The Rocket Gets It’s First Adjustment
I, along with Rob Avis from Verge Permaculture, conducted a rocket mass heater workshop this past August. We discussed the work of Dr. Larry Winiarski and Dr. Baldwin who have been researching combustion and stoves for more than 30 years and we talked of Mr. Ianto Evan’s who took these ideas and transformed them from a simple cooking device into a super-efficient wood burning heater.
We discussed the sizing of systems for different spaces, and we covered fast rotational coppice systems, which will be the key to fueling these things without deforesting our planet. We built mockups of our design, laid everything out, mortared it together, tested it, mixed our insulation with clay slip and built mountains and mountains of cob. We ate the most delicious pizza from the cob oven and we shared some great stories and had some great laughs. At the end of the final day it was about 90% complete.
I finished cobbing everything together the next day and a few days after that I applied some high-temperature silicone on any pipes that were exposed to the inside, making sure no toxic fumes were going to enter the space. And just 2 days ago I fired it up again, the first time since we tested it during the workshop. It took off but not like it did during the test. It puffed, smoked and the fire was eventually climbing into the room. It was drawing but I knew something was up.
We had put a fresh air intake into the feed tube, so I fiddled with that but no better. Then I looked through the three different cleanouts we installed along the length of the system and noticed an immense amount of condensation and even a 2-inch deep puddle at the bottom of the cleanout closest to the barrel. At first I figured things were just drying out but after a day of firing (things were pretty dry at this point) things still weren’t improving. It looked as though I would have to dig a little deeper – literally.
There must have been a bottleneck somewhere and it had to be one of two things, or a combination of both. Either the gap at the top of the heat riser wasn’t large enough or the manifold – the place where the barrel connects to the ducting – was not roomy enough. So I began investigating.
The beautiful thing about cob is that as strong as it is, it comes apart almost as easy as it goes together. The cob around the barrel was rock hard but a chisel and hammer handled that breezily. The rest was still moist, so it came apart without hassle. Once everything was loose, I pulled off the barrel to have a look. Nothing seemed out of place but I could see how we may have created a bottleneck at the manifold. Also, the top of the barrel had become quite discoloured, meaning that it was getting quite hot. We had given 2 inches between the heat riser and barrel but I figured adding an extra inch or so wouldn’t hurt.
I used builder’s mesh to bubble out the area connecting the barrel to the ducting and I raised the barrel 1 ½ inches more, for a total of 3 ½ inches. I added a bunch of water to the chipped off cob, let it sit for an hour or so and then remixed it with my boots. In 10 minutes I had a beautiful batch of cob, made entirely of the stuff I chipped off. Try doing that with cement! I packed it around the barrel and covered the builder’s mesh.
Then I held my breath, lit some paper, tossed in some kindling and crossed my fingers. Immediately the thing was sucking harder and you could hear it. Smoke rose but was instantly sucked down into the combustion chamber. I tinkered with different types of wood and burned the thing hot for a good 3 hours. When a little smoke wanted to climb up the feed tube, I simply covered 1/3 of it with a firebrick to restrict some of the airflow (this is a trick I learned on Donkey’s Rocket Mass Heater forum – an incredible resource for everything rocket mass heater you can imagine).
So, though it didn’t go quite as planned, it’s working a heck of a lot better. For those thinking of building their own systems, my advice is to allow extra room at the top of the heat riser than you think you need and make a nice big space around the manifold. And don’t get discouraged. These are experimental stoves and though we try to follow best practices, every situation is different. After all it is just cob. It’s as flexible as you allow it to be.