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

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  1. Great articles, good stuff. Seriously considering building this model, I love it!!. A few questions:

    Would it not be much simpler/less costly to let the inside flue liner in the first bell crack? It won’t be visible and it would appear performance won’ t be adversely affected. Let me know your thoughts.

    Are the fire clay bricks absolutely necessary? Don’t understand the use for them, as it says “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.” Decoration? To hang pictures?? Would you please explain.

    Also what do you recommend for the foundation (ordinary concrete slab?). What are you using to cement the bells together (ordinary mortar, or something else, where can this be purchased?), and lastly, where can one obtain the bell tops?

    • Ryan

      If you let the flues crack on their own, they will crack in multiple locations, seriously compromising stability of the stack. It might be okay, but personally I wouldn’t do it, it doesn’t take that long to cut them and put the gasket in.

      Regarding the fire clay bricks, you need something to create a gas tight channel between the heat riser and the 1st bell. As you see from part 4, we reduced the use of the bricks to only this channel and we used 1″ ceramic blanket to protect the flue liner from the high heat from the heat riser. This works better, since it pushes more of the heat into the bells, rather than into the heat riser flue. You could even use left over flue pieces with fire clay to make this seal, if you don’t have fire clay bricks.

      An ordinary concrete slab should be fine. The flues are put together with fire clay. It works like mortar but when fired becomes rock hard and will take any heat.

      We will be offering bell tops and a complete kit of the gasket and blanket material soon.

      Thanks for your enthusiastic questions

  2. Fantastic! Let me know please when the complete kit comes out, i will be purchase one for sure.

    As mentioned in another comment, the flues are so much better aesthetically than the barrels.

  3. How much space is between the top of the riser tube and the cap?

  4. Maybe a stupid question, but could you use stainless stove pipe for the heat riser lining and fill the space around it with vermiculite? Would that work? Would solve the pass through issue from riser to bell and from bell to bell? Or am I missing something obvious?

    Considering fabricating one of these in an outbuilding and like this look. I have the means to seam my own pipe of any size, so it would be a simple matter for me to fabricate the flue pieces. Kind of a hybrid of this design and a cob bench perhaps?

    • I am not clear on what you are asking. Are you suggesting replacing the clay flues with insulated stainless steel?

      • Yes. Sorry. Long day. I realize that I may have not been as clear as possible.

        And using pipe to make connections between the bells and from the heat riser to the first bell? Pipe combined with castable refractory mortar perhaps?

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  6. Douglas Pohl says:

    keep the content coming – good ideas!

  7. Bernadette says:

    Thanks for these posts… gives some real fuel for thought!


  8. This site is a must for all researching masonry heaters. I can’t complement you enough.
    I have a question. If it has already been covered please forgive me.
    With regard to the use of flexible stainless flue to create the bell cavities and flue ways. Is there any reason why thermal mass should not be provided by encasing the flue with conventional brick work.
    Providing an expansion gap with stainless scrubber sponge between the brick mortar and the flue.

    I already have a flexible flue chimney from an old oil fired Aga which is encased in bricks as it passes through an airing cupboard – works fantastically as a radiant heater.

    Stainless flue is easily found second hand as are old bricks. Might be a solution for those on a tight budget.

    Again brilliant site.