Searching for 600 – My Experiences With A Rocket Mass Heater

Part 1

For those of you who have not been following along since this journey of mine started a year ago, I have been working towards a year-round aquaponics growing environment that would supplement our household groceries. That said, a 1400L / 275gal aquaponics system just cannot feasibly go in the house. However, we live in Zone 5 and our last winter saw 100cm / 3ft of snow with a couple of days that were -45c / -49f including the wind chill, so a heated greenhouse is a minimum.

In the January to February window, our most bitter part of winter, the day time temp is usually 10c to 15c warmer during the day than at night. So, I know that if I can keep the greenhouse above 4c / 39f at night, then the sun will do the heavy lifting during the day. So, what I needed was a way to generate that heat in as sustainable and inexpensive a way as possible.

Kerosene and propane are expensive if you have to run them as water heaters every day. They also fail the sustainability requirement. A conventional wood-block or wood-pellet stove would set me back several hundred dollars at least, and almost 60% of the energy goes out the flue. As a result, the fuel requirements for a wood-block stove were work-intensive … hours of cutting and splitting wood, or paying $200 a cord for someone else to do it. Pellet-stoves cost between $4 and $7 dollars a day in pellets; that adds up in a hurry over a four-month Canadian winter.

I needed a better solution.

A friend of mine in Montreal told me about the concept of the Rocket Mass Heater system, and I started on my research. What I found bluntly seemed too good to be true. Inexpensive build costs, 80%+ efficiencies, zero exhaust products other than steam and CO2, and capable of storing heat for dozens of hours. More over, there were several forums and videos of people openly discussing using Rocket Mass Heater (RMH) systems in their own greenhouses and loving the results.

So, I decided to try it. I am not, by any estimation, a “handyman”. I’m a telecom geek and fantasy-fiction writer. So, the prospect of building a system that would handle interior temperatures that reached from 650c / 1200f towards 1100c / 2000f with radiant surface temperatures of 150c / 300f to 315c / 600f was a bit daunting.

Did I mention that my greenhouse shares a common wall with the coop of my sweetheart’s prized chickens? To say that incinerating her hobby flock as a result of a “design flaw” or “implementation issue” on my part would be a source of marital discontent is likely an understatement. Besides, I would feel absolutely horrid if that happened.

So, I read every blog post I could find, and watched every related video on YouTube. Hours upon hours of research, Sketchup designs, and spreadsheet calculations went into it. I spend about $100 on firebricks and built my “Version 1” Rocket Stove system out on our concrete walkway with about a 112mm / 4.5in cross section. I did two testing burns, on two adjacent days. A Rocket Stove (RS) is different from an RMH in that there is no heat-storage mass … they are used as extremely efficient cooking heat-sources. I figured if I could boil water, I was likely heading the right way.

The short version is that I shattered almost a third of the firebricks. I talk about how RMH / RS systems work internally in a subsequent blog post, but suffice to say that the “chimney” component is called the “heat riser” and stands about 1m / 3ft tall. All of the bricks on one side, as well as a couple on the roof of the “burn tunnel” were either cracked, or outright broken in two or more pieces.

This was hardly the outcome I was looking for. I was pretty upset, in fact … we are a busy house with lots we want to do and I cannot afford to wreck $100 in firebricks in the name of “Mad Science”.

“Oh no, not another learning experience” — Les McCallum, IT & Business Mentor

I went back to the drawing board at my sweetheart’s encouragement. I found out what it was I had done wrong on forums talking about home-made firebrick baking ovens and kilns … thermal shock and water vapour. Essentially, the first time you light something like this, you need to do so in progressive steps to “slowly” ensure that all the water is out of the brick and mortar. The recipe that I saw a few times was:

“Burn for ten, cool for ten. Burn for twenty, cool for twenty. Burn for forty, cool for forty. ‘Safe’.”

Water that goes from 20c to 200c in a few seconds explodes. Hundreds of water droplets being rapidly and violently converted to steam will shatter firebrick and thermal cement. Lesson learned.

My renewed research also revealed that a common wisdom was that there was no “economy” in building a system with less than a 150mm / 6in cross-section. Smaller systems just required more attention to keep them running, in the form of more time physically feeding wood to them. In fact, the trend was to just build at the optimum upper limit of 200mm / 8in to maximize feed capacity to thereby minimize the time required to bring the thermal mass to the target temperature.

So, I threw out my design and started over from scratch. I bought some replacement firebricks and spent likely almost a dozen hours staring at various layout designs I concocted in Sketchup. Finally, I came up with something that required only cutting 2 bricks; everything else “just fit” using what are called “boat-builder’s corners”. Four bricks cemented together formed a “cuff”. Cuffs were then stacked horizontally for the burn tunnel and vertically for the heat riser and cemented together. The effective system size was a cross-section of 210mm / 8.4in; technically unpredictable, but still viable.

I went a step further with this design than I had previously. Instead of using an adobe / “cob” bench as my thermal mass, I decided to use the water of the aquaponics system and floor of the greenhouse themselves as my thermal mass.

Water has seven times the thermal capacity as clay; 1400L of water thus has the thermal capacity of over 9800L of clay. As well, everything I am trying to keep from freezing is in the water … fish and plants. I do not need to keep the air in the greenhouse above 4c, I need to keep the water above 4c. It thus made little sense to me to try and heat the air to heat the water. Instead, I’d first heat the water.

If all went well, a water-filled copper coil would be heated and that would circulate to the aquaponics system. The remaining heat in the exhaust gasses would then be used to heat the clay floor, essentially as a bit of insurance, to make it less likely that “frost” conditions would occur in the greenhouse.

My son and I excavated a “heat trench” through the clay that would accommodate a 210L / 50gal steel drum, cut in half length-wise. Two drums gave four halves that were placed end-to-end in the trench. A piece of sheet metal ran 90% of the length down the centre of the barrel-tunnel. Hot gases from the combustion system would travel down one side of the barrel tunnel, around the end of the sheet-metal baffle, and back up to an exhaust point. A standard stove-pipe would then take the much-cooled exhaust gases out of the greenhouse.

In my next post, I will talk about how Rocket Mass Heater systems work and explain the math that drives the magic. After that, I’ll talk about my first and subsequent testing burns that I have done with the “Version 2” system and a few surprises I have learned along the way.

If you have any constructive comments, questions or suggestions, please leave them in the comments section below this post. I would absolutely love to hear from you! I hope this series of posts helps someone else get into this fascinating technology.