Experimental simple barrel stove from a coffee tin.- Searching for the ‘holy grail’ of an ‘overnight’ ultralight tent stove warmer.

This post describes a series of eight experiments with a simple barrel stove that is of the downdraft type for heating. A series of micro stoves were made from coffee tins and the main variable was the size, shape and number of fuel feeding holes that allow bush stick fuel to automatically feed the combustion.

The ease of fuel feeding and sustenance of combustion were the principal observations. The test showed that the simple barrel stove could be miniaturised and still maintain the expected clean, hot and stable burning characteristics.

They also show that the stove can be conveniently fueled with long and thick bush sticks through one or more fuel holes without any risk of reverse burning. However, the overnight stove ‘holy grail’ was not found.

Introduction

The ‘holy grail’. Often tent-stovers would like to have a small ultralight and robust stove that can run all night within a small tent and only requiring occasional refuelling. For most conventional small tent stoves, a big fuel load does not result in a clean slow and long burn. Instead, it results in a fast, hot, inefficient and smoky burn over a disappointingly short time. So size alone will not reach the ‘Holy grail’.

My success with clean and hot burning of less than 400g of bush sticks per hour often leads fellow stovies to think that I am well on the way to finding the ‘Holy Grail’. Unfortunately, this destination remains the territory of wood pellet stoves or corn stoves that are not particularly applicable for a backpacking tent warmer.

My current best tent stoves. My little tent stoves can provide abundant heat by efficiently and cleanly burning only about 400g/h of long bush sticks with minimal fuel preparation. Such a low burn rate is attributable to the tiny inverted ‘J-burner’ that is formed by a simple combined fuel stick/burner tube that is located in a hole in the top of the stove body.

In this type of burner, there is a magical but delicately balanced intense burning of charcoal and wood gas. This occurs at the bottom of the tips of the loaded long fuel sticks that feed in automatically. This is where the similarity with wood pellet or corn stoves ends.

Although feed-in is automatic, it does require some strategic human intervention (jiggling from time to time to prevent fuel stick hang-ups) to help maintain the balance of the combustion process. This is particularly the case when camping and found fuel sticks can have widely variable combustion properties and less than ideal shapes. ” These sticks are a far cry from the sawn fuel sticks as seen on many demonstrations.”

Burn time from a load of sticks. Let’s say that a 4-hour burn without refuelling could be considered to finding the ‘Holy Grail’. “This timing could easily coincide with pee breaks, especially if a tent is shared by two people with youthful spirits and old bladders”.

With my current tent stoves, it would require a fuel load of 1.6kg. Even on my test bench with well-selected sticks, my current stoves cannot practically be preloaded with this amount of fuel stick/s and have it reliably self-feed into the stove over 4 hours.

Effect of fuel stick size. One big stick that was: uniformly thick, dense, dry, straight, without side branches or splinters or taller than my tent (like a machined 40 or 50mm diameter hardwood curtain rod) might just do the trick. However, such rods are not often found in the bush.

Furthermore, even the most perfect single stick by its self will not usually sustain the combustion. “It may do so if it is very dry, rotten, and soaked with diesel, but then it will not provide the required energy or last very long”.

As described later, a single dense stick of wood must be split into 2 or more parts to have a chance of sustaining continuous combustion. This splitting increases the sticks cross-sectional area, creates splinters and it will not easily fit into the fuel feed tube or easily fall down it when required.

Downdraft simple barrel stoves

Information about downdraft barrel stoves. I had a distant memory a ski chalet barrel stove from my youth (Described later). I could not find the same or similar design in my web searches. I am sure there are others, but I just can’t find them.

A kind research colleague said to me more than once….”Tim, Tim, Tim (looking over his spectacles in a very wise way)…………..There is nothing that is absolutely new in the world! You just have not searched the literature well enough!”

The nearest search results that I could find were:

A 44 gallon simple heater/cooker by Steven Alsop. In this brief and informative YouTube post, Steven states that it is a 200-year-old design (without references). He indicates that it is clean-burning and effective. It is so far, the most similar design to the ski chalet stove that I have found. It is still very different and it is not particularly suited to fueling up with large ‘ugly’ bush fuel as with the Chalet Stove.

The air entry device is a long thin pipe that is passed through the barrel top and down to the top of the fuel. No mention of how it is kept on the top. There is a second (rough and modestly sized) hole cut into the barrel top for fuel addition. The hole is provided with a crude metal plate cover that appears to provide another restricted opening for air entry.

Pocketrocket stoves- Ianto Evans and Leslie Jackson. This is an informative and detailed post. Their stoves are similar to the Chalet Stove, but the fuel feed is very different. It uses a wide long internal fuel stick feed tube of about twice the diameter of the flue pipe. It is designed like my small ‘J-burner’ stoves to burn only the small portion of the fuel sticks at the bottom of the tube.

Their design should provide robust combustion. Their fuel tube is within the tall and wide burn barrel (burn chamber). Their flame/hot gases can immediately move upwards toward the flue pipe with no risk of reverse burning. The whole barrel is acting as a giant heat exchanger and also as a powerful heat-riser that supplements the flue pipe draft. (Heat-risers are the driving force of Rocket Stoves that have no flue pipe and Rocket Mass Heaters that work with minimal flue pipe temperatures and draft.) In contrast, in my inverted ‘J-burner’s, the flame/hot gases must move down then sideways then up the flue pipe. There is no heat-riser effect and this introduces the risk of reverse burning.

