Working on my BB RMH and wondered what the recommended distance is from the floor channel vertical secondary air tube and the heat riser port entrance?
As regards the concerns with the granite slab, I have an option to use a larger, thinner slab as the bell ceiling. I could protect this with 25mm Superwool and cut out a section that would line up with top of the heat riser. I could then use, say a 10 inch square, thin or insulating firebrick to withstand the intense heat from the riser. Would that work better than the granite slab protected by a layer of Superwool?
Many thanks Peter for your detailed reply and figures for my application.
I'm factoring in easy access to the top of the bell so that I can monitor how the 'Superwool protected' granite slab performs and also to keep a check on the Superwool liner of the 5 minute riser. I'm planning to rely on a friction fit of the Superwool in the riser - would that work or do I need to fix it in place with some sort of adhesive or hardware fixing?
I was going to roof the top of the bell with a 3" granite slab with a one inch (25mm) layer of Superwool between the riser and the granite.
The 'chimney' for the whole system is about 11 feet (3.5 metres) high of which over 7 feet of it is insulated. I'm hoping that this will provide a good draw so I could possibly compromise on the riser/bell clearance and the heat riser length without too much affect on performance.
I'm working on the core of my outdoor Batch Box RMH/heated bench project - it's a 6" (150mm) system.
On the Batchrocket resources site it lists the heat riser length for a 6" system as 43 3/16" (1080mm). I think this may be too high for my project if I allow enough clearance for the exhaust from the top of the riser to the ceiling of the bell. I'm proposing to build a '5 minute riser' using a 25mm 'Superwool' blanket inside an 8" ducting pipe.
I have two questions :-
1/ What is the minimum clearance between the top of the riser and the ceiling of the bell?
2/ Can I shorten the length of the riser and if so, by how much?
The bench top temps of around 22-44 C you mentioned are what I was hoping for. The lower section of my indoor BB RMH Bell get to around this figure after a decent firing.
The reason I want to heat the benches is so that on cold nights we can sit out and watch the stars in some comfort..
I was thinking that the actual temperature at the top of the benches would be fairly low, hopefully much less than 100 degrees C. If it's going to be more than that, it's probably going to be too hot to sit on...
I'm building an outdoor 6" Batch Box RMH with two dead end benches feeding off the main bell.
I've obtained some nice limestone slabs that are 20mm (0.7 of an inch) thick. I plan to use these as the bench tops, they're nice and smooth and feel quite comfortable to sit on
My question is: will they be able to take the heat without cracking or suffering any other damage? Does anyone know what sort of bench top temperature such a system would generate?
Mark, I would say yes Have a removeable lid. The ability to easily check if there are any issues with your system is well worth it.
I have a J tube in my workshop which drives a masonry/steel bell. The 'lid' is a cast iron section that relies on gravity to hold it in place. I have a fibre gasket which provides a basic seal and it works well. The only downside is that it is a very heavy thing to lift off to allow inspection/cleaning.
In my Batch Box rocket firebox I've used board similar to this stuff (it's widely available in Europe)
Vermiculite board
In my experience it works well but does abrade after a while even when loading fuel carefully. I've also used it in several woodstoves as firebox lining and baffles and it does wear after prolonged exposure to high heat. It really needs to be treated as a consumable part.
Incidentally, My BB firebox uses the insulating board for the walls and ceiling and I use firebrick splits for the floor (backed up with superwool insulation). The splits cope fairly well with the logs being loaded into the firebox - they survive longer in that application than the insulating board.
As your mason says, the preferred option would be to use a high temperature/high alumina/Fondu cement mixed with the perlite. It can be hard to get hold of and is fairly expensive stuff - over here in the UK anyway...
If you can't get hold of high temperature cement I'd use a mix of clay and 'normal' cement (OPC). If the 'normal' cement gets overheated it can crumble and fail but the clay should still hold things together.
I've used the front, door and sides of a small woodstove and have successfully built a 'sidewinder'batch box space heater that also heats water.
I followed Peters dimensions very closely and it works well. To create the correct firebox size and geometry I used a thin steel frame lined inside with a combination of hard fireclay and lightweight insulating firebricks.
Mark - for my BB rocket brick bell I used a cast iron oven door as an inspection door on the front. The added benefit is that it starts to radiate heat almost as soon as the fire is lit.
I also incorporated an old Georgian cast iron oven (with door) into the bell structure which also radiates heat quickly as well.
