6″ Dragon Burner Masonry Heater using chimney flues – Part 5

Here is the masonry heater with stacked stone applied.

Dragon Heater Castle Build - Rocket Heater without a barrel

Dragon Heater Castle Build completed with stacked stone. (Click to enlarge)

The feed tube box is rose wood. The caps are cast refractory that have been painted with high temperature – Rich Metalic Brown. The steel top plate and ash clean out door were painted the same color.

Frankly the pictures don’t do it justice. We were very pleased with how it came out, but we had a hard time photographing it. Hopefully we will get better shots in the next few weeks.

We left the backside open so we could show visitors how it is built, so here is a shot of the back. Kits will be available soon!

Dragon Heater Castle Build showing how the stone and chimney flues relate

Dragon Heater Castle Build showing flues


Outdoor Kitchen – Part 1

Rocket Heater Outdoor Kitchen

Click to enlarge

We are gearing up for our next build, an outdoor kitchen, shown to the left. It facilitates a wood fired white oven, cast iron cook top, and space heater.

This model will use the 4″ Dragon Burner as the heat source. We may have to upgrade to the 6″, but the 4″ has a shorter heat riser, making it a better match for height on the cook top piece.

It will use 24″ x 24″ chimney flues, covered in stone for fast construction. The oven and oven door we will custom make for this model.

It will have 2 bells, the top one is lined with 2″ ceramic blanket for heat retention. Exhaust enters the upper bell from under the cook top and is exited in the back bottom of the bell. Here is a view with the side transparent.

Dragon Heate Outdoor Kitchen sideview

Click to enlarge

As you can see the oven has 2 integrated tabs at the back to support and secure its location. There is lots of room for the hot gases to circulate around the oven. The oven is 12″x12″x14″ deep, and allows shelves for cooking multiple dishes.

Topview showing oven suspended in 1st bell

Click to enlarge

The lower bell has no lining and is designed to radiate heat to the surrounding space, especially through the arch shaped panel.


The cook top will have an optional bypass, so that all the heat can go to the oven if desired.


Copyright 2013 Dragon Heaters


6″ Dragon Heater Bell Design – Part 3

We did some brief testing of this design, but did not really like the drafting. To be fair it was still very hot, 95F. It did work well in terms of lowering the exit temperatures in increasing the bell heat storage. However, it did not draft near as well as the chimney flue build. So for the moment, we are tabling this design until the fall for more realistic test.

6″ Dragon Burner masonry heater using chimney flues, part 4


As we noted in the conclusion of Part 3, the flue liner which was at the top of the heat riser reached temperatures over 350F. This degree of heat could cause problems with the mortar or other materials used in skinning the heater.

We want users to be able to apply tile, plaster, or stone to the exterior of the flue liners to improve their looks. Consequently, this test is to see whether replacing some of the fire clay bricks with ceramic fiber insulation would bring the temperature down.

The Old Version

The area of concern is the inside of the 13″ x 17″ flue liner which surrounds the heat riser of the Dragon Burner. Here is what it looked like for the results in part 3. As you recall, the board is there temporarily to support the brick at the top of the opening to the first bell.


The New Version

You can see in this image that all of the fireclay bricks except the ones around the opening into the first bell have been removed. Replacing them is a 1″ thick blanket of ceramic fiber. This material has extremely low thermal conductivity which means it is an excellent insulation material. It is very quick to install and is stiff enough to stay in place without any fasteners.


The mortar which is binding the flue liners together and the fireclay bricks onto the flue liner is actually premixed fireclay. It forms a gas barrier. That is why we did not remove the bricks which line the opening to the first bell.

The Results

The outside of the heat riser never exceeded 185F. A much better number. It took just short of 3 hours to achieve that temperature. So it was a nice slow warm up. If you wanted it to be even cooler a layer of insulation could be placed below the cap on the heat riser.  This test left the heat riser warming the cap directly.

The other 2 bells had max temps of 165 for the 1st bell and 138 for the 2nd.  The chimney exit temperature also increased about 20 degrees overall, which we were hoping to see. For most of the burn the exit to the chimney temp was 150-160F.

