Not sure if this is the kind of thing you guys normally experiment with but I guess you may be able to help. This is totally new tech to me but I am keen to try it.
I have watched a few vids this week & as a newb, wanted to ask you experts about some ideas I had.
These 2 vids caught my attention
- can the idea of flame jets via perforation & external draw inlet pipes be used in a reburner?
- this kind of stove (radiant rather than attached to a mass) was something I wanted to try. (But it would also be METAL!!!)
I am short of space & want to try a pipe under the floor, would having the flue step down into the floor from the exhaust outlet create a problem? Given heat rises I have to question what that would do to the draw. I envisage a vertical pipe as the exit point for the exhaust, 6-10ft high.
diameter of the flue - e.g. what fx does it have if made 1-2 " instead of 6-8"?
addition of holes in the centre riser (like Takeshi Ueno's design had) - if anyone has tried this - how does it influence the internal reburn?
position of exhaust flue - does it need to be a 180 from the input? From above - could it be 12 o'clock in & 3 o'clock out I'm also wondering about under & over (exhaust below to share heat with the burn chamber)?
Given the underfloor nature of my notion, I have also had thoughts about using 'batteries' filled with oil or water, lying within or adjacent/connected to the exhaust pipe. These would be totally sealed containers e.g. >70% full, to allow expansion & maximizing localized mass density.
I thought I could use waste engine oil (being mineral it would of course need to be totally sealed as despite the urban environment, I don't need any more toxic ground; or vegetable - say from local fast food joints)
FYI I'm in the UK - currently temperate climate - don't know how long until global climate change impacts that
You can perfectly well have all radiant heat extraction instead of the typical mix of radiant barrel and thermal storage mass. This is recommended practice for spaces like shops which are used intermittently and don't need to stay warm between uses.
Be aware that it is not good practice for a continuously warm space, as trying to damp down the burn rate to avoid overheating in other than the coldest weather will cause inefficient combustion and creosote buildup, just like an ordinary woodstove.
The big thing to remember is to have a highly insulated combustion zone so you get complete combustion and elimination of pollutants, followed by heat extraction. For a large space, you can use a stack of two barrels for a big radiator.
You can do all kinds of fancy experiments with reburners, but you should build a standard RMH by the book first so you have an idea what is achievable without fuss. You can't know if you have an improvement without knowing what you are trying to improve. Generally, a well-built J-tube with P-channel and tripwire (two simple enhancements you can google for details) gets close to as efficient as it is physically possible to be, and it would be a stretch to do better without considerable skill and experimentation.
A metal enclosure is fine, as long as the surfaces exposed to flame or high heat are refractory/ceramic. Apparently the guy in the video used to build all-metal units, and has taken to lining them with insulating material.
An underfloor mass/duct system is possible as long as the rest of the design follows proven principles and proportions. You do need a good chimney to ensure constant draft. 10' would probably be a minimum for effectiveness, and it needs to end at least three feet above the roof where it exits, and two feet above any roof within 10'. These are code minimums, and in some cases might need to be increased for good performance.
The smaller a RMH system diameter is, the more surface-to-volume and drag effects degrade its performance. 6" and 8" are common, easily-built sizes. 4" requires good siting and very careful construction, and still might not work well; it is considered an advanced build. Smaller than 4" has not been found to be viable for J-tube systems. 10" or larger is a serious beast, only appropriate for heating very large spaces or extreme climates.
A J-tube gets several times more air than needed for full combustion, so improvements will not come from introducing more air, but from better turbulent mixing. The simple P-channel and tripwire, added to the sharp bends in flame path in the J-tube, do this very well.
The position of the exhaust from the barrel base relative to the burn tunnel is immaterial, as long as you have good smooth airflow. It can go in any direction you wish.
Plenty of people have built greenhouse RMHs with fish or other water tanks above the buried duct. You just need to ensure that the water cannot get above boiling without an easy, NON-PRESSURIZED, steam escape path in case of overheating.
Waste oil has been used as fuel, but requires special handling. Search to find a number of experiments with it.
