Second and larger build

Hello rocket people.

So I've managed to get a 2nd stove built over this summer. It is an improvement in design and increase in size from my first attempt. I have fitted it at work in the same position as the initial build, swapping it to fit the same final flue. It is modelled even more closely on Ppottys large gas cylinder design, being more or less the same general size.

I have used some more of the same riser pipe as I did in my first build - that is, a Schedule 80 steel pipe @ 5" outside diameter (4.25" bore). I have however internally sleeved this pipe with stainless steel - just sheet about 1mm thick. The riser has an outer shroud of stainless steel ducting pipe at 8" diameter. The space created by this shroud is insulated with ceramic wool (around the base and up and over the burn tube) and then filled up with Perlite.

The heat exchanger body is an old 47kg propane cylinder. The top plate is 10mm steel plate. On the underside is a steel disc welded onto little legs (an inch) to deflect the main up-draught heat off the underside of the top plate - to avoid distortion. I have cut out an inspection hole on one side of the exchanger body and covered this with a removable plate, rope-sealed. This inspection plate is on the same side as the secondary air pipe, allowing me to inspect this and attend if anything goes amiss. The flue box is at the back and has a generous (rather deep) spigot pipe that accepts the final flue. The gap between the top of the riser pipe and the underside of the heat-deflector plate is 2 inches. The heat exchanger has a hollow base that I have filled with ceramic wool to provide insulation beneath the exchanger body. The total height of the exchanger is exactly 40", plus a 10mm top-plate.

The horizontal burn tube is fabricated from 10mm steel plate and fully welded, inside and out, to the cut-out riser pipe. There is a 'kick' to one side again to help with a vortex flame as it enters the riser pipe. It is about 8" long and has internal dimensions of about 4" top to bottom x 6" wide, angling down to about 3.5" wide at the entrance to the riser.

The firebox is again 3mm tread plate, although the lid (door) is 6mm plate. The glass viewing window in the lid is one of those pieces that come with a typical (inexpensive) security light - the type that take up to 500W halogen bulbs and usually come with a motion sensor. These are a good resource just for that piece of glass, which is nice, thick stuff - about 5 or 6 mm thick. I wasn't sure if this glass would take the heat, but so far (just 3 burns up to now) it has held up fine. There is a front air flap, low on the front panel, rope-sealed when closed and provided with a screw clamp to tighten it shut when not required. This flap is used to kick-start the fire upon starting or will be useful if ever a difficult burn is experienced (which should be never), or when any re-fuelling is done and it is desired to get the fuel going quickly. The air supply when this flap is open is immediate and promotes rapid burning. The firebox has a removable grate set 2" high off the base. The front air flap is situated on the lower 2" of the front panel so the air intake at this point is just under the grate.

The firebox is a generous size, mainly in depth, so I can fit long pieces vertically. The outside dimensions are about 14" square x 20" deep at the rear x 18" deep at the front. Discounting the 2" of grate space, one can easily feed a 16" log into the box. The front to rear internal dimension remains about 14", while the side to side dimension is now about 9"-10", given the hollow air chambers to the sides.

I have routed the primary air via two square tubes (50x50x3mm SHS) that pass through the heat exchanger body. There are spinner plate controls on the input end of these two tubes. The tubes enter the exchanger at the rear, pass through the body and exit and are then routed around the sides of the firebox. There are slots cut into the firebox side walls and the primary air tubes (now reduced to 40 x 40 x 3mm SHS, with one of their four sides now cut away) are welded directly over these slots. Because of a gentle up-slope to these tubes, rear to front, the side entry slots to the firebox are about one third the way up the box. There are side chambers formed from stainless plates inside the box and the air, once entering the slots, is forced to rise up these side chambers and out and across the top of the firebox - all the same as my last build and how Ppotty has designed his air delivery to the fire.

