Being a Chimney Sweep with a Rocket Mass Heater

The leaves have turned color and the air has become cool. This also means the house is getting chilly, needing some heat for comfort. During these edge seasons when the skies are not overcast and the sun is shining bright I’m generally able to take advantage of all the excess electric power my off grid solar system generates and heat my place with a couple electric space heaters. Once my battery bank is full this surplus power just goes to waste otherwise, so I might as well utilize it. However, the days are going to be getting cloudy and overcast in my region due to the proximity of Lake Michigan and the effect it has on the weather. It’s also going to get much colder. So I’ll soon be feeding the small dragon in my living room that is my rocket mass heater.

(Please note some of the links in this blog post are affiliate links. What this means is that should you click through them and make a qualifying purchase I will receive a commission which I’d certainly appreciate since it helps support this blog project. However, this shouldn’t increase your cost any, and certainly don’t ever feel like I’m pressuring you to buy things through the links I offer or anywhere else. I’m all about being frugal first!)

For those new to reading this blog, or as a reminder to the rest of you, the rocket mass heater is the new wood burning system I built late last fall to replace my traditional wood stove as the primary heat source for my home. I wrote a post about rocket mass heaters and shared some images from my build which you can find here. It has been a totally awesome thing!! Other people who have switched from wood stoves to rocket mass heaters (RMH for short) find they are using from 50 to 90 percent less wood to heat their homes!

In my case last winter I started using it a bit late because I was still building it when I would have liked to be using it. I also failed to properly get my wood split and stored ahead of time for best burning efficiency. A RMH is designed to burn hot, as in really HOT, so hot you would ruin a traditional wood stove, something like 2000+ degrees F hot. Part of what helps to get that level of heat is having nice dry wood that is split up into small pieces exposing more surface area to burn at once making for a short, hot fire whose heat then gets stored in the thermal mass to be slowly released over many hours or even days.

Last year I was burning wood I had mostly stored outside under a tarp, split a bit more with an axe, and was managing to get indoors to better dry out for only a few weeks, if not days, before burning. This year I made a kindling splitter to more safely and easily split up the wood and have been stashing it away all summer in my old metalsmithing studio, which has now become the wood shed. I expect this will increase my burning efficiency this year. There are still changes I’d like to try making to the construction of my RMH that I think will increase its efficiency but they are bigger projects and I didn’t get time to tackle them. I know my heater will never reach the full awesomeness these units can get to simply because I lack the physical space to make the heat exchange/storage bench as large as it could be to fully draw out all the potential heat before sending the cooled exhaust up the chimney.

Last year my estimate was that I reduced my wood use by 50% to 55% bringing it down to about 1.25 to 1.5 cords overall. Additionally I drastically reduced my supplemental propane use by 80% to 90%. I do have a regular propane furnace, but I’ve made this my backup system for use when it gets too cold at night or when I’m away from home for long periods. Since using the RMH the propane furnace really never turns on at night unless maybe I hadn’t burned any wood during the entire day. With at least one fire a day the mass of the bench easily releases enough heat during the night to keep me comfortable. My hope this year is to get my wood use down to a cord and keep the propane use about the same as last year. We’ll see what happens.

So even though the rocket mass heater is totally awesome for heating and reducing greenhouse gasses it is still burning wood which means it needs a good cleaning out to start the season fresh. With my old wood burning stove this involved cleaning out any creosote that had built up in the chimney in order to prevent a dangerous chimney fire which could burn my whole home down. This past spring at the end of the heating season I did eagerly climb up on the roof, remove the chimney cap, and look to see how much build up there was. As I hoped, there was nothing to speak of! With a RMH there shouldn’t be any creosote build up in the exterior chimney. This is because the system is designed with an internal heat riser, essentially an internal chimney, that gets so intensely hot all that potential creosote build up is burned as fuel to heat the home. Put another way, the system is designed to safely have a chimney fire every time to prevent any build up from happening.

So I don’t have any creosote to clean out, but unfortunately that doesn’t mean there isn’t any annual cleaning to do. When burning wood, even when burning it fully and efficiently, there is ash produced. The vast bulk of this get cleaned out of the initial J-tube burn chamber every couple days. This is the equivalent of shoveling the ash out of a wood stove. However, a RMH does have one extra cleaning challenge. All the hot exhaust is run through a series of tubes where the heat is pulled off into the storage mass. These tubes and spaces become difficult to access zones where fly ash builds up, slowly restricting and clogging the air flow. Fortunately these tubes and inner chambers only need to be cleaned out once or twice a year, depending on how much and how efficiently you burn.

