Rocket Mass Heater

Hey all!

I am planing to build my first rocket mass heater and I thought I upload the plans, questions and eventually the build, knowing there are very experienced people here,
I´m hoping you could help me through it.
I live in Athens, Greece. I´m on the 6th floor of an 8 floor building and it can get cold here. it´s an 85sqm open space. the building is industrial so it can hold the load and     It´ll be placed mostly above a beam.

My first question is if I should do a 6" or 8" system. the 6" (15cm is wildly available here but the 8" seems hard to find and more expansive. All the explanations I find are for the 8" and the space is large. Just making sure, it´s better to insist on the 8"?
2. I cannot put a chimney through the building so it´ll go out through the west side wall, then maybe 4m horizontally (I´ll add upward slope) and then two floors up. this part would have to be insulated, right? would foam/rockwool with some silver tape around it be ok? The insulated pipes I found online are real expansive... anybody has experience with that?
3. I´m pretty scared to install this chimney by myself. I spouse professionals could do that?
4. For the heat riser, I see people use ceramic blanket. Could I use rockwool/ perlite slip or better just order some blanket already?

An 8" system will certainly be more capable of ensuring you get enough heat. But a 6" might be fine, depending on conditions. 85 square meters is around 850 square feet, not large by US standards, and you will have no ceiling heat loss. How much wall exposure do you have, and what kind of construction is it? Any south exposure? North exposure? You say "cold"; how cold is it? What is common winter day high and low temperature? With this information, we can give better advice on 6" or 8".

The 4 meter external horizontal chimney run is problematic. Can you make any of that run inside your own space? It would both improve the heating and increase the draft. I presume you can't get a closer location for the vertical chimney. Homemade chimney insulation can be effective, but may be against building code. What kind of regulations do you have? In your high location, I would use materials that will for sure not have to be repaired or replaced in a few years.

Pieter,

Athens is rather a warm place in winter.
Is your building insulated?
Is the 85 m2 open space or it has rooms? If yes, are they separated with masonry walls or something lighter?

Hi Pjeter,
First things first I would check wether there is a safe and legal way to run that exhaust through (someone else’s?) part of the building.
Also it being 8 stories rings my alarm bells. In Germany (where I know the code better) things get very complicated in anything over 5 floors.
Asking a professional is a good first step as you mention.

Athens dosn´t get bellow 0 Celsius but actually can get pretty cold inside the buildings because they are not really designed to deal with that.. My place in one space (expat the bathroom so the room is actually more like 70sqm) It´s brick walls and terrazzo floors that turn into a freezer in winter.  my apt in the center of Athens is built in 1965. Its 6 floors of factory spaces and the other two floors are apartments. It used to be a typography factory and some other floors still got those massive pressing machines so I´m less worried about the weight.
My walls are not insulated and the wall exposure is north west and east with very large windows on the west and east  (I´m planning on double glazing them as first part as this winter project). It´s also get extremely hot due to this orientation. The chimney would go on the west side where I have afternoon sun at most days and because of that, I also thought of using the chimney as a solar chimney in summer : make a opening/ cleaning port at the part of the chimney that's inside the house and close to the ceiling to open and draw the warm air out in summer...  Maybe it could be even better without insulation for this reason? If it´ll be isolated I guess I cannot do that.

I can´t run the chimney inside the above properties. The part that would run vertically, I also prefer outside but maybe I´ll compromise if it is very important. Can you tell me why this is an issue? I´ll also mention that above the part of the chimney that would run horizontally, there's a balcony of the floor above so It might add insulation. I am honestly most worried about it not having a good draft.

About regulations.. I lived in Germany before moving here. I am not sure if  it´ll be completely legal. I´m not going to try to get a permit.. I live in a yet very rundown area of Athens and from my window I can see countless building violations.. Also a few of my neighbors has a fireplace. I doubt if its with any permit... I thought best is to locate it above the beam and keep size to minimum.

I´ll add a floor plan of the placement I had in mind. measurements are in cm.

