I've been living with a traditional two-door Hunter multi-fuel heater (central-heating plus hot water ... in theory) for the last 15 years, and although I've been spending a small fortune on fuel, the house has remained cold, as pretty-much all of the heat went straight up the chimney.
Having learned of the principles behind the rocket heater, I then set about making one using whatever materials were at hand. Now I fully appreciate that ceramic is the preferred way to go, but I had steel and can weld, so that is what I've used.
I cut-up a 47kg propane tank, and fitted it with a 6" x 6" x 15" firebox made from 7mm mild steel ('cause that's what I had), and made a slanted front with hinge-down door flaps.
The 'engine' was made from a 4" s/s riser tube inside an offcut of 6" s/s flexible flue pipe - with a loose perlite filling between them.
The gases leave the propane tank through 8-9ft of 5" s/s flexible flue pipe which is poked through a blanked-off window for now, out to a 6ft vertical flue (ex dis-used greenhouse oil heater).
I've purchased nothing except some fire cement which I mixed with some perlite at the base of the riser, and some glass rope to seal the propane-tank join which I haven't actually fitted yet.
It's running as I type this, with it's background roar telling me that all is well. When the roar becomes quieter, then I know to feed it ...
I'm very pleased to have chosen only an 4" riser tube, for the 8-9ft of 5" s/s flexible flue pipe is getting very hot indeed - far too hot to touch - and I guess could easily be extended to 3 times it's length to recover that lost heat.
But - no complaints ! I'm really warm for the first time in 15 years - so many thanks to all.
I made the firebox from lengths of odd-angle (2" x 1" x 7mm) which I have lots of. It's very heavy stuff, and should last a long time despite being mild steel.
In this next shot the firebox has been installed into the propane tank, and shows the 2 bands welded around the top to hold the glass rope seal - also the s/s firebox tray which has tubes welded on it's underside both to support it (as it's only thin sheet) and to route air in towards the base of the riser to provide a secondary burn.
I then welded on a 4" s/s riser tube:
Which I surrounded with a 6" s/s flexy tube, and dropped some fire cement down to the bottom to hold it in place, before filling the cavity with perlite. This shows a small fire being lit in order to 'cook' the fire cement:
Although this was only a gentle burn, I noticed there was no smoke after a few minutes warm-up ...
So I quickly plonked the top on, and lashed up a flue - although this didn't appear absolutely necessary.
So, after a trial run in the polytunnel, I moved it into the house. Here's the door in the closed position:
and in the normal running position:
and finally in the 'fuel loading' position:
Having the door fold fully back like this will allow me to insert a vertical feeder into the firebox - something similar to this:
Here's the QD (Quick 'n' Dirty) flue arrangement - simply poked through a blanked-off window. Ex-greenhouse oil heater, just quickly cleaned-up and given a lick of paint.
And finally - here's a shot of the glass rope installed today. I've put some pieces of wood in shot to show the sizes I'm burning. Typically 2-3" square, nails an' all, and sometimes a few 4" wide planks. It all burns ok. Nails are removed with the ash.
A very scruffy build - but it's doing the job so far very well indeed.
If you have a properly functioning RMH, its temperatures combined with oxygen will quickly destroy any metal burn tunnel and heat riser, even stainless steel. If the metal in the combustion core lasts, you are not getting hot enough temperatures for complete combustion and high efficiency. Bare metal will suck heat from the burn tunnel and radiate it away, and you will never get the high temps you want. The well-insulated riser will fail from overheating and corrosion. There are numerous youtube documentaries showing this, usually starting with "this is my great idea" and ending with or followed by a video saying "look at how the stainless steel or structural steel buckled, spalled and crumbled". This doesn't count the ones showing how it was built but not how it failed. Don't trust any unconventional build video that doesn't show how well it held up over a whole heating season.
What temps are you getting on top of the tank and on the exhaust?
You can add some fins to the tank to help dissipate heat.
With regard to the feeder - that diagram was just the basic concept. I had already considered that some kind of bleed hole might be required within the otherwise 'sealed' cap, as you have commented. If I do proceed with this (which very much depends on how the steel holds up), I'll probably make this bleed hole of adjustable size.
