How much ash from a rocket mass heater?

This is the amount of ash my RMH produced last year. After sifting, actually about two gallons of it was biochar. I know this doesn't provide much scientific data: I didn't record how much wood was burned, temps of the house, insulation of the house, types of wood burned, etc. This is just a simple visual representation of how little ash is produced from keeping an average sized, poorly insulated house in zone 7b comfortable enough to live in through a full cold season just using reasonable means.

I don't have pictures, but a few years ago I kept all of the winter's ash from my 8" J-tube that heats the main floor of my house. (The walkout finished basement is heated by infloor radiant heat kept at 60F.) I had a five gallon bucket of ash, no charcoal or anything combustible at all. I tend to burn about a full cord of wood in a season.

I'm curious how much ash gets into the pipe runs due to starting fires with cardboard or paper, compared to starting each fire with a feather stick, where you aren't using any paper. Are your examples including the cleaning of the mass, or was this just from cleaning out the feed area?

This season I opened up and inspected the horizontal flu runs (26 feet in length) of my RMH. After five consecutive seasons without any cleaning, I didn't find any significant fly ash accumulation in them. There is between one to two inches of fly ash that managed to pass up through the heat riser and settle in the spacious open area at the base of the steel barrel (1/2-barrel manifold). A scant handful of fly ash had drifted a few inches into the manifold to horizontal flu connection area. I was pleasantly surprised, having put off inspection / cleaning for several years, since the stove is still working just as well as day one. I'm burning one to one and a third cord of wood annually.

Mark Brunnr wrote:I'm curious how much ash gets into the pipe runs due to starting fires with cardboard or paper, compared to starting each fire with a feather stick, where you aren't using any paper. Are your examples including the cleaning of the mass, or was this just from cleaning out the feed area?

I don't use paper for this reason, mainly. Like Byron, mine had 2.5 years on it and when I checked the horizontal pipes, there was shockingly little ash. Not enough to try to clean. I imagine when it enters the drum area, the speed slows down so much it mostly falls and settles around the riser.

This is actually something I've been very curious about. To me, it sounds like RMHs produce way more ash than I would expect. I got this impression from only off hand comments initially, but a couple comments in this thread kind of confirm it to me.

We have a very basic wood stove. The back half of the stove is slightly taller than the front half to allow for the stove pipe exit, but it's basically just a box. We burn one and a quarter to one and three quarters cords of wood in a winter, and produce anywhere from three to five gallons of ash, depending on what kind of wood we're burning and how much we keep it damped down (it's usually a lot). We also burn all our paper and cardboard waste. It seems like that amount of ash is at least comparable to what's been reported here.

I know we're not losing a shit ton of ash up the chimney or anything cause the snow around our place would be dirty all the time if that was the case. What am I missing?

It sounds like similar amount of ash per cord, which is expected, the ash should just be the nonburnable minerals in the wood. RMHs shine in that the heated mass means far fewer cords of wood are needed to heat relative to a regular wood stove, with far less smoke and particles like creosote going up the pipe.

I've seen some photos of people cleaning their mass and it seemed like a lot of ash, like an inch thick, while others show a pic where the metal is still clean and no cleaning is needed. I just wondered whether the latter avoided paper products or perhaps other volatile stuff that could condense once cool enough, and then that would trap other particles.

When I have a stick and a knife I enjoy goofing around making feather sticks and I hope to avoid lots of mail and food packaging, so I was just hopeful that I'd see far less cleaning of the RMH mass by sticking to mostly wood when lighting it. When I used the minnie mouse heater in the Love Shack at Wheaton Labs, I would burn maybe 2 pounds of wood and the next day there was zero ash in the wood feed, the coals were totally totally consumed the night before.

Mark Brunnr wrote:It sounds like similar amount of ash per cord, which is expected, the ash should just be the nonburnable minerals in the wood.

I always thought that if you burned hot, you'd be burning the wood more completely and therefore have less ash. I guess I was thinking about it wrong. You'll just have cleaner smoke?

A "cold burn" or incomplete burn can leave you with bits of coal in addition to ash, but those bits of coal should still be burnable. Once or twice I have left a wood feed fully open even though there wasn't much wood left in it, and the thick-ish stick went out when it was mostly a coal coating around a wood core if that makes sense. I would leave that in and add smaller wood bits the next firing, and that wood was all burned up. If I only wanted to do a single burn/load, once it's running I would stop by and close the feed opening a little every 10-15 minutes, as I heard the "rocket" diminish. Once closed a bit more to match the volume of remaining wood, the "rocket" sound would return and doing this towards the end ensured no unburned sticks remained. That RMH had a different design than the normal firebrick wood feed, which might have accounted for some of that issue. A brick feed will heat up after a bit, and I think that additional radiant heat might help towards the end.

Yeah, that makes more sense now. I might clean the stove out once over the winter. Otherwise I wait until spring. That probably means we're burning up all the partially burned stuff from prior fires, where people who clean out the stove more often might be scooping that stuff out and bulking up the ash.

Ever notice how incredibly strong your RMH drafts on a windy day or as a windy cold front passes through? I imagine those living in areas with consistently windy conditions day to day, would be more at risk of having fly ash sucked deep into their RMH's horizontal flu runs - depending on their chimney system, and whether they are using a wind resistant chimney cap etc.

Interesting thread! For myself personally I do get a fair amount of ash. I use paper bags for the typical start up which probably accounts for some of it. But mine comes from what wood I'm burning. I have an abundance of red oak, maple, and ash trees (even though they are all dead standing from the ash bore bugs, sadly) on my property. The ash tree wood, like its name, produces 3x the amount than the oak and maple. The oak leaves, it looks like, a dark bio-char, but not much is left, easy cleaning. The ash wood gets drawn up over the riser and into the first 2 foot of the mass. So I'm interested in what type of wood people are using. I'm in northern Michigan so my heater runs for half the year. This year I'm going to try and not run my basement propane heater at all this year, since my rmh supplements my heat in the house. It's my third full year burning and I still really enjoy the rmh, but I like to tinker, so it fits my personality.