Downdraft tent stove- the pudding bowl stove

This post is about an experimental downdraft tent stove that is made from a stainless steel pudding bowl. It is tiny, very simple, clean-burning and easy to keep fuelled with bush sticks. It produces steady intense radiant heat for a small tent and can boil water in a 700ml mug.

Introduction to the experimental downdraft tent stove

Most of my previous tent stove designs have used ultralight ‘J-burners’ that are somewhat complex to run effectively. I thought it was time to investigate a small and simple tent stove that may be a little less efficient, but would be much cheaper, simpler and easy to operate. At least in the early development, it would have a very limited cooking surface.

Design origin

The design was in part inspired by simple downdraft barrel stove/heaters used in ski chalets in my youth. I call them a ‘Chalet Stoves’. They ran quite hot on the top, hot enough to constantly keep a big kettle boiling. The design also was a little like the Pocketrocket stove by Ianto Evans and Leslie Jackson. A video of a Pocketrocket from another source shows the stove running very hot on the bottomand less hot at the top.

I like stoves temperatures the other way around with a hot top for good cooking and snow melting. As well, I like hot walls for strong radiant heat for directly warming of the tent occupants bodies.

Well, I expect a lot out of my designs, as I also like only a moderately hot stove bottom so that it can thaw and dry stored fuel sticks on a rack below the stove. Alternatively, it is nice to be able to safely seat the stove on a light foil guard on the ground without making smoke or setting organic soil particles on fire.

All this ramble means that the wonderful Pocketrocket design was not mean for my tent, but some features of it live on in my new design.

I have reported extensively on the experimental miniaturization of barrel stoves for backpacking in another post (coffee tin stove). These stoves used waste coffee tins, that are fabricated from tin-plate, for the experimental stove bodies.

The current experiment uses a more robust material of a stainless steel pudding bowl to contain the intense combustion. I speculate that the poor thermal conductivity of the stainless steel makes a stove considerably hotter than an equivalent stove made of mild steel.

In contrast, to the coffee tin stoves, the inverted pudding bowl (or fire dome) of the new downdraft tent stove is made of tough and durable stainless steel. It has a shallower depth and wider diameter to allow it to have a small but functional hot cooktop and a fire dome wall with a large surface to radiate generous heat to tent occupants. The stove size is such that it would fit nicely inside a large pot such as a Trangia dinner pot while backpacking.

The downdraft tent stove

The bottom of the downdraft tent stove (or the top of the bowl) was covered with heavy aluminium foil and covered with a thick layer of ash for heat protection. Two 40mm dia holes were cut into the base of the pudding bowl. One hole was used to fit a tapered conical stove pipe connector to which a 37mm dia*2000mm long rollup stove pipe was connected. The other hole was for feeding in fuel and a supply of air for combustion. The fuel hole was fitted with a short conical tube that could support and feed in a small bundle of bush sticks.

Unlike the Pocketrocket stove design, the fuel tube has minimal penetration into the fire dome. I speculate that this is why the new downdraft tent stove would be much hotter on the top than the bottom. It is the other way round for the Pocketrocket according to a video posted by rich991980.

Downdraft tent stove testing, results and discussion

Easy startup

The stove was very easy to start with some dry twigs, grass and some ‘waxie fire starters’. This was followed up with wood chips and small sticks. The stove temperature rapidly became hot enough to easily burn larger sticks.

“Once full power was reached with a few large dryish sticks, the stove would happily run on damp fuel sticks. The downdraft tent stove heat output could then easily thaw and dry fresh fuel sticks in preparation for easy combustion under the most adverse weather conditions.”

Me

Easy fuelling with sticks

Using long broken bush sticks for fuel made the operation of the stove very easy. “Luckily, the fuel requires minimal preparation.” The fuel sticks just fell down into the fire dome as their lower ends were consumed by the fire. With this stove configuration, a ~60mm long portion of the fuel stick is exposed to combustion conditions where wood converts to charcoal and produces wood gas by pyrolysis. This pyrolysis zone is substantially more than the ~30-40 zone in my prior ‘J-burner stoves’.