With regard to using this feeder, my 'plan' (as at Nov.2014) is:
The drill being - start with ash pan flap closed and the top cap in position. Start a fire above the grate, and push a few lighted sticks towards the riser tube if necessary to start the draw.
When a good draw develops, then close the front door, open the top cap and load the feeder with sticks. Replace the top cap and open front door. Regulate air intake as necessary.
When re-fuelling: close front door, open top cap s_l_o_w_l_y to allow any smoke to be drawn downwards, and then re-load feeder. Replace top cap and open front door, regulating as before.
Good theory - but dunno what will happen in practice.
As you can see from that last line - I'm not taking anything for granted until it's been tested and shown to work.
for the open ended Vertical Feed for a rocket mass heater RMH !
With a Vertical feed only the bottom tips of the wood burn and the RMH is continually fed new fuel as the ends turn to charcoal and collapse !
With a Horizontal air supply located at the bottom of your Feed tube the fire at the tips of your fuel will travel up the wood until al the oxygen above that space is used up.
At that time that space will fill up with highly flammable Wood gases , that are so fuel rich that they are not ignitable in that low oxygen environment !
Removing the Top cover will create a slight vacuum sucking those flammable gases out to mix with fresh oxygen, depending on your luck you could lose the hair on your
hand and lower arm or loose all the hair on your head ! This is the condition made famous by the movie Flashback !
Finally, All iron / steel metals at that location are doomed ! Please Take the time to google High Temperature Hydrogen attack and Hydrogen and/or Steam Embrittlement !
This is where I strongly recommend that you go to Rocketstoves.com to download A PDF Copy of the Brand New 3rd Edition of rocket mass heaters !
For the good of the craft! Big AL
Dan Stolarczyk wrote:Have you thought of adding a secondary air inlet along the top of your firebox? That helps add a vortex and increases heat production (from what I have read, I haven't tried it).
That's what the tubes are for, underneath the firebox 'tray' - well, in theory, anyway.
What temps are you getting on top of the tank and on the exhaust?
I only have a cheap (£5/$7) temperature meter from China, which is supposed to measure up to 1500 deg.C but the display gets confused when over 200 - so I can't measure the top temp. It's enough to burn the paint off, that's for sure ...
The exhaust is 150 deg.C when leaving the tank, 100 just prior to exiting through the window.
david james wrote:I just want to say thank you so much. I didn't think I'd be able to build one of these this winter, but I have almost the all the exact same scrap around and your post has given me just enough details to try to build one. I just need a beater to get me through one winter. Can I ask you a few questions? How does your ash box work? Can you show me a picture of that opening? Is it possible you could take a picture with a tape measure in front of the firebox opening? Are the pipes inside the insert for the firebox for air intake? Do they go all the way back, or how far? Can you show me a picture of your inside exhaust install? What is fire cement and where can I get it? Thank you.
I'm more than happy to give you measurements - but I really feel the need to say that I didn't 'design' this thing with any accuracy or any proportions in mind - I just knocked it up as quickly as possible to get some heat for this winter.
My thinking was to have a BIG firebox so that I could feed pallet wood in big chunks rather than feed thin sticks regularly, and to use a smaller diameter riser tube than the exhaust, to hopefully get some kind of venturi effect (maybe). The perlite insulation isn't brilliant at all - there are much better ways of insulating a riser tube than what I've done - but I had the 6" tube offcut and I had the perlite - so that's what I used.
Q's in reverse order:
Fire cement - builder's supplies. Every country's different - in the UK, one such chain is called 'Wickes'.
Here's a shot of the exhaust outlet. Because I needed to weld a thin wall s/s tube with an arc-welder, I started by making a couple of internal 'collars' and 'blobbed' weld metal onto the edges in order to double the wall thickness - so that when I came to weld the tube to heavy gauge mild steel, I was welding metals of roughly equal thicknesses. A bodge - but one which worked. I've drawn red marks to show one collar which is difficult to see otherwise.