There is copper pipe secondary air delivery routed into the heel of the burn tube/riser pipe. However on this model I have managed to run the pipe inside the firebox, laying along the side wall, underneath the chamber for primary air on that side (the primary air chamber only begins about one third the way up as explained above). I have covered the copper pipe inside the firebox with a loose stainless steel channel section, folded out of sheet, just to protect it from crushing from any logs dropped in there.

The following areas are rope-sealed with new fire-rope: Heat exchanger top-plate; firebox lid; exchanger inspection plate; front air-flap; intake and exit points for two primary air tubes and a secondary air copper pipe.

Thoughts so far:

As per usual, these can be tricky to light if one does not follow the principle of starting a good stick fire inside the burn tube. Allowing a fire inside the firebox too early is fatal for preventing smoke production and leakage out of the firebox lid and even the primary air intakes, right at the back.

It is a little more difficult to get going than the smaller previous model. However there is more mass of steel and pipe to get hot before the burn begins properly.

I did fail to get things going adequately before allowing the fire to burn back into the firebox on the first burn - and even worse on the 2nd burn. Impatience really and inattention to detail. I had quite a good bit of smoke leaking into the workshop space until I could coax the fire into the burn tube again and get the stove heated properly. It wasn't a problem as it is a large shop and I can open a wide front door and have a large extractor fan on the rear wall and can thus replace a large volume of air quickly.

Yesterday, on a 3rd burn, I lit the stove properly (at the mouth and inside the burn tube) and was more patient. The riser is a thick steel pipe (10mm wall thickness) and takes time to get properly warm. The 10mm top-plate is stone cold (or steel cold) initially and it is a good indicator that the riser is warming adequately. When this top-plate is noticeably warm to the touch seems about the right time to allow the fire to burn back into the firebox.

What I actually did was stack crumpled paper and sticks to the front two-thirds of the firebox while creating an definite firebreak area between the burn tube intake and this main fuel pile. I allowed the burn-tube fire to crackle away for a few minutes (until the top-plate is good and warm) and then I lit the main fuel pile by pushing a long spill of newspaper into the front air-slot, under the grate and lighting it at the front. The draw through this front flap is very positive and the flame is sucked inside, along the length of the paper spill and ignites all the main pile of paper and fuel. At this point the system is up and running.

On the 3rd burn yesterday, following the above procedure, it was very successful. I had no smoke exiting the firebox lid (although I did improve the rope seal a little before lighting - I used slightly fatter rope) or the air tubes. The glass has sooted up quite a bit and so it was difficult to see, but this was caused with poor starting on burns 1 and 2. I addition I haven't had the firebox anywhere near full or the stove going full tilt as yet.

My 3rd burn yesterday used at most about half a 2-gallon bucket of wood, plus a few initial sticks for the burn-tube fire.. A good part of the main fuel was just dry sticks and small dry boughs needed as initial kindling. There were at most about 5 pieces of 3"x2" lumber, no more than about 6" long. This fuel caused the heat exchanger and top plate to become sizzling hot. The firebox too gives off a lot of heat. I can directly feel the heat from the stove from about 6 feet away. This small fuel pile was still burning 1 hour after ignition and had a fair degree of life in it still. At this point I added 2 more pieces of 3"x2" at about 12" long. This fuel and the remainder of the first small load was still burning 2 hours later.

I am totally made up with the stove. I think it will only get better as I learn to tweak it properly and find the sweet-spots. I am mildly apprehensive about the result of a full firebox and some decent dry tree logs. it could go nuclear.

I realise there is one more process to accomplish - that is to line the firebox interior with fireboard and to make a sheet metal sleeve for around the exterior, into which will be added outside firebox insulation - probably ceramic wool.

Isn't fireboard expensive to buy? I aim to make some of my own in the next week or so. I can't decide whether to make a fondue cement and Perlite mix, or one of sodium silicate (water glass) and sand. Any thoughts appreciated.

I hope these pictures upload ok. In fact I will put all the pics in a 2nd post.

Thanks for reading.

Here are pictures hopefully and hopefully in some sort of order of build.

Who has ever found these kind of nails in old timber they are burning?

It's a zombie apocalypse.

