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

Posted by Sandy Mathieu on 8th Aug 2013


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