Installing Rocket Heaters Safely

Rocket heaters work efficiently, in large part, because they maintain very high temperatures 1,200°-2,000°F in two sequential combustion chambers. This helps insure that all the volatiles and most of the particles are consumed.  The first and hottest combustion chamber is also usually located close to the ground. This means it is critical that the burn chamber be extremely well insulated to protect the substrate from intense heat.

Our tests showed that the area under a 6″ burn tunnel with 2″ of perlite insulation can still be over 800°F. This temperature range is much hotter than the feet of cast iron stoves which are near the floor. Consequently, it is important to mitigate the heat from a rocket heater in a different way than traditional stoves, which only require a hearth pad.

Installing a Rocket Mass Heater

If you have your burn tunnel surrounded by a lot of clay cob like a traditional rocket mass heater, this mass helps absorb the heat. However, be sure that there is enough mass to accommodate your firing cycles and intensity. This issue is probably at the root of some buildings being burned down from rocket heaters.

Installing on Concrete

Concrete can work like the mass in rocket heaters by absorbing and conducting heat away from the site, throughout the slab. Depending on the size of the concrete area under the burn tunnel and what type of concrete it is, degradation may still occur. For example, if the concrete area under the burn tunnel is not very big, at a certain point, the heat begins to accumulate and raise in temperature.

One solution is to mount your burn tunnel container on a platform that allows an air gap between the burn tunnel and the flooring. Heat, which might otherwise accumulate to high levels,  is taken away via air convection.

Another option is to place a layer of 1″ refractory board on the cement, this will lower temperatures from above 800°F to below 300°F.

Installing on Wood

Wood requires more protection than concrete. Over time wood’s combustion temperature can be lowered due to prolonged exposure to infra-red radiant heat. So while wood may start at a spontaneous combustion temperature of over 400°F, this number can be halved due to prolonged exposure.

This danger applies equally to wood that is below an otherwise non-combustible top layer, such as tile and backer board. The non-combustible layers can accumulate dangerous levels of heat which are conducted to the plywood underneath. Over time, the spontaneous combustion temperature of the wood is gradually lowered and the heat accumulation from the tile layer can cause the plywood to ignite.

An air gap above a non-combustible surface can work if there is sufficient additional insulation below the burn tunnel container. The size of your burn tunnel should dictate how thick the insulation will need to be.  You should test your burn tunnel prior to installing to insure your wood is being exposed to temperatures no more than 125°F.

Standard Hearth Pads

Standard hearth pads are typically constructed of a non-combustible top board over a layer of mineral insulation. They are designed to combat radiant heat from a cast iron stove many inches away. A rocket heater is much hotter, usually much closer to the floor, and has larger conductive surfaces in contact with your floor.

They can be used as a part of your floor protection system, but don’t rely on them exclusively. For example, a hearth pad on top of some bricks or other spacers to allow air flow under the burn tunnel might be sufficient, depending on the size of your burn tunnel.

Testing

We recommend installation of a permanent temperature sensor of some sort to be installed under the burn tunnel next to the surface of the floor to monitor the heat level there. Compared to not knowing that your floor is getting too hot, the cost is minimal.

NOT SPECIFICATIONS

Be safe! This post is not meant to be specific installation instructions, but as a starting point for your own testing and verification if you are thinking about building your own rocket heater.

 

 

Rocket Masonry Heater Oven

Dragon Heater Masonry Heater Oven for Rocket StovesThe Dragon Heater masonry oven was tested with a 6″ Dragon Heater in the stair castle build. It  demonstrated;

  • Even Heating
  • Temperatures to 560F
  • Ability to maintain a given temperature
  • Quick recovery after opening the door

This oven has a 12V light kit, door and stainless steel rail for hanging an oven thermometer. It is also compatible with standard oven probes that go inside the meat or dish being cooked. It is 12″x12″x14″.

Dragon Heater Masonry Heater Temperature Chart

Oven Temperature Heating up and Cooling Down

Oven Temperature Profile

The chart to the left is starting the Dragon Heater cold. No more fuel was added to the heater once it appeared the temperature had stabilized at around 550-560F.  At this point it is  losing as much heat out the glass door as new heat is being added.

