Derrick – 8″ Rocket Masonry Heater – Part 2 Data Log 3 Hour Burn

Here is a short video of the construction with pictures of the data logging equipment hookup.


This stove worked well and considering its output and inexpensive construction it definitely has a place. Overall efficiency was an even 90 Percent, with average CO emissions of 778 ppm. These numbers would beat almost all the commercial stoves on the market. The maximum stack temperature was 175F, which means it is doing an excellent job of heat extraction. 6 hours after the last wood was added the surface temps at the top were still over 130 and bricks over 220. The heater was sitting outside, so convection losses were probably higher than those inside a building.

This is design offers complete heat capture of a highly efficient combustion system in a small foot print which can be easily assembled and disassembled and is inexpensive.

Exit Temperatures from Masonry Rocket Heater

Exit Temperature

Exit Temperatures

As you can see it took well over an hour to reach target exit temperature of over 140F.  For the 1st hour the temps are very low. So be careful about your chimney. Low exit temperatures with a large thin walled chimney located outside, could result in stove stalling. We ran this with an 18ft insulated chimney.

8" Derrick Rocket Heater Efficiency and Emmssions

Efficiency & Emissions


Efficiency and Emissions

As you can notice the efficiency at the beginning of the burn do not seem correct. I believe this is the by product of ballast gases being flushed 1st, while the ombustion gases collect at the top of the bell. It is not until the burn is well underway that these equal out. But as you can see it burns at a very nice even high efficiency.

The spikes in the co correspond to wood loads. As the fire burns down the CO number increase until a new load of wood is applied. The emissions levels also are generally lower as the system heats up. Other than the fuel loading spikes, you can see the back end of the burn is much lower than the 1st.

Heat Capture -Temperature Logs

One of the factors that makes this such and effective design is that firebricks store heat 7 times faster and retain twice as much heat as clay flues or cob. This means we need only half as much mass and fewer hours burning to store the heat as with a cob or clay flue based approach. The logs are shown in 2 parts because the computer locked up midway.

Dragon Rocket Heater Tower Temps Air Temperatures

The most surprising number on this chart was how hot the top of the Heat Riser Tower became. The sensor was over 20 inches from the top of the heat riser. It takes about 30 min. for the temperatures to really start rising.

8" Dragon Heater Derrick Thermal Mass Log

Brick Temperatures

The thermal stratification in the system can be easily seen in the Brick temperatures. These bricks can be heated up to 2000F so they can absorb a lot of heat. As you can see the temperature increase was fairly constant at 100Degrees/hour for the middle bricks and 200 degrees/hr for the upper bricks.

8" Derrick Rocket Heater Data LogSurface

The radiating surface area of this stove is way over 80 sq. ft. The temperatures are in the range of radiators, except for the very high locations, such as the barrel caps and top barrel. The red line is about 4 ft high. These low surface temperature makes for a safer stove. The large radiating surface are provide a lot heat evenly radiated.

Dragon Heaters 8" Derrick Heat Loss 1

8" Rocket Heater Heat Loss hours 4-6

Heat Loss Temperature Log

Our design goal for this stove was to be able to radiate heat for 2x as long as the stove was burned. So we monitored heat loss for 6 hours after burning. The charts are in 2 parts because the computer locked up in this second series. So there is about 2 hours of data missing in the middle.

Notice how much quicker the heat is lost from the cap (the purple line), versus the bricks. This is probably due to the fact that the system was outside and the caps would be exposed to more wind and air movement, and thus convective heat losses. Also notice the nice even surface temperature from the barrel.

After 6 hours the system has lost a lot of heat but the top barrel is still at radiator temperatures of over 130F.


© Dragon Heaters 2013, All rights reserved.



Derrick – 8″ Rocket Masonry Heater – Part 1 Description

8" Dragon Masonry Heater - Rocket Heater for Greenhouses, Shops and Barns

8″ Dragon Masonry Heater – The Derrick

The Derrick is designed for larger spaces where looks are not a criteria, such as greenhouses, barns, and shops. Like the Castle Build, all the heat is stored into thermal mass (firebricks). Exit temperatures did not exceed 170°F. Its main features are:

            • It can be easily assembled and disassembled
            • It uses steel drums and firebricks rather than chimney flue liners for construction
            • Less expensive materials than the Castle Build
            • The heaviest individual components are the steel drums
            • It has a small foot print
            • It stores 50K-60K BTUs per hour of burning
            • It radiates about 18K – 20K BTUs an hour
            • Maintains lower surface temperature at the height of people, than a rocket stove barrel.  Temperatures are more similar to radiators rather than cast iron stoves, for safer and more even heating


This build is similar to the Castle build, except there is only one bell or chamber. The two black barrels house the Dragon Heater Core; we refer to the stack as the “Heat Riser Tower”. The set of 2 red drums and 1 black is the “Bell Tower” where the heat is captured. The connecting passage between these two towers is called the “Exhaust Collar”.

The wood is loaded vertically into the 8″ rocket heater core (brown box) where it is burned horizontally through the burn tunnel and then up the heat riser. The exhaust is routed via the collar to the Bell Tower. Inside the bell, hot gases naturally rise and the cooler ones sink and exit the Bell Tower at the bottom via the chimney pipe.  Although the gases can be over 800° at the top of the bell tower, the chimney exit temperature did not exceed 170°F.

Heat Riser Tower (Black Drums)

Dragon Rocket Heater 8" Core in Hardiboard Container

Dragon Rocket Heater 8″ Core in Hardie Board Container

As in the Castle Build, the Derrick’s 8″ heat riser is insulated on all sides. The sides of the barrel are lined with ceramic fiber blanket for insulation. This way,  more of the heat goes into the Bell tower and is not lost through radiation.

Dragon Heater Derrick Heat riser top view

Exhaust Collar

You can see the collar at the top of the Heat Riser Tower provides a path for exhaust into the Bell Tower.

Dragon Heater Derick Collar Side View

Closeup of collar from the side

View from Heat Riser to Bell Tower before adding exhaust collar

View from Heat Riser to Bell Tower before adding exhaust collar






Dragon Heater Derick exhaust collar viewed from Bell Tower

Dragon Heater Derick exhaust collar viewed from Bell Tower

Rocket Heater Masonry Build using Firebricks

Top view of the inside of the Bell Tower


Bell Tower

The Bell Tower shown in the images with the 2 red drums and 1 black one, is filled with stacked firebrick. There are openings at the bottom for the chimney exhaust and on one side to connect to the Heat Riser Tower. Otherwise, the entire height of 3 barrels is filled with firebrick.

55 gallon Drum Barrel Cap for Rocket Heaters

55 gallon Drum Barrel Cap

Barrel Caps

At the top of each tower we have replaced the metal lids of the barrels with caps  made from 3,000°F heat-absorbing refractory. These refractory caps serve two purposes.

First, the caps store heat; when firing, they get to be over 600°.

Secondly, since the air temperature at the top of the heat riser tower can be ~1,200°F for extended periods, the caps will handle the heat better than steel during repeated firings.

Without these caps, the underside of the steel tops should be insulated with ceramic fiber  blanket, particularly in the heat riser tower.

Next post will be the data logs from our three hour test fire.


© Dragon Heaters 2013, All rights reserved.