Glenn's right. You need to have a very hot, highly insulated firebox/riser to burn the wood cleanly first and then target the heat generated towards your heat exchanger.
Any cooling of the combustion area will result in a dirty burn that will smoke like crazy and deposit tar and creosote.
I'm using a conventional 'wrap around' wood stove boiler as a heat exchanger on top of a 5" BB rocket riser to help heat my hot water and provide central heating. The boiler is rated at 45,000 btu/hr and this output, it appears, can be achieved if you keep the firebox well stoked with decent dry hardwood mixed with a little softwood.
The BB also heats a single skin brick bell.
When I get the chance, I'll post details of my build and how the system performs.
If the firebox was left uninsulated the high temperatures needed to help promote clean combustion would be hard to achieve.
Additionally, the firebox (if made from steel) would degrade very quickly under the prolonged exposure to high heat. There are quite a few threads on the forums showing catastrophic failure of metal components in rocket stoves.
Nick Kendall wrote:I am new to this but really like the idea you have using old stove as batch box had anyone else done this? I would like to try this but would insulate the riser as well
Yes. I've used a Charnwood Country 4 as the basis of a BB Rocket.
It's quite heavily modified though... To follow as closely as possible the dimensions on the Batchrocket Resources site, I had to cut off the rear of the stove and extend the depth of the firebox. The whole of the original stove is heavily insulated with insulating firebricks and superwool - there is no metal from the original stove (apart from the cast iron door) that is in contact with the fire. It's a 5" sidewinder system that I use to heat water and a brick bell. It works really well and is into its second winter with only a few issues.
The brick bell on my 5" RMH gets warm right down to floor level when heated for some time. The top is appreciably hotter but the whole bell is providing useful heat.
I'm sure a 'thin' bell (essentially a hollow wall) would be very effective.
I think your idea should work, but I would put a 'proper' insulated chimney on the exhaust side to ensure you have enough draw for the whole system.
I would also include an area at the base of the chimney exhaust flue where you could light a priming fire to get everything pulling in the right direction, especially when the system hadn't been used for a while.
I agree with Thomas. A Batch Box with a tight fitting door would be a good solution.
The added bonus is the amount of heat that the glass on a BB door puts out. It quickly warms the space while the main mass of the bell/bench/whatever gets up to temperature.
Travis, I was worried about the flash steam possibilities too. That is why, when my system was piped up by our plumber, I made provision to easily retro fit a pump on the hot water flow from the boiler if needed.
My relatively small boiler has a capacity of only 28 litres or so. In nearly two winters of operation there has been no evidence of flash steam generation or 'kettling' - the simple thermo-syphoning system has worked unassisted.
I have a generator (we're out in the wilds of Wales and we sometimes loose power in stormy weather) but the electrically powered central heating pump only operates on the radiator circuit after the water from the BB has been injected into the hot water system. If the power fails, the thermo syphom from the BB keeps working, circulates hot water and prevents an overheat situation.
There's some interesting comments being posted on here.
Just to re-iterate what I've mentioned before -
I have linked a combined Batch Box rocket mass heater/water into an existing 'conventional' oil fired central heating hydronics system, and it works...
The BB rocket is located inside the house and provides mass heating storage for the room it's located in (and to some extent, adjoining rooms) as well as hot water.
The boiler I use with the BB is rated at 45,000btu/hr.
The boiler is located around 10" above my 5" heat riser and it doesn't flash steam.
The boiler thermo-syphons into the central heating system (via a 'Neutralizer' multiport device) without a pump.
The whole water system side is open vented - in the event of a power failure there is no pressure build up and excess heat is soaked up by a gravity fed 'heat leak' radiator.
Mine is not an ideal system. If I had the finances and the time I would build perhaps an 8" BB Rocket to drive a large boiler that would thermo-syphon via large diameter piping to a 2,000 litre (or larger) highly insulated thermal store. The BB Boiler would be on an independant open vented water coil loop that would only require a small volume of water. The water in the thermal store would store this heat and domestic hot water and the central heating would be fed from this indirectly - that is, these systems would be in their own enclosed pipe loops. The water in the store would just exchange the heat into them. (Tim Barker has done something similar with a domestic hot water/shower rocket)
It's nothing new. A lot of mainland European countries already use this type of arrangement and have done successfully for many years. The difference until now is that most of the existing systems use pellet or wood gassification boilers to heat the stores which are very expensive and relatively complicated. They rely on a lot of electronics, sensors, forced air fans, etc to ensure they function and the combustion is correct.
