How To Build An L-shaped Rocket Mass Heater
In this article I will explain you in 6 steps how to build this simple rocket mass heater made of bricks and rock blocks.
Take this plan as a hint and adapt it to your needs or to the material you already have, remember that the sizes are approximated to the nearest inch, naturally you can change the shape of the bench or cover the bricks with cob if you prefer.
- heating your space
- warm couch
- heating sanitary water
- cooking on the top of the barrel (boil water, prepare soups, chestnuts)
- cooking inside the burning chamber (pizza, bread, barbecue, etc.)
- possibility to use long wood logs (about up to 24")
- high burning efficiency
- thermal battery: the bench stores the heat and releases it slowly also when the stove is off
- low temp exhaust gases
- about 200 common bricks 9" x 4 1/2" x 2 1/2"
- about 70 refractory bricks 9" x 4 1/2" x 2 1/2"
- expanded clay or similar insulant
- 1 metallic barrel 55 gal
- 1 water boiler 4 gal about
- anti-draft chimney cowl
- rock blocks
- thin metallic wire mesh about 65" x 40"
- metallic wire
- clay-rich soil
- fine sand
The burning chamber
The picture shows how to place the bricks to build the base for an insulated burning chamber; this is where you put the fuel and where it will burn. The dimensions in the picture are approximated to the nearest inch for convenience. The disposal of the bricks is studied to reduce the contact between the burning chamber and the other bricks allowing the maximum space for insulation.
I used 9" x 4 1/2" x 2 1/2" common bricks to build the base and firebricks with the same dimensions for the areas subjected to high temperatures in order to avoid breakages. I choose thick bricks because they can store a large amount of heat necessary when you want to use your stove as a oven.
I didn't use any clay or cement inside the burning chamber because it would surely come off after the first use but I used the clay mixture to secure the bricks from the outside.
I filled the empty spaces between the bricks with the expanded clay to ensure the maximum insulation of the burning chamber from the floor and from the rest of the stove. Instead of the expanded clay you can also use another inexpensive material as wood ash for example or insulating firebricks but in that case you will need to change the position of the bricks.
The feeding chamber is almost concluded, it's 35" long with an 8" x 7" inlet. Remember that to guarantee a good operation need keep about the same section for all the ducts of the stove.
The heat riser
The heat riser is the most important part of a rocket mass heater, this is the engine, it creates the draft necessary to suck the air from the feeding chamber inlet and push the gases through the exhaust duct until the chimney. As showed in the picture I placed the bricks using a pattern in order to achieve the same section of the burning chamber inlet by cutting only one brick that you can recognize by the different color.
I placed the bricks over the burning chamber rotated by the shortest side because this area is subjected to breakages and I left the necessary space to host the water boiler.
The height of the heat riser is a fundamental parameter as it must create draft and it will determine the total height of the stove.
To secure the bricks of the heat riser I totally covered them up with a thin layer of clay mixture as you can see in the picture.
Build the shell
I used common bricks to build the external shell that wraps up the burning chamber: the empty space between them must be filled with expanded clay to avoid heat dispersions.
Heat riser insulation
As the word suggests the heat riser must produce a rise in temperature so it deserves an accurate insulation.
An easy and inexpensive way is to use a thin metallic wire mesh to wrap the heat riser and leave adequate space to fill with expanded clay.
The net lay on the base of the stove and over the burning chamber. I secured it around with metallic wire to contain the pressure of the expanded clay that will give the net almost a cylindric shape.
As you can see in the drawing I also placed the water boiler; the empty spaces under it must be filled with the clay mixture to avoid leaking smoke from the burning chamber. For more info about the water boiler read my post on how to heat the water with a rocket mass heater.
Then I covered all the refractory bricks and water boiler with expanded clay to ensure the maximum insulation and efficiency. The rocket mass heater needs operate at very high temperatures to work correctly so it's vital to not dissipate the heat from the combustion chamber.
Clearly the water heater dissipates heat but you can choose when to use it just by moving the wood inwardly of outwardly; when the combustion point is after the water boiler the wood will burn at the maximum efficiency.
The radiator essentially consists of a metallic barrel that instantly transfers the heat from the hot gases coming up from the heat riser to the ambience. The high thermal conductivity of metal gives to the rocket mass heater the ability to heat the room within few minutes after start up.
The top surface of the radiator can reach very high temperatures making it suitable for cooking.
It's very important to clean accurately the barrel before using it in your stove to avoid the emission of toxic gases from paint residues. Initially I set a fire inside the barrel and I let it burn during some hours then I used a grinder with a metallic brush to remove any residues.
