I have been using this forum for reading for quite some time but became a member today. Planning on building a batch RMH for my small house and need some support and encouragement in this. I have purchased the Erica and Ernie book "the rocket mass heater builders guide" and fully read the Peter Van Der Berg website batchrocket.eu. Very nice sources of information!
So, house is 60 square meters, built on a concrete base (approx 15cm thick). Single story building. It was used only as a summer house before I got it but has proper isolation (approx 150mm rockwool in walls and 100mm in roof between house and cold attic) and windows and so on so it is well worth to make it an all year residence for me and my two kids. Swedish winter here is from november to march and is sometimes as cold as -30 degrees but not for very long. A few design things I want some advice on. I'll go through it in order here with questions:
Fire box - According to recommended measures from batchrocket.eu website for a 200mm heat riser. Built with bricks/firebricks and a metal door with window. P-channel for secondary air, easily changable in some way. I build it with bricks directly on floor with fire mortar close to the fire and standard mortar when its not in contact with fire. Should I isolate the fire box from the concrete floor or is it ok to build it on top of the floor?
Heat riser - Standard fire bricks isolated with kiln isolation on outside. Octagonal shape, i cut the bricks with an angle grinder where needed. 200mm diameter (8"). Fire mortar to keep it together. Isolation to be held by metal fence/mesh. Length is according to recommendation, I have calculated 1440mm.
Bell - I have a 720mm diameter metal barrel that is 1250mm long that I may use. Or maybe make a custom bell with sheet metal and some welding. Base of bell from standard bricks. My main concern is the ISA, internal surface area of the bell, the recommendation is 9,4 square meters and this would make a too big bell (or too many bulky bells) for my small home in my opinion. Practically i could reach barely half of that ISA with my existing barrel and brick base. I guess the trade off is not enough cooling effect and from that less drag?
The Mass - An approx 12m U-shaped (6+6m that is) ventilation duct piping arrangement with proper cleanouts at the right places. 200mm piping to match the 200mm heat riser. Mass from clay, stones, dirt (non organic) and horse shit that seems to be recommended. Shell from bricks and standard mortar. that will also be part of the wall between the two bedrooms. Put directly on top of floor. Can the length of this compensate for the small ISA of the bell? Do I need to seal the pipes in some special way? There is a rubber lip seal on standard ducting, is this ok or will it melt? Screws/rivets to make the pipes stick together?
Chimney - The existing chimney is made from bricks and did earlier have a kitchen wood stove and an open fireplace but they where in bad shape and are now gone. 3 channels in the chimney and the one I will use is the size of a brick and the cross section area is equal to the 200mm heat riser CSA (how about that!). Height is approx 5m. Just got it approved by the local chimney sweeper that it is not leaking (very nice!). Will equip the top of the chimney with a scoop that is quite common in these parts of the world and stop rain and the crows from populating the chimney and hopefully help the draft(?).
In general, I read someplaces about expansion joints (manifold to mass for example) but don't understand how this is accomplished in practice?!
So, please feel free to answer questions or make remarks on my written thoughts on this. As stated earlier I have read alot on this but not yet fully understanding it all so if you see any faults in my plan please feel free to say so and give me good ideas on how to do!
Regards, Thomas in Sweden
I wonder, if only a 6 wouldn't fit the bill. I have a 11 / 220mm. And heat 165m2 on two storeys.
Badly insulated. At 1500m elevation, in the alps.
So imho, a 6 could be all right.
It's the -30, which i have too. But not that often either. Which i'm worried about.
There is solutions too. Like multiple bells, you retract 15% of the total isa for each new bell.
You van also combine bells ans benches.
Or you could also make a bell wall.
Against an existing wall. You can also alternate the orientation of the bricks. To make stubs, ans increase isa.
Stuff like this. For example.
Thomas Rubino, I understand that the ground slab will consume some energy, but will it not also give it back as part of the mass to some degree? Or is it a case of wishful thinking from my part? Mass gets too big? Or too cold? Isolation best made with air gap or sand or any other mean of isolation?
With a normal triple bell, you can go down to 6.8 m2.
