Rocket Mass Heater L-Shaped Profile VS J-Shaped Profile

A J-shaped rocket mass heater with its vertical feeding conduct is looking great but after some time of usage I decided to switch to a L-shaped design featuring an horizontal feeding chamber. Below are the reasons of my choice, take it as an hint and do always your own considerations.


With a J-shaped rocket mass heater during the lighting process there is an high chance to experience smoke back especially if the stove is totally cold. Conversely I never experienced this kind of problem with an L-shaped design because you can directly light the fire inside the feeding chamber. When all components of the stove are correctly dimensioned there should be enough draft also when the stove is very cold.


The feeding chamber can be used as a wood oven to cook some little pizzas, bread or using a suitable barbecue grid fish and meat. And what about a good coffee?

Do you want to cook pizza? Just keep the burning charcoal over the bricks for some minutes and close the feeding inlet to let the temperature increase further. After push away the charcoal and use a metallic tube (I use copper) to blow away the ash from the bricks; now you can put the dough directly over the bricks as in a true wood oven. Close again the feeding inlet and wait for your pizza.

No smoke back

Just ensure that the wood logs are entirely inside the feeding chamber and you don't need to worry about smoke backs. Conversely using a J-profile you can experience this type of problems especially during startup or when the wood logs inside the feeding chamber are interlocked.

Heating DHW water

You can exploit the first part of the horizontal feeding chamber to add a boiler to heat the sanitary water. This is a big advantage of using a L-shaped profile because you can very easily build a water heating system without upset all stove.

Moreover you can decide whenever heat the water by moving the hot charcoal under the boiler. This can be an extra security when you leave the building because you can push away all charcoal from the boiler to ensure that the water will not reach dangerous temperatures if some malfunction with the circulation system occurs.

Loading capacity

Being the horizontal feeding chamber longer than the vertical one used in the J-shaped design it's possible to use longer wood logs without incurring in smoke back problems. Using the "burn it all" combustion mode you can burn a great quantity of wood. You don't need always to check if the wood logs are interlocked as within the J-shaped design, you can just load and forget.

Combustion management

With a L-shaped rocket mass heater you can adopt a lot of different burning configurations:

Hellfire setup

Wood logs are pushed deeply inside toward the end of the combustion chamber to reach the highest temperature inside the heat riser and ensure the maximum draft.

In this configuration the wood will burn very fast and the temperature over the top of the barrel will reach the maximum levels; perfect to boil your soup or to heat the room in a matter of minutes.

To continue burning at the maximum speed you need often push the wood towards the end of the combustion chamber.

Burn It All

When the wood is totally burned you can spread the remaining hot charcoals along the whole surface of the combustion chamber and put new wood on top of it. After some minutes the wood will entirely start to burn throughout its full length and you will not need to push it deeper. As result you get a great burning process that runs by itself where you can manage the combustion speed by reducing the feeding chamber aperture.

Water Heating Mode

Just collect all the burning charcoals under the water boiler and the temperature of the water will rise very fast because the hot charcoal emits a great quantity of heat by irradiation directly towards the surface of the water boiler. Therefore in this way you can concentrate the heat on the water boiler even more effectively than the flames from the burning wood do. You can encourage the process by closing the feeding inlet with an aluminium foil; it will be sucked by the draft and it will remains in position, that will increase the temperature inside the feeding chamber because no cold air will enter.

Sleep Mode

After ensuring that the combustion is quite bright fill the stove with big wood logs and accurately close the inlet. Almost without air the combustion will runs very slow and the fuel will last many hours; by doing so I avoid to turn on again the stove every morning because I find the hot charcoal remained from the previous evening.


Cleaning the feeding chamber can be a tedious task with a J-shaped rocket mass heater because it's difficult to cross the curve after the feeding chamber and reach the end of the combustion chamber. Otherwise with the horizontal feeding chamber it's a piece of cake to remove the ash using the right tool.

And you? Which one do you prefer?

Last modified on 02/12/16

Written by Davide Buldrini

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