Here's a shot of the firebox 'tray' - it doesn't need to have tubes to raise it off the floor of the firebox. 3 or 4 (say) 1" strips of steel would do the same job. I used tubes a) because I had plenty of tube, but no suitable strips !, and b) because I wanted to form a 3-D structure beneath the plate, as it's so thin (c.1.5mm) - and one which would stay rigid even if the plate itself 'sagged' from the heat.
I fashioned a 'kick-up flap' to hopefully induce a vortex, but I don't think I got that quite right, as there is a vortex, sure, but it rotates in the opposite direction ! (go figure)
Here are the pics you asked for:
Firebox 7"x 5.5" (should have been 6" - don't know what happened there ...) and which is 16.5" long at the bottom, and 13.5" at the top - so that 3" difference gives you the slope for the door-flaps - which isn't critical - something around 25 deg from vertical, I guess - something like that.
Question: are you going to use a propane tank ? And have you cut one open before ? If not do say, 'cause I've got a couple of tips to make life easy (and safe) when doing so.
john little wrote:My thinking was to have a BIG firebox so that I could feed pallet wood in big chunks rather than feed thin sticks regularly, and to use a smaller diameter riser tube than the exhaust, to hopefully get some kind of venturi effect (maybe).
Look into batch rockets. Easy to make, and burn shit easy! I'll repeat what Allen said,
Metal is doomed!
Dan Stolarczyk wrote: I would love to hear your suggestions on cutting a propane tank as I think others would also
Take off the tap, leave to vent outside a few days, cut with an angle grinder or plasma cutter, even a oxy cutter would do! Gases are volatile at ambient presure and temp, so they escape the tank easily.
Well, it's rather anoying that you call everything "gas" in american, not making the difference between gaseous products, and blady petrol!
- X 2
Dan Stolarczyk wrote: I would love to hear your suggestions on cutting a propane tank as I think others would also
Ok - there's one video on YouTube of a guy belting the hell out of a propane tank valve with a hammer and chisel - that is SO barbaric, dangerous (the hammer head came flying off at one point) - and so unnecessary ....
Here's the problem - there's no room to 'swing a cat' with the valve shroud in place, and it's impossible to get a spanner with any thickness onto the valve flats:
Solution ? Easy - if you're welding steel, then you'll have an angle grinder - so - fit it with a thin 'zip' disk, and cut away the shroud ...
Then - with the valve CLOSED - cut away the lower portion of the outlet boss, so that the 'big dog' spanner can then access the valve flats:
Enlarged, for clarity:
Then - drop the tank to the ground and sit on it - or get somebody else to sit on it ! - and give that spanner one good whack with a lump hammer to break the seal. That's all it takes. Then - observing anti-spark precautions (steel on brass ok, steel on steel not-ok), unwind the valve a few turns until you can unscrew it by hand.
With the tank vertical, insert a garden hose and fill to over-flowing with water - but - 'second tip' - place the tank on a raised surface before filling, as putting water into the tank is easy ... but getting it out afterwards is not-so-easy, and a simple hose syphon works so much better it the outlet is significantly lower than the water level.
So those are my tips. There was a guy in rural England not long ago who killed himself by cutting into a propane tank which he'd left empty for a long while, and which he'd assumed had been vented of gas during that time. Not a good idea. Water is your friend.
Article here about dangers etc.
Lovely and warm for the first time in years, and by burning old pallets it cost me absolutely nothing to heat the house - and - despite the "metal is doomed" merchants, the stove held up perfectly ok. No sign of any deterioration at all - although that could well be due to not reaching the highest temperatures due to the wood not being as dry as it could have been, and also having the smaller 4" riser. But - no smoke, except at start-up - a few drops of creosote was observed having dripped from the base of the smoke-stack indicating a less than 100% burn (presumably during start-up), but not enough to get excited about.
Although minimal, in view of that creosote I'll be checking the interior flue pipe for residues, but I can't see any reason to change anything going into this winter, with the singular exception of the external flue. I'm about to make a rotating cowl, as I did experience some smoke blow-back during start-up when the wind was >F7. Once up to temperature, it was ok - but getting there could be a tad 'smoky'. I did rig-up a QD (Quick 'n' Dirty) wind deflector as a field-fix which worked quite well - I now just need to make a proper cowling for the smoke-stack.
