Autopsy of a rocket mass heater

Winter in tiny house was cold. We didn't put much effort into building tiny house because we only needed it for one winter and we were expecting the winters we've been having for most of the last decade: a few snow storms but mostly in the teens with just enough t-shirt weather to melt all but the plow-piles of dirty, gravel-filled snow. If we'd had that winter tiny house would have been swell.

The polar vortex of 2014

Mike trying out his new drawknife January 2012

Back in the early fall we were trying to decide between building a rocket stove mass heater and installing a cast iron wood stove. Spoiler alert: this post could also be titled "the winter of my epic failure,"as Mike didn't want to build the rocket stove but, for better or worse, I can be very persuasive.

January 2014

The rocket stove we installed kept us warm until outside temps dropped below 10 degrees, which, anyone in upstate NY during the polar vortex knows, was so often as to make one believe spring was never going to come. For much of the winter tiny house hovered around 45, which isn't so bad in the spring, but in your home, when outside chores are done in subzero temps, 45 degrees means going to bed cold and waking up cold. It means so many blankets that shifting their weight as you toss and turn in your sleep wakes you up, and the two dogs and cat you let sleep in bed to help everyone keep warm wake you up too and in the morning you're still tired. So that was our winter.

Great info on building rocket mass heaters is available all over the internet, so I'm not going to cover that. Rather, this is an account of our difficulties with the heater and some guesses as to what went wrong.

Dry run of the mass heater showed excellent draw.

Fire being drawn through the burn chamber.

Mixing the cob

Laying it out in tiny house

refractory mortar for high heat applications

Cobbing it together

I had pictures of the finished product on a cell phone that lost a fight with the dog's water bowl. Oh well, it pretty much looked like every other simple mass heater.

Autopsy report:
1. Getting started: For the first two months the stove started up quickly but it got progressively more difficult to get the wood to catch and stay lit.
            -this could have been an issue with wood wetness as we cut our own firewood from 6" diameter black locusts we felled two summers ago but didn't chop up until this fall/winter--might have been too wet?

2. Wood usage: Online you'll find countless reports of the mass heater's superior efficiency and subsequent decreased wood usage. We did not find this to be true. We used about half the wood as our neighbor's conventional wood stove and they heat a rather large home (around 2000 sqft). Also, other rocketeers report very little ash to clean out of their stove, by contrast we were removing roughly a full paint can a week.
            -again this could be a wet wood issue
         

smoke couldn't get around the build up

3. Smoking: By January (two and a half months into using the stove) we were getting heavy smoke back for the first half hour of trying to start the fire. It completely stopped working in the last week of March, smoke billowed into the house and we couldn't get a draw. We periodically cleaned it out and fed a shop vac hose through the pipes, which usually helped but in the end this is what we found:

charcoal and ash build up between barrel and heat riser

        -my understanding was that this much ash shouldn't have made it to this point (up through the heat riser and settling between the barrel and the riser), having been so efficiently dissapated in the heat of the burn chamber, which would suggest that our fire didn't get hot enough. However, I find this unlikely as the fire got so hot that within the first month the refractory mortar was crumbling apart.
        -we probably used more paper to start the fires than advisable, this may have caused an ash problem.
        -I'm not satisfied with either of these explanations.

4. Warmth: Rocketeers report warming their bottoms on mass heater benches a day after the last fire. This certainly was not possible for us. The bench only warmed after hours of running a fire and was stone cold in the morning. When temps dropped below zero, which it did frequently, we took turns waking up every 1.5 hours to refresh the fire throughout the night (ok, mostly Mike because he's a mensch).
           -this one is up for some debate: one of us thinks there wasn't enough sand in our cob mixture, the other thinks there was too much.
           -not enough of a mass (tiny house is 9'x13', we could only fit a 5'x2'x2.5' bench), maybe the heat was drawn through the small system too quickly to heat the cob
          -tiny house was not "buttoned-up" the way the house will be due to previously mentioned expectations of a mild winter--maybe we were losing heat faster than the rocket stove could replace it.

Conclusion: I could not recommend a rocket mass heater, but I also wouldn't argue against them. I think user error played a role and I believe online rocketeers who say they love theirs. Aside from the functional aspect, I have to say that after the novelty of having built it ourselves-- and it looking vaguely steampunky-- wore off, the aesthetics were not our scene. The industrial look of the barrel, the bulkiness of the bench, and not being able to watch the fire burn are enough to dissuade me from having one in the house.

For the house, I'm keeping my fingers crossed for one of these: