heaters, the exhaust is routed through masonry lined flues. Often these flues or channels are larger than the exhaust chimney to allow additional time for capture of their heat, but they are still considered “flue” designs since all the gases move together.

In the case of rocket mass heaters, the exhaust is routed through steel pipe that is matched in size to the chimney exhaust and is typically covered with “cob” a clay based building material. This is the heat capture technique developed and outlined in the book “Rocket Mass Heaters” by Ianto Evans and Leslie Jackson.  The gases heat the pipe which, in turn, transfers the heat to the cob where it is radiated back into the room.

Rocket Heater


An alternative to the flue approach is the use of chambers or bells. A specific version of this approach is called “Free Gas Movement”. A lot of the basic research was done by V. E. Grum-Grzhimailo (1864-1928) in Russia in the early 20th century. Subsequently, Igor Kuznetsov has been developing and implementing masonry heaters using chambers also in Russia. He has also written about the physics of gas movement to maximize heat extraction and put much of his findings in the public domain.

In a bell system, the exhaust is routed into large chambers where the gases are allowed to collect. They will then, by process of physics stratify by temperature, with the hottest gases being at the top and the coldest at the bottom.  The exit point for the chamber is then always positioned at the bottom so that only the coldest gases are removed and the hottest gases remain. If two or more chambers are put in series, the hottest and coldest gases for each chamber will be successively cooler.

Double Bell

This approach has a number of important advantages.

Hot Gases are not swept out with cold gases

In a flue based system, both the hot and cold gases are intermixed and carried at equal speed to the exit. By allowing the gases to stratify, only the colder gases are being evacuated and the hotter ones are trapped and remain in contact with the thermal mass until they have cooled.

Prevents damper induced rapid stove cool off 

Because flues sweep all the gases together, if the damper is not closed “in time” the remaining hot gases are swept away along with in residue heat in the flue. With bells, the hot wood gases collect and cannot escape until they have cooled, preventing rapid stove cool off from a damper left open too long.

Gas velocity losses reduced

As gases move through flues, they develop drag. Each turn creates even more resistance reducing the chimney’s ability to pull the gases out. Too many turns or flue runs which are too long can result in a stalled and failed heater.  Conditions are not always uniform, so when designing a flue system a “draft reserve” is needed to insure proper stove operation. The problem is that providing for additional draft margin, often means compromising on heat extraction capacity.

When heat extraction is done via bells, the travel distances and directional re-routing of gases is minimized, allowing heat extraction to take place without large frictional losses. Gravity separates the hot and cold gases without introducing any form of drag on the chimney’s draw.

Improved performance during prolonged firing

In a flue approach, the longer the stove is run the hotter the flue walls become, decreasing their ability to absorb heat. However, a second chamber (or bell) will always be cooler (than the first one) and thus allow better heat extraction.

Faster removal of ballast gases

Exhaust gases from burning wood are comprised of those gases which were part of the combustion process and those that were merely heated by proximity to the combustion. Gases that do not directly participate in the combustion are called “ballast gases”. For example, nitrogen, which comprises approximately 80% of atmospheric air, is a ballast gas. Ballast gases are not as hot and cool off quicker. In a bell system where gravity naturally separates the temperatures this allows the ballast gases to be removed 1st, providing more time for the higher temperature gases to transfer their heat to the thermal mass while not slowing down the overall gas velocity. If all gases are expelled at an equal rate, as in a flue system, this is not possible.