In the last post on the Rocket Mass Heater (The Joy of Rockets), I shared some of the challenges faced from the initial build, which required that I pull the barrel off to expand the size of the manifold and to increase the gap between the top of the heat riser and the barrel. This proved to make a huge difference but, in all honesty, I had higher expectations and knew that something was still not quite right.
My dissatisfaction came from the fact that after the initial start, which had great suction, the system would become a little finicky after about an hour. The signature rocketing sound would taper off and, in some cases, back draft, causing smoke to billow into the room. My suspicion was that the chimney, which was little more than some stovepipe poking out the side of the wall, was not quite cutting it. The chimney, if you could call it that, exited in a spot where air was trapped not only by our building but also by a neighbours garage. The air hardly stirred in this particular location and, consequently, the chimney was insufficient to move the smoke up and away.
Apparently this situation is not uncommon to those living with rocket mass heaters (RMH). Because a RMH has a chimney right in the combustion unit you generally get great suction from the start. But as the entire system warms up, essentially moving closer toward equilibrium, the draft created by the chimney in the combustion unit no longer serves to push the combustion gases, heat, smoke, through the system. What is needed, at least in my case, is a chimney that provides secondary draft.
My first experiment was to simply take a 6-foot section of HVAC duct and attach it to the ducting (aka the chimney) that lackadaisically exited the building. This not only took it up above our roofline but also above the neighbours garage. The improvement was instantaneously noticeable. It was like day and night. It was the difference between rocketing for the first half hour and rocketing for the entire time the rocket was burning. This was the final tweak that was needed and now all I had to do was make a proper insulated chimney, such that the flue gases would remain hot enough to evacuate without creating too much condensation, which would surely happen in an un-insinuated 6-foot chimney.
The beautiful thing about a RMH is that they can be pieced together safely and efficiently for a few hundred bucks. A proper insulated chimney, which is rated to deal with temperatures in the thousands of degrees F, would cost about $700 when you factor in the elbow, supports, and 6-feet of insulated chimney. This seems a little overkill, especially because the exhaust on a RMH is hardly above 100 degrees F after it winds its way through 25 or 30 or more feet of mass. My solution was to continue the 6-inch stovepipe into the chimney, surrounding it with 8-inch galvanized ducting to create a space that could be filled with vermiculite for insulation. All said, this chimney cost little more than $100 and has served its purpose wonderfully. It’s insulated, sturdy, and is more than enough to deal with the exhaust temperatures that exit the building from the rocket.
The new chimney was installed in late January and since then – about 40 fires later – the RMH has been an amazing addition to our lives. There is nothing more comforting than radiant heat and to receive so much warmth from so little wood is truly remarkable. It has taken a while to fully appreciate the nuances of the RMH but that’s the joy of creating something with your own hands. When you create something from scratch, you create a connection to it and you are ultimately responsible for it, working or not. When, after using your senses and intuition, you work out all the kinks, well, it’s just that much better. Pulling something that is already finely tuned from a box can be convenient but rarely does it provide you with a life lesson that shapes you and makes you feel a real sense of pride and accomplishment. I guess that’s why we’re attracted to this stuff. It keeps us sharp and it makes life fun, and always interesting.