This cleaning is something that needs to be considered when designing and building the heater to begin with to provide for reasonably easy access ports. This is something I tried to do, yet there was a nagging concern I’d get it wrong since I never read any clear description of how others went about the annual cleaning of fly ash. So this is what I really wanted to share today, the approaches I tried this fall to take on the task of being a chimney sweep for a rocket mass heater.

The first thing I wanted to do, both to see for myself and to share with all you, was to discover just how much ash build up there was. I started with the barrel. When building my heater I made a point to get a barrel with a removable lid specifically to make this cleaning task easier. Without the removable lid I’d have to break the cob seal and carefully pull the whole barrel up and off. That’s certainly not an impossible task, but it’s a bit of a challenge I’d rather avoid needing to do often. In my case all I had to do was loosen a bolt to remove the binding band and pop the lid off to see that indeed fly ash had been building up. It was probably worse than you see in the photo below too since this spring I had a bird that managed to get through the small spaces in the outdoor chimney cap looking for a good place to build a nest. It eventually ended up in the barrel, no doubt knocking some ash down and around. (please note this is not a problem new to the RMH. I’ve had the birds get into the wood stove in the past too.)

In addition to making sure there was a removable lid on the barrel I also built three different clean out port holes in the bench section. The first one is right after the tube starts out from the manifold section below the barrel. I expected the worst build up in the tubes themselves to be here. Indeed there was a fair amount of build up.

My second clean out port was designed to be at the very end of the first horizontal run of the heat exchange tubes in the bench, just before it makes the 180 degree bend for the run back and out the chimney. I made the port here so I could both see and be able to easily run a brush straight down the entire length if needed. I expected the ash build up to diminish as it made its way down the length of this tube but it looks fairly consistent. What you don’t see well in the photo is how the build up dropped off quickly as the tube turned the corner. This makes sense to me as it’s a point where things slow down as the exhaust gasses meet more resistance resulting in the drop of the particulates just before the turn.

Finally we have the last clean out port just before the heat exchange tube makes the bend to vertical, becoming the exhaust chimney heading out of the house. The ash was less here which was nice to see. The block you see in the photo was a chunk of firebrick I put there deliberately. Because I have such a short run of heat exchange tubing and a strong draw to the exhaust I decided to impede the flow a bit with this block, slowing things down and hopefully transferring a bit more heat into the storage mass. My testing equipment was not really precise enough to know if this worked or not. If I found I had trouble with too weak a draw in the draft I figured I could always go and remove the block.

I feel like I should probably note that you see some black residue lining the tubes in these photos. My concern was that this would be creosote building up. There really shouldn’t be any creosote at this point. It should have all been burnt up in the firebrick heat riser. I believe this black residue is just soot. It doesn’t feel especially sticky or anything, nor is it built up to a thick coating. I do plan to keep an eye on it though just in case.

Ok, so that is a visual tour of the mess I needed to clean out. The question is how best to do the actual clean out? The first technique I wanted to try was one I read about on some RMH forums. Some people recommended using a leaf blower to simply blow all the fine, powdery ash out. If this worked it seemed like it would be the easiest. I wouldn’t really have had to even open the whole thing up. So I put the lid back on the barrel and closed all the clean out ports up tight, and got out the leaf blower I had bought just for this purpose.

Now if you know me you probably already know I’m not one who is likely to use a leaf blower to chase leaves around the yard. If I feel the need to collect leaves I’m going to do it with a much simpler tool, the humble rake. A rake costs less money, has significantly fewer parts to break, and has so much less of an environmental impact over its life cycle. Better even than a rake to me is simply leaving the leaves where they fall so that they can decompose in place, feeding and building the soil. Staying out of nature’s way and doing nothing with my annual crop of fallen leaves is my general approach.

So for the RMH I had to go buy a blower with this use in mind. It seemed like a very good theory for cleaning. A high rate of air flow should pick up and blow out all the fine ash build up. I knew I was going to be using this indoors so in no way did I want a gasoline powered blower! The cordless electric blowers looked pretty cool and would certainly be easier to work with out in the yard. However, they come at a much higher cost which makes sense because they are more complicated, which also means there is more to break down and a higher environmental impact to produce them and eventually dispose of them. I have no intentions of using this on any sort of normal basis in the yard. So the ideal type for me was a corded electric blower, fairly simple, lightweight, and cheap. I went with a basic Black and Decker model.

What I had read about the leaf blower technique of cleaning very strongly suggested that one first remove the chimney cap! They didn’t say why, only that you would know why if you didn’t. I’m assuming that impeding the strong flow of ash laden air at the top of the chimney would result in said ash finding its way back into the house in a potentially dramatic fashion. I opted to just follow the advice and remove the cap.