“Stick with me here, don’t nod-off, as this is a long sequence.” This means that extra exposure can gasify the wood and convert the sticks to charcoal more thoroughly and quickly. It also means that there is a fast air blast down through the downdraft burner. This action preferentially attacks the charcoal and makes it burn up more thoroughly (Than in my J-burners.). This then means that, unlike the J-burner stoves, it does not accumulate charcoal to the point where it can block the entry of fresh wood for sustaining the production of wood gas and the essential wood gas flame and stove draft. “Phew.”

“By luck or intelligent design or otherwise, he has found the Goldilocks design balance!”

Mothy The Elder

Robust and easily sustained combustion

The downdraft tent stove rapidly produced and then consumed the glowing columns charcoal on the ends of the fuel sticks. The collapsed charcoal columns formed a bed of charcoal below the downdraft zone. This residual charcoal was easily able to re-start the wood-burning process if the stove re-fueling was neglected.

The use of a flaming waxie could instantly light the wood gas flame to greatly shorten the ‘smoky combustion phase’ that would otherwise persist until there was spontaneous ignition of the wood gas to once more make the hot combustion self-sustaining.

The tips of the fuel sticks are exposed to intense heat within the burner. This means that even damp fuel sticks would easily burn when required during alpine camping.

Simple and strong flue pipe draft

The vigorous combustion, as seen in the photos and videos (below), ensured that there was a strong stove pipe draft to drive the combustion at all times. The simple, direct and upwards path for the hot combustion gas to the flue pipe meant that there was no likelihood of reverse burning as experienced my other stoves with ‘J-burners’.

“More than likely, it is considerably less thermally efficient than my J-burner stoves. However, most people would agree that this a reasonable trade-off for simplicity and robustness of the burn process when it burns only ~7g/min (0.24oz/min for those over the big ditch).

It also eliminates the dreaded hazards of reverse burning that a J-burner stove will have if its fuelling is neglected or mismanaged. Nodding-off in the warmth of a tent stove to die of carbon monoxide poisoning is not an ending that I am searching for. I have too many Telemark turns left in me for that! “

Me

Radiant heat output

The stove ran delightfully hot (350-450C) and occasionally ran even hotter, as shown in the above photo. It provided abundant radiant heat that would be welcome in any small backpacking tent under very cold winter conditions.

Intelligent or un-intelligent design?

At first, the closeness of the fuel/air entry hole to the flue pipe exit hole may look like a very poor design feature. Much like the close proximity of the anus to the birth canal in humans. We now know this helps the mother impart immunity to her baby and is a process that is unfortunately bypassed in caesarean births (https://www.sciencemag.org/news/2016/02/how-give-c-section-baby-potential-benefits-vaginal-birth). I wonder if they will one day say; “Look kid, expect to cop a lot of shit when you go to work, so let’s get started now.”

My stove design was this way to maximise the available cooktop area and make provision for ‘pot overhang’ when need. As I hopefully explain next, the design is not so dumb inside.

Flame guide.

The downdraft tent stove has a crude flame guide within the fire dome. It stops the flames and hot gas taking a ‘short-cut’ from the inverted burner to the stove pipe exit. This makes the stove burn effectively, cleanly and efficiently while distributing the heat to the stovetop for cooking as well as to the fire dome walls for radiating strong heat to the campers’ bodies.

The fire dome with a flame guide also acts as a very effective spark arrestor

Fuel consumption/efficiency

The fuel consumption rate was about 400g damp bush sticks/h. This is equivalent to only 7g/min or 0.24oz/min. This makes this powerful little stove a very efficient one.

Stove draft

The very bottom of the flue pipe reached ~200C or more and there was little or no smoke coming from the flue pipe which was about 100C at the top. This indicates, to me, that the combustion was probably efficiently completed in the combustion chamber (not in the stove pipe).

The 100C temperature at the top of the flue was about double the 50C which I typically measured with my ‘J-burner’ stoves. This indicates to me that the stove is less efficient than my J-burner stoves. However, the ‘heat lost’ up the pipe is used to maintain a very stronger stove pipe draft. It is a trade-off that I am happy with, particularly if the combustion is much more robust and eliminates reverse burning. “If it is hot enough, it does not need to be hotter and the miserly amount of fuel is free for the ‘picking up. Even the heat from the pipe can dry your socks and gloves and other small items up high in the tent.”