If you do the stub trick, two bricks laid normally take 264cm2 it goes to 638cm2 with one brick perpendicularly for the same original surface. 141% more than the original surface.
So, that would make the bell 7.5m2, since you can't do the stubs on the bell cap. For a single bell. Which would be 9.4 originally.
If you bother to do a triple bell with the stub system. You could end up with a 4.82 m2 bell. Well , no, i'm daft. Forgot the caps. 5.3 may be. A cube of bricks 1.2 x 1.2 x 2m outside. Would do the trick as a single bell with stubs. I would say you could also consider the new shoebox batch. And a plunger tube may be.
thomas rubino wrote:The slab will warm up some BUT the earth will never warm up and it will continue to steal heat from your core. Your firebox can be build on a perlite/clay base. Your mass can be simply built over several inches/MM thick of cob, with plenty of straw mixed in to make it insulating. Your horizontal pipes lay on top of this.
Well, i agree and disagree. The earth insulates some. And if the construction is by the standards, there should be either a gap under the slab, which can be insulated. Or insulation.
Plus, there is the speed of travel of the heat in the ground. The heat making a cone or sphere of heated ground under the heater. If this is placed in the very middle of the house, it can be of some use. Since the ground warms up , the more you go towards the center of earth. There is a moment heat from the house, and heat from the ground equilibrates. Ok; may be very far! :D
The new version looks promising. I really wish to have the possibility to cook on the wood burner. Will look into it! Thank you!
Then the heat transfer from metal to mass isn't well studied either. My opinion, stay with bricks.
And another opinion of mine. if you increase the inside surface. I think it would be interesting to do something like a double skin of bricks. To accumulate more heat. Instead of releasing your heat fast. Increase the temp of the mass, and it's release time. If you want fast heat, a bell metal top or window in front, to release a bit of fast heat would be in order.
You can skip the barrel and have just masonry bells of proper ISA, and put a metal access panel in the first bell for heat riser maintenance; this will give some instant heat as well. If you have a good chimney, its draft can eliminate the need for whatever boost the barrel gives.
I am considering all input by you guys of course. I am considering a second masonry bell but don't know how it affects the 12m duct and mass that i think I need to spread the heat.
Since i am not too skilled with brick. I used metal in a different way!
If i make a bench/bell and made it long, how low can I make it? Would 350mm be too low? Instead of the routing with HVAC pipes.
Still missing an answer to my biggest concern, what happens with a 50% ISA bell and then a mass with HVAC pipes as the J-style builds? 😁
I understand that a too big ISA can stall the heater but not much info about small ISA and a mass bench. Maybe I'm just not getting it, he he.
If you want a bench, i think using Matt's half barrel bell system is good. You only count your half barrels as ISA, not the floor.
And iirc, a 8 incher J can cope with 4 to 6 m² approximately. So 1.86m² for the barrel, half of that 1.86m² per half barrel following that, may be 1.80 since you cut the ends. 1 barrel over the riser, 4 half barrels, as a single bench in the bench. And you should be there. You make a plunger tube at the end of the bench, reaching the chimney, and rise the tube from the floor, is it's not working too well. To adjust for the proper ISA.
I do like the simple idea of the half barrel or the dead end bench, it could work in my place and it would be easy to calculate the ISA to have a balanced system to avoid too much experimenting. I know I would need a cleanout anyway, due to restrictions where I live. I could work that out somehow I guess. I like the idea of ducting being sealed, and easy to clean and it would be easier to make my planned room divider and kitchen part using ducting.
I have seen baking ovens and cooking plates on various rockets, is there a best way of incorporating those in a heater? I saw the cooking plate on your firebox Satamax, did that work? I have a few cooking plates like that.
The baking oven just a brick void in contact with the hot gasses in the first bell? Or just the bottom of the oven? Should the hot gasses be allowed inside the oven? I have seen different approaches for this.
Maybe I'm just procrastinating. 😂
Oven wise, i know some are having problems of low temps right with white ovens.
White oven, no smoke or hot gases allowed in. Separate from the bell.
Black oven, smoke or even fire in the oven itself. Works hotter. That's pizza or old bread ovens. To me, the best idea, i think, is to skip any after riser oven, and use the firebox as an accumulation oven. But only doable on big systems.