Outside temperature of all 3 towers

In the next graph you can see that 6 hours after the fire, the inside of the 1st bell was around 135 F, the outside 120 F.  The ambient temperature was very hot, tipping over 100 F. The dip and rise was from stirring the coals a bit.

In effect the heater works like a giant radiator,with  approximately 60 square ft of surface area, it is the equivalent of  12 medium size cast iron radiators, still pumping heat 6 hours after fuel was loaded.  (Radiator surface temps range from 112 – 200, 140-170 F is a common design range).


Dragon heater castle build bell temperatures 2-6 hours after fuel

Bell temperature 2-6 hours after fuel


Click Here to Continue to Part 5

6″ Dragon Burner masonry heater using chimney flues – Part 3

The re-design of the flue solved the thermal stress issues and improved both the thermal curve and exit temperatures.  The temperature profiles and Testo numbers are included in the charts below, but showed low stove emissions and high efficiency.

Testo Gas Analysis

Here are some charts from the Testo 330 Gas Analyzer. We only ran it for 90 minutes of the 3 1/2 hour test. The analyzer is not really designed for long test periods. The filters and readings drift and go wonky after a while. We start all charts when the O2 number drops below 20.


The 1st chart here graphs the increase in negative pressure vs the stack temperature. Starting at ambient, (around 94°F) through 155°F or so towards the end, you can see what a big difference the temperature differential makes to the drafting.

Dragon Heater ( Rocket Heater shippable core ) Efficiency and Emissions for 6 in. Flue Build

Dragon Heater Flue Build #2 Efficiency and Emissions (Click To Enlarge)


CO was impressively low. Its carbon monoxide levels were low from the very start.  Cooking on a gas stove is an exposure rate of 100 ppm, and smoking a cigarette is 400-500.

Efficiency - Once again even the efficiency numbers were very good from the very start, over 92% efficient and never dropping to even 85%.

Dragon Heaters are exempt from EPA requirements. It is considered a constant burn stove because it has no option to reduce the levels of oxygen. Many cast iron stoves attempt to control the heat by controlling the amount of oxygen allowed. However, it’s performance would clearly exceed even the most stringent efficiency requirement of 68% efficiency in catalytic converter stoves.  Masonry heaters require 58% efficiency. It is not unusual for plain old cast iron stoves to only operate at 25% -30% efficiency.

Dragon Heater ( Rocket Heater shippable core) Oxygen and CO2 data, Flue Build

Dragon Heater Flue Build #2 Oxygen and Carbon Dioxide (Click to Enlarge)



As you can see in the chart of all temperatures, the fire was allowed to die down at about the 9000 second mark, about 2.5 hours into the burn. We left the temperature probes on until the 4 hour mark. So the last portion of any of these charts is after the main wood has been consumed.

 Outside surface temperature of both bells

In the chart below you can see that both bells have a similar surface temperature even though the 1st bell has an additional column of flues that the heat must migrate through before reaching the surface.  After the fire is gone, the 1st bell outer surface continues to increase in temperature as the heat from the inner flue migrates to the outer flue surface.

Rocket Heater Masonry Heater built with Dragon Burner and chimney flues

Surface Temperature of the top of the Bells

Temperatures inside the 2 bells

Temperatures inside the 2 bells show that the bottom readings of both bells is fairly close to each other and the exit temperatures.  The 2nd bell continues to collect heat from the slower draft of the dying fire.

Temperature chart of a double bell masonry heater built with 6" dragon burner

All Temperatures

This final chart shows the temperatures of the heat riser vs all the other probes. You can see how much the heat is dispersed across the 2 bell resulting relatively low temperatures everywhere except the outside of the heat riser.

Chart of double bell masonry heater built with a dragon burner and chimney flues

Shows the temperature as it leaves the heat riser versus various locations in the stove. (click to enlarge)



We think the exit temperatures could be a bit higher, but we were running this at over 93°F ambient, not exactly cool weather. Draft should increase during more realistic winter conditions.

The temperatures outside the heat riser column rose to over 350°F even with the firebrick lining. It was not logged well because the adhesive on the tape holding the thermocouple failed and it would not stay on the flue.