Wow what a monster, what are you going to heat this with, sorry if I missed that part if you spoke about that.
Nice fabrication work man.

Hiya T2H and thanks

The stove is now fitted in my workshop, replacing the smaller one I made before last winter. The workshop is 80 feet long x 20 feet wide = 1600 square feet of floor space. It is 8 feet to the roof trusses and another 6 feet to the central ridge. My calculations say there are 17,600 cubic feet of space. In addition the place is full of metal, from machinery and tools and benches and from steel stock. In December, January and February it can get particularly cold indoors and when it falls below zero degrees (C) outside it can often get down to minus 10 degrees C indoors. I have a small running joke over the years that sometimes I have to stand outside for a short while - to have a warm. .

Now this stove is unlikely to heat all that space, especially in the far reaches at the back end of the shop. The stove is set centrally across the width and is 30 feet from the front door, leaving 50 feet behind the stove to the rear gable wall. But it will make a good difference all around the centre and for some distance beyond. I shall have to see. The shop is not perfectly sealed against draught (under the doors etc) and will have small losses at the roof (around skylights mainly) but I don't mind this because, being a welding shop also I don't want totally sealed premises, either upper or lower. I want any gases that may build up from work processes to be able to escape. I think that if I make another one of similar size and site that somewhere down the back end, then I may well heat all the shop noticeably. I may well do that one day but have no plans to do so atm. Being a little chilly in winter keeps me moving and getting work done.

If anything I could have made the firebox a little smaller, especially less deep but I deliberately want to be able to get a 16" log(s) (max) in there. My saw buck is set to produce 16" cuts of logs.

The smaller one I made was adequate for the central area and pleasant to sit by (not getting work done) and for placing cups of tea (actually coffee for me) on top to keep warm. I could have put a skillet on the top and cooked on it but if I do any bacon frying I prefer to cook on a small trivet over the forge coals (small coal forge). The same will apply for this stove - nice flat top around 16" diameter. I will certainly have a kettle on there however.

The main problem (not really a problem as such) with the smaller one I made was that the exchanger was unable to liberate all the heat before some of that heat passed into the final flue and up and out. The flue is vertically within the building till the roof exit giving about 13 feet length of hot flue, but of course at an increasing height. On the small stove the flue was untouchable at operating temperature.

On this new stove the flue can easily be touched. I have had a hand palm half wrapped round the flue when it is up to temperature and it is just pleasantly warm - less than a hot bath I say. My temperature gun is broken and I need another one for accurate measurements. I will get one shortly.


My next project is to make another one of similar dimensions but far more aesthetic and take it home and fit there. I will not use another gas cylinder for a stove - all the curved surfaces make fabrication time far greater than if I was dealing with easier flat surfaces. It's more economical to use new 3mm (1/8") plate and save lots of time and tooling. I have some nice ideas to make a prettier rocket stove yet just as efficient as this one. I will include descriptions and pictures here when I get to making it - soon, because it's already mid-September.

Thanks again.

very interesting build is this paul

i know i will end up coming back to read this again

Paul I would be interested in seeing what the results of the stove in use.
Very much interested to see if you can keep the system running smokeless with such large loads.
That is quite a scale you have built this on.
Very nice work man.

I have to admit when you go over kill you certainly do that with your builds

i do hope that thick steel dosnt burn out quickly or flake off badly

you may find that 1mm stainless steel will fail quickly like ppotty did with his build due to the joint in it i havent used mine as much but on the cutdown in was still in good working order so i have planned to use it again as my usage is only in winter or when its getting cold

i dont know what grill you use but i have upgraded my own to a thicker fire grate to handle the heat much better

i like the light glass idea id like to know how well you cope with it on your build and how it manages up to the heat whilst in use in the box of yours

also before you get too far id put some tubes inside of the stove like ppottys just some box section or even some rectangle box section this gives a far better blow out of heated air also fitting some box section improves the systems heating output

ive considered this and going garys way of build to provide more heat yet a much smaller system being used