[Sticky] Running-in of the Rocket Mass Heater  

Floris Winkelmeijer
Member Admin

Hereafter you will find the instructions for running-in your Rocket mass heater

Drying and Running-in Instructions for the Batch Rocket Stove

It is important to dry and run-in a new Batch Rocket Stove before beginning operation at full capacity. Drying means to let the stove dry without firing it. Running-in means lighting small fires over a period of time to help dry the masonry structure of blocks and clay at a constant rate without too many or too large temperature fluctuations. If the clay and stones were to dry too fast, there would be a good chance of them cracking.

After the running-in period the stove can be used normally. Please read “Using the Rocket Stove” to learn how to use the stove after this initial running-in period.

If you have any questions after reading these instructions you can comment to this topic.

Drying stage

Before lighting the heater for the first time it should dry for a certain amount of time. That time depends on a few things. Two factors are humidity and air temperature. The higher the humidity the slower the heater dries. The lower the temperature the slower the heater dries.


Things you can do at this stage to speed up the drying process:

  • Increase room temperature
  • Decrease humidity. When the cob is drying it releases water to the environment. As the inside of the heater is an enclosed space the humidity can't go anywhere but back into the clay and out the other way. By attaching a ventilator fan on the flue pipe that sucks air out of the system you decrease the humidity inside the heater and therefore it will dry quicker.


Observing the drying process

  • After a few days you will start to see the first fire bricks becoming a lighter color
  • In another few days they will all be light again meaning that a lot of moisture has evaporated into the air out of the bricks.
  • Next the joints will start to dry. In below picture you can clearly see that the joints between the bricks in the top layer have dried already but the joints below have not. It will take another few days for those to dry as well.


  • After all of the joints have become the lighter shade, wait another few days and then you can begin the running-in stage by lighting small fires in the stove.


During the drying stage little cracks might appear in the clay or between the clay and the bricks. This is not a problem but it's good to close the cracks with a sponge and water.

Running-in stage

  • Before lighting the stove, always remove any remaining ashes from the firebox and within the riser.

  • It is important to use fairly dry wood. Start the fire with some kindling or a pine cone and some small branches just in front of the port, but NOT between the bricks of the port. Start with smaller pieces and add bigger ones subsequently. You can use any type of wood but Mimosa and Willow are very suitable for the running-in period as they burn very well. Open the air intake and the secondary air valve fully, and then you can close the door.

  • Once you have a small fire burning, open the door and stack about 1kg of wood lightly on top of the fire ensuring there is plenty of air space between the pieces. Fully open the air intake and the secondary air valve, then close the door as soon as the fire is sustainable.
  • Resist the temptation to make the stove burn vigorously and sound like a rocket by putting more wood on; the temperatures will rise too much and this may damage the stove.
  • When the fire is going well you can set the primary air inlet to the Ideal setting which is at a gap of about 6 mm.
  • When the 1kg has burnt down to a heap of burning coals on the firebox floor you can add another 1kg batch of wood at any time. At this stage, size doesn't matter, but do not exceed the 1kg limit. The idea is to do a number of small firings during each day so as to minimise temperature fluctuations in the stove.
  • Be sure to keep the port and inside of the riser free from burning wood.
  • Keep adding 1 kg batches of wood at a reasonably steady rate as often as possible. A few batches a day, spaced one to three hours apart, is a good rate. This will prevent temperature shocks to the stove.
  • Continue as constantly as you can whenever at home for up to 14 days. Ideally there should be no days in between, as the stove will remain warm from the previous day and be more easy to start, while temperature variations will be smaller.
  • Initially there may be a lot of smoke from the chimney. If there is still a lot of smoke after 14 days of running in, then continue until it abates.
  • As running in progresses, it is OK for the fire to burn with the characteristic roaring rocket sound.
  • By the end of the running-in period, the fire should be burning nicely – with the batch of wood burning down to hot coals within 10 minutes without big fluctuations in intensity. This may take 7 days or up to 14 days, depending on conditions.


During the running in stage little cracks might appear in the clay or between the clay and the bricks. This is not a problem but it's good to close the cracks with a sponge and water.


  • Steam or smoke seems to be coming out from the walls of the stove. Is this normal?
    • Carefully smell the emissions to determine whether they are smoke or steam. If smoke, it will be coming through cracks in the wall. The cracks need to be fixed by wetting the cob and smoothing it out or adding a bit more cob. If it is steam, it is simply water boiling out from within the clay and masonry. This is a normal part of the drying process. Generally, steam will come out more generally from the surface, while smoke will usually emanate from one point, a crack of some sort.
  • The stove is difficult to get going. Is everything OK?
    • A feature of a Batch Rocket Stove is that the gases leaving up the chimney are not very hot and the flow is slow. This means that it can be difficult to establish the initial flow of warm gases. Be patient and you will soon learn how best to start the fire. And remember that, after the first firing of the running-in period (and, later, of each winter season), the stove should start much more easily because the mass of bricks and clay is already warm.
Posted : 26/10/2018 12:04 am
Topic Tags
Steve Cassidy
Member Admin

Latest version of this text is always available at:  https://docs.google.com/document/d/1Njpwn4IrZFBXu22oOZBESuNp7u6zErfxT2PUhEJdtZ4/edit?usp=sharing

Posted : 15/11/2018 11:17 am

Subscribe To Our Newsletter

Join our mailing list to receive the latest news and updates from our team.

Subscribe To Our Newsletter

You have Successfully Subscribed!

Please Login or Register