We fell in love with Ianto Evans’ rocket mass heater the first time we heard of it. So much so that we booked a trip to Oregon to take his course. What isn’t to love? The idea is so ridiculously simple and immensely practical it’s amazing that we haven’t been using these all along. Rather than burning a fire in a traditional fireplace and allowing much of the heat to escape straight up the chimney, with a bit coming into the room and quickly dissipating, the chimney is built into the furniture. Furniture that is built to store the heat. Additionally, it’s built in such a way that it uses way less fuel than a traditional fireplace.
If you don’t already own a copy of “Rocket Mass Heaters: Super-efficient Woodstoves You Can Build“, it’s worth picking up. True, if you’re using anything other than an eight inch system (we went with a seven inch) you will need to adjust your calculations, and the pictures are not altogether fabulous if you’re hoping for a step by step pictorial to accompany the tutorial, but the basics that you will need to understand are in there. Understanding the ‘whys’ of how the system works was critical, for me anyway, and helped with a somewhat finicky build.
The critical measurements are the feed tube, the burn tunnel, the riser and insulation, and the space between the top of the riser and the barrel. Of course the size of pipe that you decide to go with will affect those measurements. Less so, but still factoring in, are the bell chamber and the length of the run.
If you are planning to build one of these, I’d suggest leaving yourself a little extra time for fiddling around. You may have to raise or lower your barrel a little to get a good draw (and the amount of heat you want on the top of the barrel) or make other adjustments to the system. We ended up shortening the burn tunnel and widening the feed tube a little from where we started. Don’t apply your final mortar or mudding in until you’re certain the system works.
In the first pictures that accompany this article, you’ll see that we used papercrete blocks against the wall- behind what would become the bench. Less visible is the papercrete that we built into the floor (about 6 inches worth) along the wall. We wanted to make sure that we weren’t losing any of the heat to areas that don’t provide any value.
We didn’t use cob for the bench- we went with a mix very similar to that which we used for our bag work, just a little wetter. (Gravel reject: clay/soil: touch of cement. No- I didn’t keep exact measurements.) We used quite a lot of rocks in the bench. Rock stores heat more efficiently than cob, or in our case ‘mix’, so it makes for a better battery to have lots of them.
Our stove pipe will end up being eight inches from the surface of the bench. We don’t want it so high that the top super-heats and we want to ‘charge’ as much of the thermal battery (the bench) as possible, so that the system continues to release heat over a longer period of time. As the bench was put together post-building season we were short on time (it’s now freezing every day, and snowing) so we built the system to where it will work and provide heat to the area and will finish it off come spring.
The stove takes amazingly little wood. I’m impressed by how little fuel it takes to fire this puppy up and heat up that amount of mass. Although the bench isn’t done, it’s already several tons of material. Another impressive feature is how efficient this thing is. I’d seen it work at Cob Cottage, but I have to say- when I went outside and put my face to the vent to find steam being released- and not smoke- I was just thrilled. Still, we’re interested in using as little wood as possible so we’ll work on some possible biofuel alternatives over the winter.
So there it is- the basis of what will be our primary heating system for the living room area. Come spring we’ll finish building the bench and punch a hole lower in the wall so that we have a more direct fresh air intake (we’re connecting to one of our vents right now). In the meantime- we have heat!