Cooking potential

A small cooking surface was made available by locating the fuel port alongside the flue pipe port, as discussed above. Using a large 700cc stainless steel mug as a cooking pot, it could boil 500ml of water in about 11 min. This is a very limited cooking surface, but nevertheless welcome in a cold world. Clearly, a larger cooktop would increase the cooking potential, even if it lowers the temperature somewhat. Increasing the cooking area for a big dinner pot is described in another post.

Impact of the cooking pot and stove temperature

The photo below shows a thin, round and dark patch on the stovetop outlining the bottom of the almost invisible cooking pot. This is the first time that I have observed such an effect. “Probably because it is the first time I have attempted to cook on such a small hot surface and take night time photos.” It indicates to me that the pot must be significantly cooling the stovetop. This is in contrast with observations with other stoves where I observed refractory coatings heat curing more thoroughly when covered with a cooking pot. “Tinkering is full of mysteries, the more you think you know…..the more you know you don’t know.”

Surface temperatures

Normally, the top of the fire dome (or cooktop) about 450C. The top of the wall reaches 300C and the bottom of the wall reaches 200C. The ~7mm thick layer of ash in the bottom of the stove kept the bottom of the stove from getting too hot (~150C). Higher temperatures on the upper portion were reached periodically and it was on such occasions the photos (above & below) were taken. There was no visible damage done to the light aluminium foil base cover. The ash layer also would have acted as a seal to prevent excessive air entry through the stove base.

Cause of periodic temperature spikes

“The fascinating glow from the unusually high temperature made photography irresistible. I rushed upstairs to get my camera hoping that the glow would persist. It did not! However, another bright phase occurred soon enough”.

I speculate that during the ‘bright burning phase’, charcoal has temporarily become the dominant component of the combustion process. The alternative gas flame is less hot and can spread and move away from its source. At the same time, it can ‘shroud’ the charcoal to limit its combustion.

The charcoal combustion, in contrast, is; much hotter, stuck in one spot and must largely dissipate radiant heat outwards to the fire dome (There is only a little flame.), hence the glow. I have observed similar ‘bright phases’ in my ‘J-burner stoves and it is often a precursor to a charcoal choking/stall that results in reverse burn where there is insufficient wood gas made to sustain the flame and the flue pipe draft.

Self-regulation of the balance of wood/charcoal ratio

The downdraft tent stove burner configuration appears to be able to easily self-regulate the mix of charcoal to wood during bright phases’, without the need for operator intervention or detriment to continuous effective combustion. It makes the combustion strong, robust and stable without the risk of reverse burning.

Stove mounting

The low temperatures on the stove base (~150C) should mean that it could be simply mounted on the ground on a thin heat-dissipating sheet of metal foil and it should not set fire to soil organic matter. Such organic matter can be almost invisible but it can make a lot of smoke if it gets hot enough. Also, a platform of stout wooden sticks could be used as a mount on deep snow. Such sticks could also be driven into a snowbank on the walls of a snow pit to provide support for the stove over the pit. Stove mounting legs are described in another post (KISS tent stove).

A day time video of the stove running and boiling water can be seen in the following links. They should make the features of the stove a little clearer. The gentle chuffing sound of the stove is enough to make the birds sing.

Instagram video

YouTube video

Conclusion

The downdraft tent stove shows considerable promise as a very small, versatile and highly functional tent heater and cooking device for ultralight backpackers, particularly those that want simplicity.

This simple stove can; produce abundant radiant heat by clean and efficient combustion of a small number of bush sticks that can be prepared, tool-free, for simple stove operation.

This new design appears to have overcome the charcoal stall and reverse burning reported for my previous ‘J-burner’ stoves

The bowl-shaped stove should fit inside or outside of a suitable cooking pot for efficient backpacking.

Lastly, would it be possible to craft this little stove, with minor modifications, to serve a second purpose as an ultralight blower stove for outdoor cooking as well? For the answer to this question please see my Three in one tent stove post. For a larger backpacking tent stove please see my latest Large and slim downdraft tent stove.

Related posts;

Micro tent stove design for strong radiant heat and robust draft 

Ultralight tent stove overview

Tim