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

Posted by Sandy Mathieu on 4th Aug 2013

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″ x 36″
  • Drafts well even in summer
  • Low CO emissions
  • High efficiency
  • Captures all of the heat
  • Can be constructed in a day
  • Inexpensive to build