In order to lower the temperature on the flue surrounding the heat riser, we will do an additional revision. We will change most of the interior liner from fire clay brick to insulation. Making this area cooler will  to allow for skinning the stove in tile or stone.  It will also force a bit more heat into the bells, which have plenty of capacity for additional heat.  With the change to insulation around the heat riser exhaust, we are hoping to push the exit temperatures a tad bit higher.

In general, we were pleased with its performance and think it is a winner. It

  • Small footprint 30″x36″
  • Drafts well even in summer
  • Low CO emissions
  • High efficiency
  • Captures all of the heat
  • Can be constructed in a day
  • Inexpensive to build

 Click here to continue to part 4

Copyright 2013 Dragon Heaters

6” Dragon Burner masonry heater using chimney flues, Part 2

Masonry Style Rocket Heater with 6" Dragon Heater Burn Tunnel

Completed Rev 2 Masonry Dragon Heater

Our redesign on the masonry heater using chimney flues included two major changes.  As you may recall from the end of Part 1, we were thinking of lining the first bell with fire clay brick in order to store the heat and keep the flue liner from cracking. Peter van den Berg suggested that we put a smaller flue liner into the larger one instead. Each piece of the smaller flue liner was also split on one side to allow for thermal expansion without cracking. We cut the smaller flue liner vertically using a diamond blade in an angle grinder and then inserted a ceramic fiber gasket into the cut. No strapping was used to hold the piece together.

Although the firebrick approach would probably perform a bit better, it would be a great deal more time consuming and expensive to do. Our goal is to design a heater which delivers most of the performance of a full masonry heater, but can be built quickly and inexpensively. The performance data of this build are very good and are covered in part 3.


Like the first build, the tallest bell is 7’. The second bell is 6’. These two bells are 17” x 17” on the outside. The 13” x 17” flue containing the heat riser is 4’ 8” tall. The burn tunnel flue liner has been reused from the 1st build and is 14¼” tall.


Foundation for Rocket Heater Double Bell Masonry Build with Chimney Flues

Foundation for Bells, 1st Bell is to the right. opening on the left is for ash clean out, the opening at the bottom is for a 12″ x 4″ to 6″ chimney adapter
(Click to Enlarge)

Notice that the opening between the first and second bells is one foot tall here at the bottom of the heater. So, the gases go up the first bell to the top, cool and fall down, then do the same in the second bell before exiting to the chimney.

Rocket Heater Masonry Build using Chimney Flue Pipe - Inner Flue

This shows the liner put into the 1st bell.  (click to enlarge)

The 2nd bell does not need a liner. Bottom right square. (Click to enlarge)

Below  is the burn tunnel-heat riser stack. The 17″ long side of the heat riser flue lines up with the first bell. (The burn tunnel is off-center; it will be fixed later.) Premixed fireclay has been used to seal the openings between these two pieces of flue liner.


Double wall flue liner bell for rocket heater made with dragon burner

Here you can see the ceramic fiber in the side of the inside flue liner.


 Rocket Heater Heat riser opening on Dragon Heater Masonry Build

This shows the top piece of the heat riser section, where the exhaust will enter the 1st bell. Notice the liner flues in the 1st bell. (click to enlarge)
(click to enlarge)

Heat Riser Exhaust

In the first bell,  both the inside and outside flue liners are cut at the same level. We need plenty of space for the gasses to flow from the heat riser into the bell.

The inside flue liner has fireclay on it and is ready for the next layer.

Notice the next layer has a slit with ceramic fiber in it on the opposite side and a huge cut to let the gases into the second bell.



Inside the bell on the left is another piece of 13” x 13” x 2’ flue liner with a slit and ceramic fiber tape. The 17” x 17” x 1’ pieces on the outside are not cut.

Inside the bell on the left is another piece of 13” x 13” x 2’ flue liner with a slit and ceramic fiber tape. The 17” x 17” x 1’ pieces on the outside are not cut.


Fire clay Brick

So, now most of the main structure is done. The goal of the next steps was to keep the

the temperature of the flue liner outside of the heat riser cool enough that tile could be adhered to the outside for decoration. In order to do this, we lined it with fire clay brick. As you can see in this image, a small platform was temporarily constructed inside the heat riser flue. The bottom of the bricks has to be below the top of the vermiculite boards of the heat riser (when in place).