Thanks for the detailed response.
The points you make have made me consider variables that I might not otherwise have figured into my newbs calculations
Perhaps I am trying to run before I can walk, but I do like to experiment. I will initially be making something that 'must' function via the info here (I think I will buy the Wood Burning Stoves 2.0 DVD set about RMH so I can see what is happening with different processes.)
Looks like I will have to go for 6-8" - where I am siting this, the free space is relatively minimal, I will need to excavate the floor or suspend the pipe work. This is going to be a small annex build with the possibility of running pipes into the rest of the building -if- I can get it to work properly.
The former seems preferable as then I have the floor mass for heat retention.
The 2nd install would be for a workshop - greater area & would be better as a radiant only build as far as I understand it, not being ocuppied 24/7. 50/50 radiant/storage might be a better way to start thinking about it though. (I.e. a limited amount of mass+a lot of radiant metal surface.)
I all cases I think the gasifier angle with firebrick/fire cement components is where I'm heading.
One thing - I was thinking of using oil filled containers as mass to store heat rather than burning it. I kinda wrote off burning even waste oil given Big Oils relentless destruction of the planet.
Is a clean oil burn even possible?
............Better head over to DuckDuckGo.
Thanks again for the detailed specifics - it's a great help!
Rereading your first post, I see that you were indeed still talking about oil as mass rather than fuel. I wouldn't advise burning waste (petroleum) oil, but a lot of people really want to do that and there are experiments out there. Oil may be able to get to a higher temperature as a storage medium than water, but I would check the specific heat capacity of the oil you have in mind versus water before concluding that it is thereby better. It would certainly be more potential mess to handle.
Water can be better than masonry as thermal mass if space and weight are extremely limited, or if you want to move the stored heat elsewhere, and if you have cheap barrels it may be less expense and work than masonry. Masonry has the advantage of tolerating high temperatures and being stable... cob or brick won't leak
For an underfloor mass, I would advise solid masonry well insulated from the earth below, as space is not significantly limited and it will be very easy to build.
What are the square footages of the spaces you are considering? Annex, existing house, shop? A 6" system is generally good for a single room or small well-insulated cottage. An 8" system would have surplus capacity that could be extended to the rest of the house.
The standard style RMH with barrel and bench has about 40% radiant and 60% storage capacity, and for a shop that didn't need overnight heating I would go with mostly radiant surfaces.
The area of the room for my first & learning build is only 5x9ft. This is a block wall extension linked to the main brick-built building by a brick & block corridor of about 5x5ft. (I was thinking of tracing the whole L shape outline).
From what you say I guess I should try for 8" then I can extend if it works out, rather than tearing the whole lot up again, for a bigger gauge. My initial thought is the unit in a corner & then the under floor pipe running around the perimeter.
Digging up the cement/hard-core (rubble) floor thus shouldn't be a mammoth task . & the various gains will be well worth it. I am planning to reduce it to a shell & re-roof/re-render walls etc so the floor being up too is OK.
I will need to figure out the relative fx of tube overall length & req'd chimney height. I did see some arguments about the use of underfloor without a stack somewhere, which has left me thinking about draw vs linear volume vs height.
The workshop is physically discrete, with all walls exposed to the weather & is bigger, can't recall exactly but estimate 45x60ft ish.
I am slowly wading thru the threads, isolating these key bits of info takes a while. I saw a comment about using a chimney pot for a riser & thought that might be a quick solution for initial trials. I could probably get hold of some quite long used chimney pots locally (3+ft).
I can see I am gonna be busy....
Glenn is very accurate with his description. by definition an all steel non insulated riser can not be called a rocket stove. it can be more accurately called an "air flow stove" but not a rocket stove. just because it has a shape and sound of a rocket stove you need temperature and burn efficiency in order to be an actual "rocket stove" many people will give you a line of crud and yada yada how their all steel stove is this and that but by true definition all steel non insulated riser "air flow stoves" cannot be called a rocket stove. i guess technically you can call it what you want but it doesnt change reality.