Heating Water with a Rocket Mass Heater

It's a pity to have a wood stove not capable of heating sanitary water; in this article I'll show you how to easily add a water heating system to a L-shaped rocket mass heater.

What we're going to do is placing a little water boiler just over the feeding chamber so that its surface will be directly exposed to the heat. Naturally the boiler has to be connected to your water heating system in order to store the hot water inside storage tanks but I'm not going to address it into this post.

I could place the boiler somewhere else, for example over the heat riser, it would be the best if you need heat a great quantity of water because of the higher temperatures. But It would mean sacrificing the cooktop and I didn't want to renounce to it.

Another place to put the water boiler would be over the very first part of the exhaust duct to catch the heat from the gases coming from the barrel. I excluded also this possibility because of the following reasons:

  • constant subtracting of heat from the bench
  • risk of water overheating in case of a malfunction of the hot water circulation system

So I decided that the best for my situation would be to have the possibility to choose whenever heat the water or not. Once you installed the water boiler over the burning chamber you can be free to choose whenever use it by simply moving the fuel hot charcoals under it as they will heat the surface of the boiler by irradiation making the water hot in a matter of minutes.


  • A water boiler or any metallic container able to contain water with 2 pipes exiting from it. Its dimensions must be suitable for your burning chamber.
  • clay mixture
  • expanded clay or similar insulant material
  • normally open thermostat
  • simple thermometer with a probe (optional)

I took an old electrical water boiler (about 4 gallons) from the junkyard, I removed the plastic cover with all the insulation and I cleaned it with a steel brush until I got a polished iron water container. I removed the thermostat too because it was normally closed and not suitable for my purpose.

I connected the boiler to the water supply under pressure for some minutes to check against eventual leakages.

The installation was very simple: I dismounted the top of the feeding chamber and I placed the boiler on the bricks. I closed every fissure between the boiler and the underlaying bricks using the clay mixture.

I placed a normally open thermostat probe in place of the original one together with a thermometer probe to show the current temperature of the water.


To properly insulate the boiler I covered it all with expanded clay.

I connected the boiler to my water heating system to store the hot water into storage tanks and it works great.

Last modified on 02/12/16

Written by Davide Buldrini

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Read step by step how to build an L-shaped rocket Mass Heater with bricks and stone blocks.
Read why I decided to convert a J-shaped rocket mass heater to an L-shaped one.
You can actually build by yourself the most efficient wood stove without being an engineer or hydraulic and using inexpensive or recycled materials .