What is a Rocket Mass Heater?

What is a Rocket Mass Heater?

A Rocket Mass Heater is a type of a masonry heater that has been designed specifically for the those who want to do it themselves, and often with many reused materials, making it more affordable and accessible.

From the Masonry Heater Association of North America website, they define a Masonry Heater as:

“A masonry heater allows you to heat your home with wood in a unique way.

The main thing that distinguishes a masonry heater from other wood-fired heating appliances is the ability to store a large amount of heat. This means that you can rapidly and cleanly burn a large charge of wood without overheating your house. The heat is stored in the masonry thermal mass, and then slowly radiates into your house for the next 18 to 24 hours.”

This rocket mass heater was a huge upgrade to the wood stove in our yurt. The rocket mass heater is FAR more efficient. It burns much cleaner since it combusts both the wood AND the smoke, and uses far less fuel as it captures nearly all of the heat rather than allowing so much of it to exit through the chimney. It was a must for keeping our baby warm last winter.

As we tend to do, @nomadicbuilder and I made this project more complicated as we attempted to maximize both efficiency and number of cooking surfaces! I love cooking with the heat being generated, and I’ll share more about the cooking features we built in as well as the construction of this heater in another post.

This is the part of the inside of the rocket stove

We began constructing our rocket mass heater after insulating the wall behind it and the plywood floor with perlite coated in clay or cement (if it was bearing a heavy load). Then built our firebox (from firebrick), adding a cooktop from an old wood cooking stove. We used insulating firebrick for our heatriser, and built our combustion chamber (often a barrel is used) out of adobes, with a thick metal plate on the front to radiate heat. We used a pottery kiln shelf as the top of our combustion chamber, covered in a layer of cob, with split fire bricks on top. If the bench bypass is open, gasses leaving the combustion chamber will exit the flue pipe - after the rocket is roaring, we close the bypass, forcing the gasses back down and into the bench (along the front). The bench is a bell design with the air chamber created by two halves of a barrel. The adobe wall separates the beginning of the chamber and the hot air fills the bench cavity, only exiting as it cools and drops low enough to be forced out the exit (slide #8) and up through the flue.  

Our door, bypass, and one of the cleanouts were purchased from @firespeaking, and we HIGHLY recommend them - especially the glass door! We followed Peter van den Berg's brick core design, which can be found at Batchrocket.eu making a slight modification to install the cooking surface above the firebox. We found so much info in @sundogbuilders facebook page (look in albums>Batch Box Rocket Stove for MUCH better step by step photos), and on Donkey32.proboards.com .

So many rocket stoves don’t allow you to enjoy watching the fire. Our little one loves watching too! You can also use the door of an old wood stove, but this one is just the right dimensions. We have considered modifying the air intake to allow for better flow - we usually leave the door cracked open. We find an angle iron frame bolted together and held tight by the cob to be the easiest way to install the door.

The airspace between a wood floor and the insulation layer is mandatory. A rocket mass heater is intended to slowly and continuously release heat into the room, and it will also slowly heat up any insulation layer. If you’re building on a wood floor, you need to create an airspace between that warming insulation layer and the wood or your wood will slowly become more likely to combust.

Our biggest mistake was complicating the build to add in the old cookstove hot plates. Doing so required us to extend the firebox a few inches longer than Peter’s design, which we believe creates a little smoke back occasionally when we first light the oven. We LOVE cooking on it, so it’s worth it to us, but IF you want to cook on the top of the firebox I’d recommend a thick metal plate cut to the size of Peter’s SketchUp plans. This also caused us to modify the secondary intake and make it trickier to install, but it seems to work just fine, bring air in from the back of the cooktop (the design calls for that air to be pulled along the top of the firebox).

This image shows the heat riser built out of insulated fire brick, and is an example of how complicated the inner workings became!

We originally topped our combustion chamber with a thick metal plate to create another cooking surface, but couldn't get it to seal well without minor leaks as the earthen and metal elements expanded at different rates, so we replaced the metal plate with the kiln shelf, and we usually keep an oven rack on top of it that we use A LOT to warm food or to dry things like herbs, fabric and cloth diapers.

While I feel that Rocket Mass Heaters are a great alternative to standard wood stoves in almost all situations, there are some parts of our mild CA climate that do not warrant the extra effort. A yurt however, or other low insulation wall system, is one of the best uses for the masonry heater that stores the heat in the mass, rather than in the air. With the old wood stove we would wake up in the morning to the indoor air temperature being only a couple degrees above the outside (which is pretty uncomfortable when its in the 20’s!). After we build the Rocket Stove it would be at least 20-30 degrees warmer in the mornings inside the yurt vs outside.