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

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 


6” Dragon Heater Bell Design – Build Part 2

After several days of testing, we have concluded that the heater needs to be re-designed. What we built does not facilitate enough heat capture. Because the exhaust is presently directly under the barrel, in the barrel supports, too much of the exhaust from the barrel was moving into the stove pipe without lingering and stratifying in the bell. Attempts to pull exhaust from the floor of the bell through the use of stove pipe elbows inside the bell resulted in poor stove pipe velocity and extensive back smoking.

The New Design

We are going to re-locate the stove pipe exit from the barrel supports to the bottom right hand side at the feed tube end of stove. This will require the exhaust from the barrel to travel to the other end of the bell before leaving. The additional time in transit should providing more opportunity for the “free movement of gases” to occur and stratify the temperatures better.

The stovepipe exit will be changed from a 6″ round to an over sized, 12″ x 4″ rectangle to 6″ circle adapter, placed ½” above the floor. By going to a wide, shorter shape closer to the floor of the bell, exhaust gases drafted into the stove pipe will come from the coolest portion of the bell. By increasing the cross sectional area for this drafting, stove pipe velocity should remain strong.

We are hoping for an exhaust gas in the 170° – 225°F range. This design would then offer all the benefits of a traditional rocket mass heater, without the larger footprint requirement of benches or other types of mass.

Other points of interest

Data Logger used for test result on 6" Dragon Heater Bell Build

Data Logger used for test result on 6″ Dragon Heater Bell Build

This was our 1st time to breakout our various data loggers and Gas Analyzers. So we had a fair bit of learning curve on them, including a number of lost data logs :<. So I do not have lovely logs to publish here. We look forward to providing detailed logs for the new design.

It was fun to see the heat profile of all the different parts in action. Here are some of the highlights.

Burn Tunnel Temperature

Dragon Burner - Burn Tunnel Sensor Locations

Dragon Burner – Burn Tunnel Sensor Locations

We placed 2 probes in the burn tunnel, one just after the tripwire and another located where the heat riser joins. The junction between the heat riser and the burn tunnel always had the highest numbers, by about 50-100°F.

Most of the burns had sub-optimal chimney arrangements, even still the temps at the burn tunnel typically ran 1500-1600°F, with the odd rush well into the 1700′s. It will be interesting to see if the new design drives these numbers higher.

Heat riser and Barrel

Dragon Burner - (rocket heater) Heat Riser Sensor Location

Dragon Burner – (rocket heater) Heat Riser Sensor Location

The top of the heat riser at full burn might be 1200°F or so with 800°F being loss to radiation from the barrel into the surrounding space.

The roof of the bell then should theoretically been in the 350F range, but instead was only around 250°F, and the floor maxed out at 160°F. The fire bricks heated up to 230°+ in the roof of the bell. Three hours after the fire was out this number was still 180°; the CMU wall stayed at 120°F.

The cast refractory barrel supports also absorbed a LOT of heat and stayed very warm for several hours.

6″ Dragon Heater Bell Design – Build Part 1

We are building a  traditional rocket heater style build, but with an single bell underneath the burn tunnel. This should provide a lot of heat capture without increasing the footprint of the stove. It is a bell heat capture and not a flue heat capture approach. (you can read more here about the differences between the two approaches)

Dragon Heater  6in w Bell

Exhaust Path

The exhaust is identical to a standard J-tube heater shown here, except that instead of exiting the back of the heater, it continues to drop down to a “bell” (chamber)  below the burner.  Because gravity and pressures naturally sort out the hot from the cold gases only the colder gases leave.

6" Bell with barrel supports made transparent, Click to view larger

6″ Bell with barrel supports made transparent, Click to view larger

Bell Components

OLYMPUS DIGITAL CAMERAThe cold gases are exhausted to the chimney from a few inches above the floor in the middle of the bell. We used a piece of stove pipe with an elbow facing the floor. The entire bell chamber, except the floor,  is lined with cast refractory or firebrick. Half size bricks are mortared to the CMU walls and 3″ full bricks cover the ceiling, where most of the heat is collected.

Between the fire brick and cement filled CMU bricks there is a lot of thermal mass for heat collection.  We considered using 2 chambers underneath, but there is not that much room by the time the bell is lined with firebrick, so we opted for a single chamber.

Design Features

I have included 3 2″x4″ steel tubes that run through the top of the bell area to both support the fire brick ceiling and provide an avenue for experimentation by block and opening the channels. The steel tubing is wrapped with 1/8″ ceramic fiber gasket material.  A piece of expanded metal painted with high temperature paint is laid on top of the steel tubing.




Materials and Construction

Construction techniques are pretty similar to the YouTube video on the 4″ build. (Yes we will have a video on this build.) The combustion system is based on the 6″ Dragon Burner. We used the 6″ Barrel Support with Bell Chamber, and the 6″ Steel and Gasket Kit. Other materials included generic 4″ thick CMU blocks for the main construction, fireclay bricks for the bell lining,  mortar, fire clay, and a 55 gallon drum.



Tiled 4″ Dragon Heater Build

We built this Dragon Heater in less than a day.  Of course the color of  the stove-pipe and barrel can be changed, as well as the tiles used. You can use the sketchup files to try out many different looks to find the look that matches your decor.  It is an easy build that anyone can do.

Here is a video to the step by step video of building of this heater.


This is a great stove for quickly heating up a shop or other space that would typically be heated by a cast iron stove. It does not have any integrated thermal mass, so if you are wanting delayed heating you will need to either add a heated bench or consider the plans that include an integrated bell chamber below.

Here are some shots from the Sketchup plans used to build this heater. A complete step by step video of this build is also included with the plans. Plans are included free with the purchase of the barrel supports or steel and gasket kit.

4intile1 4intile2 4intile3

Dragon Heater 4in Plan