So logically if I wanted to clean the fly ash out of the entire length of the RMH I would stick the blower down the initial J-tube and let it run. This is what I did first, trying to get the blower pipe directed as far down the channel as I could and minimizing the feed tube opening with a firebrick. You can see in the photo below my initial setup.

Some of you are probably laughing at me already, and rightly so. I’m not always the sharpest tool in the shed. I get by more on persistence I think. So yes, I flipped the switch unleashing the obnoxious roar of the leaf blower (did I mention how wonderfully quiet rakes are?) and simultaneously a great cloud of ash shooting into the room from all the little openings around the feed tube. Well, I was needing to dust the house anyway…

After it having been made clear that I needed to seal off the opening much better I went and grabbed a towel to shove around all the spaces to prevent any blow back, keeping all the air moving in one direction through the chambers and tubes of the RMH.

With things properly setup I let the blower run, growing to detest the noise more and more, and went outside to see if I could see any fly ash shooting out from the open chimney. I could see a weak stream, so I knew it was doing something, but I’d expected more. Maybe there was a big initial cloud blasting out before I could get outside to see. I don’t know.

After letting it run for a few minutes I turned the blower off to blessed silence and opened up the clean out ports to see what I could see.

The results were mediocre. Clearly some had been blown out, but it appeared that the bulk of the ash was still sitting happily in place. Being persistent I decided to next try feeding the blower in at the tube clean out ports to see if directly blowing there would at least clean out the tubes better. Again I let it run and ran outside to see what was rising out of the chimney. This time there was a stronger flow of ash moving out, so I moved the blower from one port to another trying to see how much I could get. The flow from the chimney was never strong enough for me to get a good photo where you can see the stream. Below is a shot from the center clean out port where the tube makes its 180 degree turn. You can see where significant progress has been made, but a lot still remains.

I then shoved the camera into the first port aiming back at the manifold section below the big barrel and took the photo below. You can see there is still a nice pile of fly ash built up where it was designed to collect.

Next I decided to stop playing with the leaf blower since it didn’t seem to be getting much more and another technique was needed. Being true to myself I moved to low tech options. First I took my chimney sweep brush that I’ve used for years to clean out creosote and ran it down the long length of tube I had access to from the second clean out port. The result was that the ash in this long section with the greatest initial build up all got shoved down to the first clean out port.

With most of the ash gathered in one place I next grabbed a piece of scrap paperboard from my recycling bin and cut out a scraper I could use to just reach in and scrape the ash out.

This low tech approach did work, though it got mighty messy. I probably shouldn’t have been wearing a long sleeve shirt, which is now likely permanently sullied with embedded ash stains. I imagine I was also starting to look like the chimney sweeps of historic lore all covered in soot.

While this approach was working it was still challenging to reach into all the spaces. With a bit more messy effort and creative adaptations to my scraper and brush this probably would have worked well enough. It’s not like these tubes need to be pristine. I just need to reduce the obstructions to air flow enough so it would function properly. However, I decided to now try another approach at cleaning I’d read about, that of the shop vac.

This is probably what I should have started with since I already had an old shop vac, and it was what most people suggested using. However, I resisted for two reasons. First many people also said they didn’t work real well due to how fine the fly ash could be. I assumed this meant it clogged up or would not be sufficiently filtered and thus blow the fine ash out into the house. The second reason I didn’t start here was that I kinda doubted my old shop vac would work. I hadn’t tried turning it on in years. It was a low end cheap unit to begin with, and the clincher was that it got submerged when many of my outbuilding flooded a year or so ago. My guess was that it was junk.

Still I went and hauled it out, opened it up to find water still in the bin from the flood. It is a wet/dry vacuum though so I cleaned out the collection bin, rinsing it down. I pulled out the mud coated fabric filter and washed it off. I thought about letting this filter dry off before trying it, but was feeling impatient at this point, and hey, it is a wet/dry vac right? Plugging it in, flipping the switch, half expecting sparks or a circuit to break, I was pleasantly surprised to have it roar to life and work just fine. I did choose that word “roar” deliberately as it is just as obnoxious sounding as the leaf blower.

Since the barrel and manifold chambers were the most challenging sections for me to clean out so far, as well as being the zones where the ash would collect first and most, I decided to again pull the lid off the barrel and try the vacuum in there.