Other than that - no complaints at all.
It is very clear Why you went the route that you did ! Also you deserve much credit for successfully building a sub 6'' unit and with almost zero
insulation around the Firebox ! Insulation on the outside of the firebox burn tunnel would have given you higher temps and a cleaner burn and
hastened the failure of your heating unit ! Hopefully you will have had time to get some wood in undercover, the dryer the wood the more heat
you get to enjoy while using less wood !
I will ask that when you start to develop problems probably with your inner metal pipe for your Heat riser you give us a report, If it seems that
your Fellow Members 'just need to be right' - remember that probably hundred of future Members and Would-be Rocketeers will see these pages
Too ! For the Good of the Craft ! Big AL !
It may be that what I've cobbled together is not so much a 'rocket stove', but a 'Rocket-Assisted Wood-Burning Stove' ! - but it does the job in keeping me warm (and for no money at all), and so I continue to be very grateful for having learned about the basic concept of an internal chimney within a wood-burning stove from you guys. That, for me, is the real game-changer over more conventional technology.
I have no doubt that there will be a failure of the riser tube at some point - but if I get 5 years service from it, then I'll be well pleased. I'm more than happy to treat it as a replaceable item, as I have enough spare tube for two replacements. The rest of the stove will see me out, I'm fully confident of that.
Wishing you well
This lash-up may be breaking many of the design-rules of the classic Rocket Heater (it's not a RMH btw - as there's no significant storage mass involved), but I wouldn't change anything if building a similar one again - except perhaps to install a thicker-wall riser tube. But I'd stay with a 4" riser - much less heat is generated than a 6", sure, but I'd rather have two 4" Rocket-style Heaters in this format than one 6". As a 'portable' (bl##dy heavy, but still a moveable item) workshop-style heater, I have found that it performs excellently.
The combustion is less than perfect, as is it's 'heat extraction' efficiency (as the exhaust vented to the atmosphere is still very warm) - and it no doubt burns more wood than is necessary. BUT - I'm not comparing it's performance with permanently-installed domestic fire-brick style rocket mass heaters - as I've said before, that's clearly the way to go for maximum efficiency, and only a fool would dispute that. I'm comparing the performance of this style of heater with it's conventional steel equivalent: the Hunter multi-fuel stoves; the Jotul box stoves, and so on ...
When comparing a steel Rocket-style Heater with the nearest equivalent steel wood-burning appliance, the performance of the steel Rocker Heater is vastly superior, due to it's 'internal chimney'.
Of course, the use of steel has it's limitations and one needs to work with that in mind, and so a lower operating temperature is actually desirable to ensure longevity. The negative aspects of a lower burn temperature is unfortunately the price one must pay for using steel - but in my considered opinion, to constantly dismiss steel as 'being doomed' is a mistake.
For portability, and for workshop use, the steel Rocket Heater cannot be bettered. It is a completely different animal from the permanently-installed ceramic-based rocket mass heater, and should - in my view - be viewed as such.
'Horses for courses' ...
- X 4
This is but a small area of the local (just 1 mile away) pallet re-cycling yard - you'd need an aerial shot to see the full size of it - it's HUGE :
This is one of their trucks loaded with selected pallets waiting to be strapped-down before leaving the yard :
These are just some of their broken pallets which have found their way over to my fuel store. Numbers fluctuate, but I like to keep at least a thousand 'in stock' at all times :
This is a shot of what happens to pallets made from useful wood:
And here are a few 'pallets' destined to be turned into beehives.
So my story begins with a pallet re-cycling yard which is prevented by law from burning their waste on site, and so some of their waste ends-up in my yard instead. Useful wood is recycled into beehive boxes, stands etc., with the offcuts and twisted planks fuelling the rocket stove.
Thus, the re-cycling yard is happy to be relieved of a problem, I'm happy to be heating the house for free from now on, the bees seem happy enough in their new homes, and the atmosphere is subjected to far less smoke than would be generated by burning those broken or non-standard pallets in conventional stoves, or in the open air - as would be done otherwise. What's not to like ?
Best wishes to all ...