You can see two dips where the door was opened to see what effect there was on the temperature. As you can see the temperature drops more gradually than it increases. The dip on the way up was a lapse in fuel loading.

Maintaining a Stable Temperature

Next, we wanted to see if a partial loading of the heater would maintain a stable temperature. Once the temperature dropped to around 280F fuel was added. 1/3 to 1/4 of the feed tube was filled and the remainder of the tube was covered to prevent excess air.  The results are shown below. As you can see it leveled out pretty nicely.

Operational Considerations

Dragon Heater Masonry Oven holding temperatures

Sustaining Temperature at 280F

Like other wood fired ovens it produces an even heat, with heat coming equally from all directions of the oven. There are no hot or cold spots.

Because the heat originates outside the oven it is called a white oven. A black oven is where a fire is lit inside the oven.

© Dragon Heaters 2014, All rights reserved.

 

Painting Flues

This is a 4″ Castle Build painted with High Temperature Paint.  Using high temperature paint is an inexpensive way to finish off a castle build.

Dragon Heater Castle Build Rocket Masonry Heater

Solid Moss Metallic Green

Mini Rocket Masonry Heater with High Temperature Painted Flues

Moss Green Metalic with Mohave Red Caps

We added some red caps to give it a little punch. The flues can be painted a large variety of colors to give your heater a modern, rustic, or industrial look.

If you plan on painting to finish off your flues, you may want to take a little extra care which direction your flues face. As you can see the top flue of heat riser column to the right shows the production imprints facing out. This would like nicer if these faced away.

Small masonry heater made with rocket heater core and chimney flues

with red feed plate

© Dragon Heaters 2014, All rights reserved.

 

6″ Castle Stair Build

Dragon Rocket Masonry Heater, stair build

Stair build with oven prior to “skinning”

This 6″ Dragon heater is in a small 2 floor apartment, so space is at a premium. This build uses a tall single bell (8 ft)  lined with fire brick. Larger 20″x20″ flues were used to maximize heat capture and allow for the oven. The excess heat from the bell is radiated from the stove pipe as it exits through the ceiling. It takes a hour and half for the heat to start radiating, but it will then raise the room temperature of this 1400 square ft apartment about 1 degree an hour.

Fire Bricks lining Dragon Heater Stair Build

Fire bricks

The firebrick stores more heat and does it faster than clay flues. However, each brick must be cut and laid. So it takes more time and expense than the inner clay flue liner approach used with the other Castle Build. The larger flues used on this build are also necessary for sufficient clearance around the oven.  Even without a door on the oven yet, the temps stay 240-300F.

Next, we will finish the oven and add a decorative finish to the flues.

© Dragon Heaters 2013, All rights reserved.

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.

Overview

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

Overview

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.

 

 

4″ Castle Build Data Log

Small masonry heater built with 4" rocket heater

4″ Rocket Heater Castle Build Test

We were impressed with the power of this 4″ Rocket Heater built with a Dragon Burner. It is a little different from the 6″ Castle Build in that it has only one bell. It also uses stove pipe to radiate  heat into the space. The short stack on the right is the heat riser. The stove pipe is on the left. We were trying to minimize the footprint for smaller spaces and provide a mix of immediate and stored heat.

4 inch rocket heaters require very small fuel — less than one inch. However, if that works for you, it is an amazing heater for the size.

How The Heat Is Used

75% of the heat is stored into the Flues for gradual release and 10% is radiated into the room via the stovepipe. The remaining 15% is used to drive the chimney.

 

Bell Surface Temperatures

Rocket Heater Heat Transfer in a Bell

shown in minutes

The inside of the bell was over 400°F within an hour. It reached 536°F in the second hour. It took about an hour for the heat to show up on the skin of the outside flue liners. You can see at minute 61 the temperature finally starts gradually climbing. Despite some very high bell temps the bell skin stays at a nice sub 130°F surface temperature.  If you skin the flues with stacked stone or similar, it will take even longer for the heat to migrate, however you will be storing more.