Accumulation, you mean build an oven on top of the firebox?
Thank you Satamax for putting up with all my questions.
Nop, cook in the firebox, with primary and secondary air closed, when the fire is out.
A load of bricks was delivered yesterday. Used red bricks in good condition. Think I may need a few more but I'm good to start anyway!
Mass is approx 2000 kg in bricks alone. Add to that cob/clay/stone filling.
Thomas Holm wrote:ISA is to spec, approx 7,7sqm.
Mass is approx 2000 kg in bricks alone. Add to that cob/clay/stone filling.
Well, explain then.
One barrel above the riser, i count 1.86m².
By the looks of your drawing, i wee two half barrels, so that's a bit less than 1.86, since the plunger tube doesn't go down to the floor. Only the active part is taken into account, not floors.
This is today's haul of bricks. Traded for a bottle of whiskey, got approx 270 bricks today so i have a total of about 600 bricks now.
Found a local source of clay yesterday so it should work well for the mass filling.
The heat riser will be made from ceramic fibre board in hexagonal shape. It is then cast in a round tube with fireproof cement and the fibre board is the centre and facing the fire. Just to make it stable and durable.
No cooking top on the batch box, it will be lined with ceramic fibre too, to keep heat in the burning process and hopefully make it as efficient as possible.
Need some advice on the mass and flue flow. Do I make the exit from the first bell as low as possible and shall i make it enter the second bell/bench as low as possible?
Your door is lovely.
Reading this: https://permies.com/t/69146/Bell-questions
..and wondering at the plungertube that I am thinking of using. I want to use half barrels at dia 600mm for the bell/dead end bench for my 8"/200mm RMH. Will use a trumpet/cone end for the plungertube but want to know the approximate distance to floor and end of bench. As i read it in the above linked thread I need at least 200mm (CSA) from floor to pipe. That puts it above middle of the half barrels and that seems wrong but it's only a feeling of mine. I have read the thread over and over but cannot grasp the meaning of formulas expressed, maybe my English is too bad. Would appreciate some help here in formulating a solution for me with the plunger.
Thomas Holm wrote:Also, it should be ok with normal bricks in the manifold area? No need for firebrick there?
You don't need firebricks there. Do it with normal bricks
I'll clarify after if need be.
A simple sketch with my case. I hope you have the patience with my lack of simple understanding... 😅
Like a window in a wall, in which air would flow. It's easy to calculate, height by width. Pish easy.
Then, to calculate the actual surface of a gap, is different. But as easy.
You keep your height, from floor to tube in this case.
So the width of the window in this case is the circumference. Your big D
We know your cross section area, or CSA and the diameter of the system. 200mm
20x20x3.1415926/4 314.15cm² of CSA
For example, if you want a 2 inch gap, or 5 cm.
You divide this 314.15cm² by 5cm. Which gives you a "window width" in reality a circumference, of 62.83cm, for a 5cm height. This is theoretical. It's better to have at least 1.5 the CSA
So, 314.15cm² X1.5 = 471.22 cm².
471.22/5 for the height 94.24 cm circumference.
94.24/3.1415926 gives 29.99cm of diameter, so 30cm is good enough for a 5cm gap. This is the very lowest gap you can have. for that diameter with a 20cm system diameter
In this case, i would make the sides of the funnel end straight, just in case you need to recut to a bigger gap, is the system is temperamental.
a bit like this
Thank you Satamax! You are very kind!
Safety margin of 1.5 is the absolute minimum. x3 is far better. So that would be a gap of 10cm.
This is necessary because the edge of the funnel creates a drag, the floor creates a drag too.
Plus, behind the edge of the funnel, the drag creates a stalling zone, with turbulences and under pressure.
Not far from jetflow patern
Will make it adjustable between 5-15cm approx. I guess once it is set it will stay in that setting.
Thanks again Satamax!
Oh, and i will use standard bricks in lower bell area. Thanks for reassuring me Ted!
Can you really tell me that we aren't dealing with suspicious baked goods? And then there is this tiny ad:
Rocket oven documentary pre-sale now availablehttps://permies.com/t/90306/Rocket-oven-documentary-pre-sale