The past few weeks have been the first real test for my rocket stove with outdoor temperatures dipping below freezing at nights, and daytime temperatures in the 30s-40s. (This week has below freezing daytimes, with wind chills in the negatives – but I am with family in NJ and April is at home tending the stove, so I keep hoping that it works well for her…) I have been firing the stove fairly regularly, and I’ve learned a good deal about its performance – or unfortunate lack of, at crucial times!
First and foremost, the rocket stove does not draw 100% of the time. When the air is still, or when the wind comes from the south (sometimes), the stove hardly draws at all. As you may or may not know, the flue pipe exits at ground level on the south side of the house, but I have added a vertical stack (6″ pipe, not insulated) to get the stack above the eaves (but not higher than the very peak of the roof). The stove is very sensitive to the wind. At times, the draw is so weak that the stove smokes into the house, and we cannot use it. The draw is strongest when there is a wind from the north or northwest. It is obviously not ideal to have to depend on the wind to be able to use the stove, because the house takes a lot of energy to get warm, which leads me to…
The stove takes hours to get warm. I am fairly certain that this should not be the case – that it should be warmer with less time (and wood). After several hours of firing the stove, the barrel just doesn’t get that hot! I am convinced that the heat riser, which is built out of firebrick, is leeching some of the initial heat before it even hits the barrel… this is my guess. I am not certain, though. With the barrel itself taking hours to get really hot, I have little hope that we can get the mass of the cob bed warm. It just doesn’t seem possible with the amount of time and energy it takes to get the whole thing really cranking. It is very difficult to raise the air temperature in the house – it’s been hovering in the low 50s, and after four hours of firing the stove, we can usually only get the air temperature up 3-4 degrees. It’s pretty cold in the house. We have gotten the house to 63 degrees inside, but that was after two days of pretty constant stove firing during the daytime (with a strong north wind).
I am not sure what to do to improve the draw. Rocket stoves are very finicky, and there are many variables to consider. Is it simply because of the two 180 degree turns in the flue pipe in the mass of the bed? Is it because the stack exits horizontally, and is only vertical outside of the house? Is the outdoor stack not tall enough? Is it something in the guts of the barrel, a tight gap somewhere where the gases slow down? Clearly the stove is capable of drawing well (such as on days with a northerly wind). But why does it not draw so well when the air is still?
And for getting it hotter faster, switching out the firebrick heat riser is only my theory. It could be because I am using a 6″ system, and the stove has a small firebox, too. There is only so much wood we can burn at a time. (By the way, it is good firewood – kiln dried oak scrap wood from a furniture place, mostly. Very small dimensional pieces.) But that barrel should get way hotter way faster!
Rocket stoves present a huge learning experience. Clearly this stove is not perfect, and it would be naive to think it would work without a hitch in its first iteration (and I have never built one of these things before, either.) But I do hope some solution can be achieved, because it could use some improvements! Brrr….