Rocket Mass Heater: Batch Style

Rocket mass heaters (RMH) burn fuel quick and hot to produce a very clean burn that eliminates almost all smoke after combustion.

Many newer wood burning stoves can achieve similar results, however where those stoves fail is in retaining heat that otherwise exits your chimney. Functioning similar to masonry stoves, rocket mass heaters use a series of channels or bells to capture heat from exhaust gases in mass that can then radiate heat for hours after the fire has burnt out. This lowers exhaust temperatures by hundreds of degrees, but due the efficiency of the stove’s burn, creosote cannot build up to cause chimney fires. The result of these stoves are slow temperature releases that create long lasting heat for your home. Those familiar with Rocket Mass Heaters will no doubt be visualizing the J-style stoves that innovators like Ianto Evans, and Erica and Ernie Weisner have successfully promoted over the last twenty years. Other great pyromaniacs have joined the hunt for the most efficient wood burning stoves and in the last five years we have seen huge improvements to the science and design of Rocket Mass Heaters.

Because Spiritwood House is much larger than our first cob house, we opted for a batch RMH design that would produce more heat in smaller amount of time. Anyone interested in the specifics of batch RMH design or function should check out Peter van den Berg’s site http://batchrocket.eu/e which is where we received our stove plans for free. The batch RMH also allows a larger volume of wood to be fed into the burn chamber at once, eliminating the need to tend the fire of the traditional J-tube every 15-20 minutes.

For simplicity we won’t go into detail about the sizes or dimensions of the actual build. For that information I will again suggest visiting http://batchrocket.eu/e as the work has already been done in great detail. However, below is a photo tutorial of the steps we took to build our own wood burning stove.

Bottle Insulation

RMHs need a solid base and insulation to support the weight of the mass and keep heat from leaking down into the ground. We chose to experiment with using glass bottles as insulation. This technique is commonly used as an insulation layer for cob pizza ovens, but with such an easy access to empty bottles, we decided it was a good way to upcycle a waste product.

Below shows the supporting cement/perlite pad that will hold and distribute the weight of the cob bench. This bench uses nearly 2 yards of cob and when dry will weigh approximaly 4,000 lbs. The perlite replaces sand in the cement pad and adds insulation value under the bench.

Insulative Perlite Cement

4" perlite cement pad

measuring each row of brick

The main structure of the RMH is constructed with firebrick and is essential to withstand the high temperatures of a stove. As can be seen, this RMH employs a traditional square riser that will be within the metal drum of the stove. A round or octagon shaped riser has been found to be much more effective at mixing gases and increasing the efficiency of the stove. As can be seen in the image below, triangular pieces of firebrick were cut to create an octagon shape and create a smoother transition for the gases to move. This experimental design was first employed by Peter Van Den Berg in Spain and can be viewed here.

riser and first barrel

RMHs use bells or piping to distribute heat throughout a thermal "battery"--a large amount of mass that can slowly release the heat over an extended period of time. Here you can see the common HVAC piping used for moving the gases through our cob bench. Though the gases cool down rapidly as they move through the system, it is still a good idea to use caution when dealing with high temperatures and combustibles. We wrapped this wooden beam with fire blanket to keep it from overheating despite the large amount of masonry between the pipe and the wood.

full length of rmh piping

full piping and chimney installed

Stove Manifold

Perhaps the biggest problem area for RMH builders is the manifold of the stove where gases move out of the barrel and into the piping. At this point in the stove the gases need to move from a relatively large space to the small 8" diameter piping. Many failures of these stoves come from not giving enough room for the gases to easily move into the mass. Extra room should be provided for an easy transition. We created a large--1 cubic foot--space to move the gases from the barrel to the cob bench and added a removable cap to clean the system if necessary. These cleanouts may not be used anytime soon, but it a good idea to provide easy access to the interior of any stove so that blockages can be removed if they occur.

Almost Complete Bench

main bench finished and cleanouts

The secret to a high performing and clean burning stove is to add the correct amount of oxygen at the right time. In our stove, the P-channel (named after Peter Van Den Berg) is the small L shaped metal tubing used to put oxygen right at the back of the stove. It functions to increase the temperature of the fire and thoroughly mix the gases. Without this additional air, the fire would not be able to burn the smaller particles that create creosote buildup.

Lighting the stove

Batch RMH stoves give the added benefit of being able to load a larger amount of wood. In our previous J style RMH, a continuous feed of small diameter wood was needed every 15-20 minutes to keep the stove burning. Batch stoves allow a much larger volume of wood and give a burn time of roughly an hour to an hour and a half. This allows users to provide many more BTUs without being hands-on.

completed stove

Materials used to build the rmh:

• 2 metal 55 gallon drums
• Enough firebrick for both the rmh’s pad, the firebox, and the riser
• Refractory cement
• Cast iron griddle for firebox lid
• Cast iron door for firebox door
• Refractory blanket insulation

Materials used for the bench:

• Metal ductwork for heating the cob bench
• Metal tape for sealing the ductwork
• Perlite
• Cement
• Lots of cob

Materials for the chimney

• Single wall chimney pipe
• Class A chimney pipe
• Chimney support box
• Chimney cap