What I love about the BB Rocket is its simplicity and its ability to keep working in the event of a power outage. It doesn't need sensors, fans or valves - just dry wood and a bit of manual attention now and then. I really think that a BB rocket would be in it's element in this application. The large volume of water in the thermal store is ideal to 'smooth out' the btu output from a BB and store that heat for many hours. Maybe one day...
I'm not really answering your question William, but I'll explain how I've linked my Batch Box rocket water heater to an existing oil fired central heating system (hydronic) over here in the UK.
The BB heats a Clearview wrap around boiler from a normal box stove. It's rated at 45,000btu/hr. I've located the boiler around 10" above and to one side of the 5" heat risers exit. It's a 4 port boiler but only 2 ports are connected (lower left - cold water in, upper right - hot water out) The flow of water is caused by a simple thermo-syphon that then connects to a 'Neutralizer' Neutralizer - a multi-port beast that allows several heat sources to work together without interacting with each other. It therefore allows us to use either the BB, the oil CH boiler or both together. Thankfully I'm really only using the BB for most of the time, but it's handy to have the oil CH backup for when were away from the house.
I have a high limit stat and a low limit stat fitted to the feed and return pipes from the BB boiler and the whole combined system is open vented - ie: unpressurised. I made provision to fit a pump onto the hot feed from the boiler in case the BB boiled the water in the boiler, causing it to flash steam, but we have had no issues like that so far after two winters use. Certainly, in our case a simple thermo-syphon transfers the heat from the BB boiler without using a pump. We have used 1" copper pipes to connect things up.
If you run several batches a day it can provide most of our heating and hot water requirements on all but the coldest of days. Plus of course the heat from the bell and the glass door easily heat the room the BB is located in.
On my J tube rocket in my workshop I use a 5mm thick steel plate as an adjustable 'closure cover' for the feed tube. It's fitted with a heat resistant handle (from an old box stove door) that doesn't get hot.
The plate itself does get quite warm after a while but, as a bonus, it acts like a small radiator and gives off extra heat to the room. It hasn't really degraded or warped yet after several years of use.
Hi Fox - I've studied ppotty's videos for a while and his latest one seems to work well with a proper glass door.
My 6" workshop J-Tube Rocket has a large feed 'area'. I lined the walls with vermiculite board to make a sort of low 'firebox'. It works really well but I have to use a steel blanking plate to cover some of this 'firebox' or else I would get smokeback. The amount of air that I admit to the fire depends on the amount of fuel and state of the burn. It's also handy for closing down the J when the fire has gone out, so retaining the heat in the mass of my brick bell.
An added bonus of this arrangement is that the steel plate acts as a small radiator and gives off a lot of heat. It is fitted with a cool touch handle so it can be adjusted without having to wear gloves.
Hi Linda. It may be worth obtaining a moisture meter. In the UK you can get one for around £20. Split a piece of your fuel and check the level on a freshly exposed section of the wood. Ideally your wood should have a moisture content of 20% or less.
If your wood isn't dry enough, try to get hold of some clean, unpainted and non-treated pallets. Cut them into small sections and make sure they are dry (they should 'ring' when hit together, not 'thud' )
Fire up the beast and once the 'dragon' is up to temperature, if your stove stops smoking your original fuel may well not have been dry enough. If the smoke persists then lets look at some other possible issues.
Jeroen - as a temporary fix for this years winter, you could try dry-stacking masonry bricks (or similar) around the sides of the metal stove to at least give you a certain amount of mass storage for the excess heat coming from the stove.
I've tried that on a couple of our wood 'box' stoves and it does help a little
I would recommend insulating the firebox of the cast iron wood stove. Personally I would use vermiculite bricks, but you could also use split firebricks with some backup insulation.
Is the stove pipe heat riser insulated? If not, again I would recommend insulating it using ceramic fibre blanket or similar.
Good luck and please keep us posted on how things progress.
I would always go for an insulated chimney if you can. As has been mentioned, it factors in a degree of safety in most types of weather conditions. You may well get days where the stove will not be safe to light with an uninsulated flue system.
Over here in the UK, Insulated SS pipe is available at a reasonable price Insulated Flue Systems - I realise it's the 'other side of the pond...' but there's probably an equivalent product available near you or via an on-line supplier.
I've used 'Insulfrax' in various thicknesses, for back up insulation behind firebricks and as a gasket material. It copes with extreme heat and does it's job very well.
A data sheet came with the products and provided sensible precautions are taken, there appears to be no serious risk to health.
