Rocket mass heater build

POSTED BY   thenelson
January 31, 2018

Last year our hot tub motor blew out and at a cost of $600 + on an older model tub, I had to ask myself if this investment was worth it. And it didn’t take much mulling over to come to a conclusion. However I wasn’t about to give up.

After some weeks of research, sketching and multiple trips to various hardware stores I had a solution. I came across a century old technic that was used for heating and cooking. Rocket Mass Heater. Basically it is a small chimney designed to heat/cook with limited materials. Given the correct dimensions, one can create one with 3 blocks and a shovel.

My challenge was to create a rocket mass heater capable of heating a hot tub. And based on my research, something that is not commonly built due to the complexity and resources needed.

Challenge accepted…

I used the plans I found on-line and increased the size. The rocket stove operates on a principle of mathematics. The relation to the burn chamber to the riser creates a draw or suction action that carries the smoke sideways in the unit which allows for use inside. Think of having a smokeless bonfire. And as the smoke and heat carry through the unit, it is collected in the main drum and circulated until the smoke and ash are converted to essentially steam which creates a temperature of over 2,000 degrees. After all is converted to that high temp gas, it is expelled through a rear stove pipe that is positioned in a manner that creates continued suction throughout. Some people extend this section for 20 – 30′ and wrap with a mixture of clay and straw for conductive heating allowing one to heat a home, greenhouse or small area.

My challenge was to take this concept and engineer a way to heat water. Solution: wrap 40′ of copper tubing up the outside of the burn chamber with an intake line hooked up to an in-line pump and an line dumping into the tub.

See the above pics of the build…

Once I had this up and running, took about a month of building and tearing down to get the spec exactly correct, I ran into an issue of volume. While the water did get warm, the volume I was attempting to heat was too large. At over 300 gallons, I will never be able to get this hot enough for a length of time for a soak. Yes that definitely is unfortunate, however the solution I’ve devised is just reduce the volume and build a small 1-2 person tub. This project is in the works so check back in a few weeks and I’ll post the progress.

Has anyone out there used this book “The Rocket Mass Heater Builder’s Guide: Complete Step-by-Step Construction, Maintenance and Troubleshooting” by Erica Wisner and Ernie Wisner. I’ve heard great things about it and thinking this is my next step towards getting this right. 




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