In the first few seconds a cloud of fine ash blew out the exhaust port of the vacuum. Initially I shut it right down. Then deciding since I already had a fine layer of ash blown around the room from earlier efforts with the leaf blower I persisted to see what would happen. Would it suck up quantities of ash quick enough to make the amounts blown into the air worth it? To my surprise it stopped blowing ash into the room and just sucked it up into the collection bin like a dream! I’m still not sure quite why I got the initial cloud and then nothing but my speculation is that leaving that fabric filter wet was a fortuitous thing. Perhaps initially openings in the filter were large enough to allow fine ash through, but it quickly “clogged” with the moisture in the chamber dampening the ash so it stuck together reducing the holes thus preventing any more fine ash from blowing out? I don’t know. That’s my guess anyway. I’ll have to experiment more next year when it’s time for the annual RMH cleaning again.

What I do know is that after that initial cloud, the shop vac quickly and cleanly sucked up the remaining ash build up, leaving what you see in these next photos.

I have also heard that there are vacuums specifically designed for cleaning out ashes, with extra filters or something. If I didn’t already have a shop vac I would probably get one of these. I might also consider trying one in the future if I find my vacuum blows a lot of ash during next year’s clean out. (Edited to add, a FaceBook friend, Steven, shared with me that there are such things as filter bags you can use with a shop vac which are designed to work with fine particle dust such as ash. Looking on Amazon I would guess he was talking about things like this. I will probably look into getting some to try using next time. Thanks for the tip Steven!)

I think for next year I’ll try starting with the vacuum, though I can see me utilizing the chimney brush to push the majority of the ash to the clean out port for easy vacuuming. Since I already have it, I might then finish off with a good blow from the leaf blower. In hindsight though I don’t know that the leaf blower was worth buying for this job. I expect I’ll make another blog report next year as I refine my system to share what I’m learning.

So that’s my report on my first annual cleaning of my rocket mass heater. While I did occupy a fair chunk of a day doing this I know it won’t take nearly as long the next time. Much of my time was spent doing extra work to document this for you all, as well as trying different approaches to see what worked best. Next year should be fairly straightforward and go much quicker.

This cleaning part is what I have been quietly dreading in fear it would be a huge, horrible task. Now that I’ve faced it I find it’s not a big deal. It’s certainly nothing that should prevent someone from building their own RMH. These are such superior wood heating systems it’s crazy! While all the links and much more info about building one is in my previous post, the rocket mass heater advocate in me feels I should offer links here to the resources that were a huge boon to me in building mine. These are Ernie and Erica Wisner’s “Rocket Mass Heater Builder’s Guide” and Paul Wheaton’s 4 DVD set “Better Wood Heat: DIY Rocket Mass Heaters”, also available for less money as HD instant view instead of physical DVDs.

The world needs more people adopting rocket mass heaters to stay warm in cold climates, save a ton of money, and reduce our environmental footprints!

Studio Snippet

If you don’t know already, I love rocks! For years I’ve resisted bringing stones into my work because I know I’d probably end up with boxes and boxes of absolutely beautiful stones others have cut, polished, and sold to me. I’d have a bunch of capital tied up in enough stones to supply several lifetime careers of metalwork.

Now, however, I am seriously considering some new bodies of work that would utilize stones. To avoid getting sucked into spending all my money on them though I’ve decided I’d get even more joy out of doing the lapidary work myself. I’m even going to start out using just rocks I’ve also found myself. To that end a few days ago a friend showed me the basics of how to cut, shape, and polish some cabochons. I had a grand old time and expect I’ll likely be investing in some new tools to do this at home before long!

These may not look too exciting to you just yet, but this is what I’ve done so far. All these stones were found on my most recent camping trip to the Upper Peninsula of Michigan. The middle one is what I started with. I have no idea what the stone is, but I’m happy with the look of it. The two on the right are pieces of slag from the old Bay Furnace iron smelter up near Christmas, MI. I hadn’t realized these could be worked as “stones” until a friend on Facebook made the connection for me to Leland Blue stones which are slag from an old iron smelter around Leelanau, MI. Funny thing is I could have picked up way more of this had I realized what it was. A major storm had washed away the shoreline exposing a new crop of it.

The two unassuming stones on the left side are something special that are really what pushed me to learn how to cut stones now, instead of leaving it for a “future” project. I’m not going to tell you much about them just yet as they will comprise a future blog post on their own. I’ll give a hint though. They have a hidden inner light that is quite stunning to see. Since inner light has been a major theme of my work for a long time now it seems fitting to play with them somehow.

I’m happy to have a site where I can again allow comments. (I had to shut them off on my main website because the spam was simply uncontrollable!) So please I encourage you to share thoughts of your own. My general rule about comments though is just to play nice. Differing views are fine, but I’m not interested in engaging in or moderating verbal fights. If I feel things get out of hand, by whatever criteria I decide, I’ll just start blocking or deleting things.