As you control the whole interior leading to the chimney location, I see no reason not to run the exhaust inside just below the ceiling right up to that corner. I also think that if you do that, the mass run can be reduced a little bit so the exit temperature is higher, and then get some radiant heat from the whole length of interior exhaust duct. This would allow for not depending on rising right next to the barrel to regain heat/draft; you could run the exhaust up near the wall so it has a straight run to the corner.

I understand keeping the mass over a main beam in the floor, but if you want a low bench mass rather than a tall bell, it will be spread out enough to not matter much where you put it in an industrial-grade building. In a climate that seldom gets much below freezing, I don't think you need a large mass. If you actually want a bench rather than a tall mass, I would advise making a half-barrel bell (or other construction that equals at least a 20x40cm internal cross section). This will reduce the mass while giving the same radiating surface in the living space, and also greatly reduce the friction in the system so that draft will be easier.

You really want to minimize bends in the horizontal exhaust, so if you can rise from the mass, run straight toward the corner, then exit through the wall at a 45 degree angle that leads directly to the vertical chimney location, you will get the most reliable draft.

Man, I thought it´ll be straightforward but there's some considerations to put in it
I looked again at how I want to set it up and I realized there's some site restrictions and I might have to do it with the insulated pipe running outside anyway because I have a concrete beam running all through above my windows and there's a column running up on the same line with the heater so I think there's only two places I can drill a hole for the chimney ( I marked it on the photo)

I thought I reduce the size of the mass to 140cm from the bell to the end of the bench and insulate the outside chimney, give the 4 meter vertical part a good 70cm slope+ the pipe would be black (It´s a sunny spot  from 15:00 and in Athens its sunny even on most cold days)
Glenn, I looked at some half barrel. it´s like a stratification chamber? It looks really interesting but I really wanted to do a simple design for my first heater..
I would like to have just a low bench.
What do you guys think- would this size of mass be ok to keep the smoke warm enough on the way out? Would going for the 8" system be the right way in this case?

Should I rise the pipe near the bell? If not, how? I thought I can put the last part of the pipe above the first part so that it can heat up a bit. would that help or just have the pipe zigzag instead of do a 180

I´ll add a few sketches of those options let me know if they make seance..

Hey all! So, I finished the RMH and wanted to share the results.

There’s about 4.5m of 8" duct inside the exchange, and after it goes out of the house, it runs 4m horizontally and another 9m straight up. I insulated all the outdoor ducting with 5cm rock wool, tarp, tape, wire and aluminum tape on top. I hope it holds up well.

For the heat exchanger, I used cob and some rocks and plastered it with a mixture of lime and marble dust. I think it looks great and feels great.

I built the manifold with construction metal grid to initialy hold the bell in place. I tried to make it as spacious as possible, so it’s kind of conical. I laid the cob pretty thick on it, so it won´t crack.

The parts that are directly on top of the bell did crack a bit when it’s firing strong. I hope I can just repair it while it’s expanded, but anyway, it’s only small cracks . I would have added fiberglass mesh if I were to do it again. I did tried to make a small gap for expansion but apparently not enough

The RMH works great. I mean, it drafts like crazy and is super easy to fire.

It’s not holding yet heat as much as I hoped, but it is getting better each time, and I think it just needs some time to dry.

I added a few more features:

  - I added a damper at the point where the duct goes out of the heat exchanger. I use it after firing to stop the hot air from drafting out, but I also found it’s good for generally controlling the draft while it’s running.
   -I painted the last two meters of the chimney black and inside the house, I added a port to the duct close to the ceiling. I’m hoping that in summer I can keep this port open and it would function as a solar chimney to draw the hot air out of the house.

Hi Pjeter;
Your build looks excellent!  Great Job!
Nice, you have no draft problems.