These 2×4 studs support the OSB on which the fireclay bricks rest.

Several more bricks have been added to the inside of the flue liner. The ones at the top of the image handle the transition to the first bell.

The leaning board is supporting the fireclay bricks which are lining the opening to the first bell.

The heat riser is shown in this image. As you can see with the fireclay brick on all sides, it’s a tight fit.

Now the temporary scaffolding is gone and the perlite has been added for more insulation around the heat riser. The white string is actually a thermocouple.

Click here for Part 3

Montreal Becomes the 1st City to Ban all Wood Burning Heaters

The pollution problem from inefficient wood heating sources has become so bad that Montreal has issued a ban on all wood burning appliances by the year 2020. Currently only Pellet Stoves are being allowed.

Here is a link to article.

This points to the urgency of using only high efficiency wood heat.

6″ Dragon Burner masonry heater using chimney flues – Part 1

We were looking for a simple way to build a bell style masonry heater. This build does not use a barrel that is commonly used in rocket heater designs with J-tube style combustion chambers. We decided to try using commonly available chimney flue pipe sizes to construct a quick and dirty masonry heater using a dragon burner as the combustion chamber.

This approach we knew would be sub-optimal due to the materials in clay flue pipe, but decided to give it a try without lining it with firebrick, just to see how it did.  Below is a picture of the mostly assembled heater.

Although we will re-configure it for the 2nd round, we were overall impressed by its strong drafting through out the burn, its ability to extract most of the heat, and its dead simple build. Our target exit temperature of around 200F (to avoid condensation problems) was maintained pretty well.

6" Rocket Heater Built with Flue Pipe - Barreless Rocket Heater

6″ Rocket Heater Built with Flue Pipe and Dragon Burner – (click to enlarge)


The left column is 7 ft tall and the right 5. Each column is a bell, the larger bell is where the heat riser is and the is the 1st bell. Each block is a 17×17 flue pipe. The feed tube is in a 13×13 flue pipe cut down to size. Below are some pictures of the construction.

Rocket Heater Burn Tunnel inside Flue Pipe

Top View of Burn Tunnel in flue pipe

Rocket Heater shippable core inside chimney flue pipe

Complete Dragon Burner – Ready for insulation (click to enlarge)

rocket heater shippable core with insulation and ash plate

Insulation installed and ash top plate in position (click to enlarge)

Rocket heater masonry bell build joint view

Joint between 1st and 2nd bell, just above ash plate (click to enlarge)

We fired the unit for 4 hours to see what kind of heat build up we would get, if the draft deteriorated and what the external temps of the pipe would be after long firing.

About an hour into the burn there were some very loud pops. Which made us more than a little nervous. After a moment we could see that the top 3 pipes had cracked. The fissure stayed pretty much the same through the rest of the burn. According to the manufacture, this is typical. Evidently 90% of the chimneys have cracks, just no one sees them since they are covered.

There are 2 ways to not have this, one is to only heat the pipes at 50 degrees an hour, or two, line the chambers with fire clay bricks. We had intended to do this anyway after the plain test, so that will be our next test. The fire clay bricks should help the unit absorb more heat quicker, so it will be interesting to compare numbers with the next build.


Here is a chart of the 1st bell.

Rocket Heater temperature profile for 1st test bell

Temperatures for 1st Bell for 1st part of test (click to enlarge)

Here is a chart of the 2nd bell

Masonry heater with j-tube, temperature readings

Temperatures for 2nd Bell (click to enlarge)

Here is the combined chart

Masonry heater built with dragon burner j-tube temperature readings

Both 1st and 2nd bell temperature readings combined (click to enlarge)

Testo Graph

The draft number which are recorded, but do not show up in the graph went from .004 at start to a high of .213 in H2o. Most of the run was over .1.

I think there was a problem with the pump in the testo so I took it out, as you can see. As the fire is dying the numbers are not as good, as expected. I am still learning the equipment and hope to have even better charts in the future.

Testo 330 analyzer chart of 6" dragon burner with masonry bells from chimney flues

Testo 330 gas analyzer chart of “a lot” of the burn (click to enlarge)

As you see the stack exit temperature stayed pretty much around the 200F target until the end of the fire when they climbed.






Click here for Part 2 


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.