Stove Pipe Heat Radiation

Stove Pipe heat loss on 4" Rocket Heater Castle Build

Heat loss from 9ft of Stove Pipe

Although the exit temperature into the stove pipe was higher, the stove pipe  up to the roof reduced the exit temperatures by over 140°F when the temperatures were at their highest.  9 feet of stove pipe were able to keep the exit temperatures under 210°F. You will also notice the greatest amount of heat was radiated into the room at the highest temperatures.

Heat Dissipation

4 Hour Rocket Heater Castle Build Heat Dissipation

4+ Hours

In this chart, you can see how the heat dissipated after we stopped feeding the fire. The blue line represents the inside of the bell, which still has some heat over 4 hours after the burn.

Option to store more heat

If you want to put more heat in thermal mass rather than in the stove pipe for immediate radiation you could do anyone of the following.

  • Add a second bell
  • Add a bench
  • Line with firebrick anywhere in the bell starting at the top. Firebrick will store twice is much heat 4 times as fast.

Click here to view the 4″ and other size rocket heater castle build kits

Copyright Dragon Heaters 2013  All rights reserved

 

 

 

Annoucing the Helix – A Horizontal Fed Rocket Heater

We are finalist in the  Alliance for Green Heat Wood Stove Design Challenge sponsored with Popular Mechanics. The contest has been under development for over a year and is set to come to completion on the National Mall in Washington DC. November 15-19th.

Our entry is based on another design developed by Peter van den Berg, the designer or our rocket heater combustion cores.  This heater works like a traditional stove with a door you can view the fire with and a batch loading of the fuel. However it has been married to rocket heater concepts to increase the efficiency of this approach. Several early versions of this heater have been built in Europe and a few in the US.  Below on the left is an example of one installed in a single bell. The photo to the right shows one burning.

Batch Fed Rocket Heater for small masonry heaters

Horizontal Fed Rocket Heater masonry heater. Installed in a single bell.
By permission Ouwehand & Molkenboer

Rocket Heater Masonry Heater Hybrid - The Helix

Burning Horizontal Rocket Heater with Door. By permission Michael Winjnja

 

 

 

 

 

 

 

 

 

 

Here is sketchup of our entry.

Wood Stove Design Decathlon Entry - The Helix

Helix stove being presented at the Wood Stove Design Decathlon

 

We have taken this design and optimized it’s flow using computer simulation to increase the already considerable power of this heater. The back of the firebox feeds into a very tall insulated heat riser via a series of specially designed curves and gates. The result is a combustion chamber with internal velocities around 900 mph with a pronounced double helix that serves to mix the gases during secondary combustion in the heat riser.  This was the starting point for our models. The final design is not being released at this time, but you can see how exhaust gases flow.

Helix air flow pattern of Horizontal Rocket Heater

Helix air flow pattern of Horizontal Rocket Heater

 

These heaters burn hotter, about 100k BTU’s an hour so it is easier to produce more heat faster. The big bonus is that you get to view the fire. On the downside they are much more expensive to build. The castings are much larger and the door and ash tray add to the expense. We will be offering these combustion chambers for sale in late November 2013.

Outdoor Kitchen – The Oven – Part 2

Wood Fired White Oven for Rocket Heater Kitchen

Here is the oven that we will use in our outdoor kitchen. As you can see it assembles in less than 90 seconds, yet ships flat. The joints will of course be filled with clay mortar when it is installed. Here is a quick video of its assembly.

The groves are for adjustable shelves. The interior is 12×12.

Rocket Heater White Oven

Dragon Heaters Hit the Road

GreenLife Aquaponics Training with Murray Hallam

Dragon Heaters Seminar and Demonstration

Dragon Heaters will be at a number of events around the country this fall. We look forward to seeing you there!

Seven-Springs

Will be demonstrating the castle build at the booth
Booth 501

MENFairLogoLawrence

SREDenver2013-3

Wood Stove Design Decathlon

Finalist in Wood Stove Design Decathlon

washingtonDc