However, the damper you added is not a good idea at all.
By slowing the flow, you may find excessive ash is generated, slowly filling your system with ash.
It will also enable creosote to form, with the possibility of a very scary chimney fire.
RMHs are only intended to be run wide open, thus creating the temperatures needed to incinerate particulates.
You are treating this as a conventional wood burner by slowing the flow.

You have a strong draft, so you might benefit by trying the method I use for lighting a strong J-Tube in that situation.
Use two pieces of larger wood to partially block the burn tunnel.
Behind them, you place your paper, kindling, and smaller starter wood.
By partially blocking the burn tunnel, there is a venturi effect, creating an even stronger draft.
The larger wood will keep your coals in the feed tube, creating a glowing base to which you can add larger wood as room permits.
Very important once your fire is burned down, cover the feed tube with brick to stop the heat from being sucked out of your chimney

Thanks for the comment Thomas! I did not think about it...
I think I´ll just use it as I first intended, to keep the heat in after I fire the RMH and I also close the feed tube with firestone

Please be careful about closing it off.
Only close it when there is no material left glowing. (I suggest during the summer)
Carbon monoxide is a silent killer with no smell. If your damper is like ours in the US, there are holes to allow CM to escape. making them safer, but...
With a firebrick-capped closed system, the damper is not needed at all.
The tiny volume of heated air that rises up your chimney is negligible when it can not draw fresh air in through the feed tube.

However, your "cooling"  vent might work exactly as you are hoping!

EDIT) Perhaps a fan might help draw the hot summer air into and up your chimney?

Very nice build Pjeter,

I will have to "second" what Thomas said, about your damper beyond the fire. Actually using it the way you described, I am surprised you wouldn't get into trouble on "heavy air days"   That is simply, being on a edge of a weather front, Everything starts perfectly and you decide to damp it off like you described, and the front moves over a few miles to your area.   This heavy air, tends to be high in moisture, warmer than when you started the fire, and well "not conducive to great drafting"  If your gone for a an hour, you could come back to a house full of smoke.

That really won't happen, if the fire is simply running wide open.   With no restrictions.

If you find this is spiking your house temp more than you like, you may find your self adding more cob, (more mass)  which I think you find as a double win with even more even heat in your beautiful home. Win-Win

Lastly, a barrier at the beginning of the stove will be more effective than one after the stove to maintain heat in your mass.  Thus preventing the saying,  " you will only forget once" on your damper setting.  Better to never forget.

Hope this makes sense,   excellent looking build.

Hi pjeter - Well done! That looks like a beautiful heater, I'm glad it is working well for you.
I found Paul's video on using the thermal mass for air conditioning in summer - I hope this helps

Hehe, ok, you made me rethink it. I’ll probably just remove the damper or keep it open. But why do you think the amount of heat escaping when the feed tube is closed is negligible? The hot air will continue rising up the chimney even when the feed tube is capped, no?

Regarding the cooling function, I’ll update here in summer. I also want to build a wall-sized swamp cooler to accompany it. Athens is dry and hot in summer, so I think it could all function together.

I watched Paul’s video. It’s cool. I think it’s different from what I’ve done because he’s trying to cool the mass by getting cool air to go into the feed tube, and I want to keep it capped and only let the hot air go out from the ceiling area. If it doesn’t work as expected, I might try his method.

I don’t think I’ll need an extra fan for that because the sun in Athens is intense, but I might add glass on its side to make it a more efficient sun collector.

Yes, it is true that your warm air will drift upwards even with the feed tube closed off.
The difference is the volume of air and its velocity.
With the feed tube wholly closed, the air inside is relatively stagnant, and not much will go up the 8" pipe.
If the feed tube is left open, then there is cooler room air being drawn down into the mass where a corresponding amount of your hot air can now easily get sucked up the chimney.
The damper you installed will be fine, left open, and would be ok being closed all summer.
The problem comes when you are not there and someone else is running your stove.

A good choice is to hang a tag on the damper explaining its use.
Another option is to create an "operator's" manual for using the whole system, which should include an explanation of why the damper should only be used open or closed.
The best thing would be to completely remove it.