It's time to build a solid base to support the radiator: I placed the bricks around the heat riser to form an octagon leaving about 2" of empty space between them; this is where the hot gases coming from the top of the heat riser will flows and will channel through the exit duct. For convenience I didn't show it in the picture but clearly the burning chamber and the water heater must be totally covered with the expanded clay.
In the picture above the barrel is set in position and I made the top surface transparent to show you how all elements are positioned inside. The gap between the top of the radiator and the heat riser will significantly influence how the rocket mass heater works, its draft and the maximum temperature of the upper surface of the barrel. In the example I placed the barrel with a 2" gap; increasing the gap the temperature of the top surface of the barrel will decrease and vice versa.
Note that you cannot use a too short gap because you will choke the smoke flow causing problems with the draft. The area of the gap must be same or greater of the section area of the heat riser.
Around the barrel I built a curb where I can pour inside fine sand to seal up the radiator in order to avoid smoke leaks; this solution can be very practical for inspections because you can easily remove the barrel without dismount anything. Alternatively you can just seal up the barrel using the clay mixture if you prefer.
The purposes of the exhaust system is leading the smokes out of the building while extracting as much heat as possible from them. Choosing the right material to build the exhaust duct is vital; the best would be to use one with high thermal conductivity capable of transmitting the maximum amount of heat to the bench.
Usually the majority of rocket mass heaters builders use stainless steel chimney pipes covered up with clay and others heavy materials such as bricks or stones. I think the steel pipes are too way expensive and It's difficult to ensure a good thermal transfer between the surface of the pipes and the stones. Usually the clay mixture is used to connect them but clay is a not an exceptionally conductive material compared with some kind of rocks as you can see in the table below:
Thermal conductivity of common building materials
|Clay, dry to moist||0.15 - 1.8|
|Clay, saturated||0.6 - 2.5|
|Granite||1.7 - 4.0|
|Solid rock||2 - 7
|Marble||2.08 - 2.94|
The thermal conductivity is the property of a material to conduct heat and the higher the coefficient the more it's capable of transferring heat.
Another problem of steel pipes is that they are prone to get clogged with soot involving the installation of several inspection points along the duct raising even more the price (and the environmental impact). A masonry exhaust system is prone to clogging too but building some inspection points with bricks and clay wouldn't costs so much.
I built the whole exhaust system of my stove using simple bricks with clay and during 3 years of usage I didn't experience any problem of draft or smoke leaking nor I needed to do any maintenance. Naturally you have to check up the condition of the duct every year and perform appropriate cleaning when needed to prevent any possibility of fire.
I guess the best would be to use directly solid rock to build the exhaust duct as it can be pretty inexpensive and it does have a very high thermal conductivity. In order to build the best thermal battery It's also very important to use rocks of the right size to make sure to capture all heat from the exhaust gases; using too small rocks after some hours of operation they wouldn't be able to accumulate more heat resulting in a rising of the temperature of the smoke leaving the building.
In the drawing I used nice squared blocks but in the reality, unless you are willing to spend a considerable amount of money, you will have to deal with rocks of all shapes and sizes; to avoid smoke leaks you can seal them with the clay mixture. Remember that the most important thing to avoid smoke leaks is the presence of a constant draft; it not any sealing will be useless because the smoke can escape everywhere or at most from the feeding chamber.
It's advisable to build a longer exhaust duct if you have the space or you can even twist it as a serpentine to occupy a wider space and extract more heat.
Since the bench will reach high temperatures especially near the radiator you can cover it by a layer of cob or clay mixture. If you like to spread the heat faster to the ambience you can leave some areas uncovered or if on the contrary you want to release the heat very slowly you can cover it all by a thick layer of cob or even wood (but pay attention to the temperatures).
In the drawing I connected the exit of the bench to a brick chimney. I prefer to use bricks instead of steel pipes because they can extract more heat from the exhaust gases and they don't fear the condense produced by the low temperature smoke.
A rocket mass heater could work also with side-exit exhaust but it should be avoided when possible because it can bring a lot of problems under different conditions such as cold outdoor chimneys or wrong winds. To ensure the maximum draft in the worst conditions or when the stove is totally cold is better to use a conventional through-roof exhaust, if will pay off at the end. There is a nice discussion on this topic at Permies.com: rocket mass heater - exaust can be simpler than chimney
The last but not least important part of the exhaust system is the chimney cowl. A smoke back can be very dangerous with a rocket mass heater; it can very fast lead smoke inside the room through the feeding chamber or even make the flames escape from the inlet opening. The chimney should be installed in a position sheltered from the wind gusts and the chimney cowl must be wind proof. You must be sure that draft inversions will not occur. I have to say that I had a bad experience using spinner anti-draft chimney cowl because after few days the hub was galled by the condense so it's better to use a fixed anti-draft chimney cowl.
Last modified on 02/12/16
Written by Davide Buldrini