Their diagrams also indicate that the stove can be scaled down successfully. However, it is not particularly suited to bulk fueling for overnight burning. Elsewhere on their website, they also comment on the difficulty and unsuitability of using damp wood for fuel. Interestingly, this is the very fuel that my stoves can and must be able to burn while backpacking camping.

A small drum stove by Mansoo Lee. This stove is an elegant, clever and refined drum stove that is geared toward burning wood pellets but has some attributes of a simple drum stove. It also has limitations as an overnight stove to be fed with bush stick fuel.

[My small search result could just be a reflection of my inability to search the web effectively, but if anyone has any links to information about such stoves, please let me know.]

The simple barrel stove inspiration

My initial experience with downdraft barrel stoves. I have had a long life of ‘playing with fire’. The simple downdraft drum stove (Ski Chalet Stove) is the only stove that I have encountered that could happily burn a single block of wood cleanly efficiently and robustly.

Ski chalet barrel stove. The stove was made from a 44gallon drum. It burned dead wood for fuel for heating as was the norm in those days. The wood came from ski run clearing, bush fire damaged trees and tree cleared from from road making and maintenence.

We were Greenies before the word was invented by burning this truely renewable carbon fuel that would otherwise decay on the ground.

I can remember Sir Henry Bolte responding to environmental protesters from a CSIRO laboratory where I had a vacation job. “The wind will blow it away”. We have made progress since then, but I am afraid not enough.” Back to the stove….

I speculated that a miniature version of the simple-downdraft-drum stove may efficiently burn wood sticks or charcoal of any shape, size, quantity or proportion (wood/charcoal). It may also provide a self-regulating and hot clean burn that would last for some hours without refuelling or tinkering. Also, it may avoid the dreaded reverse burning experienced by my inverted J-burner stoves.

Success in this could be another step toward the ‘holy grail’ of an overnight tent heater stove. “To make it clear, I would not sleep or snooze in a tent with a stove that could reverse burn and make it my last long sleep!”

The Ski Chalet Stove had a vertical barrel, with a removable lid and had a side-mounted flue pipe just below the top. The lid could be hinged back open so that huge ‘ugly’ blocks of wood (that no one could split ‘or even wanted’), could be loaded through it.

The barrel lid had a large round hole in it that allowed an air inlet cone to be placed in it. The cone had an insulated handle welded across the large diameter (It may have had some sort of shutter disk to regulate the airflow rate when required. In my workshop stove based on this design (described later), I used a simple scrunched blob of aluminium foil as an air restrictor.) for this purpose.

The cone could be tilted somewhat to direct the air jet at a particular fuel target when rarely required. The cone could also be removed so that regular split firewood could also be easily added to supplement the ‘ugly’ fuel blocks.

The barrel walls and base were lined with refractory cement. Once heated up, this simple stove could cleanly burn a barrel load of wood. Even a large big block would happily burn on its own. Unlike the above mentioned ‘box style’ tent stoves, it burnt cleanly, regardless of the quantity of loaded fuel.

Demonstrating robust burning. “With the help of a ski friend I remember gleefully putting a huge block of snow gum into the hot stove. The block was frozen and had ice on it. Then we watched from above, while standing on chairs, as the hot air/flame jet from the cone just bored its way into the heart of the frozen block. Maybe it needed the Saturday age to help it along? As part of our backup plan, we knew that we could simply build a ‘new fire’ with paper and a few sticks on top of the frozen block to get rid of it if it did not immediately burn.

I still get the same pleasure when I set my tiny USB fire blower to work on destroying a tree stump on my farm. See…. there is another kind of stove that can burn one piece of wood, but I would not have it in my tent.”

Self-regulating burn rate. The simple-barrel-stove had the exceptional attribute of self-regulating the rate of combustion when filled with fuel. It pyrolysed and burnt just enough of the wood component of the fuel, at any time, so that a strong clean burn was maintained until the wood was used up. Then it went on to consume the accumulated charcoal, with no involvement of flames, if no more wood was added.

Wood protecting charcoal from burning. From my growing understanding of combustion that is described in wood and charcoal combustion, I think that the ‘magic’ of the self-regulation of a barrel stove comes from the oxygen in the air preferentially burning the wood (or more correctly the wood gas from the wood). This is because the air only enters from the top of the barrel and meets the shroud of burning pyrolysed wood gas moving upwards. The shroud substantially protects the charcoal from rapid combustion. “The wood gas gets ‘first-dibs’ of the oxygen in the air because it can move to find oxygen, while the charcoal must sit waiting for oxygen to come to it.” If the wood gas shroud reduces or is depleted then the charcoal can increase its participation in combustion.

The slow oxidation of the charcoal still provides intense and valuable heat for sustaining the pyrolysis of the wood and the clean combustion of the smoke. I think this is why a fully loaded barrel stove can burn cleanly without significant flue pipe smoke emissions as experience with other fully loaded simple box stoves.

This means that the simple-drum-stove will just keep burning cleanly with any amount or ratio of wood to charcoal. Importantly it will not reverse burn as my earlier ultralight ‘J-burner’ stoves can.

Delayed charcoal burning to extend the burn time. Effective burning of only charcoal, after the wood runs out, is natural and somewhat slow in a simple-drum-stove. This attribute could be exploited to greatly extended the ‘burn time’ if a large volume of accumulated charcoal could go on providing gentle heat without refuelling.

The burn duration could be extended even more by appropriate restriction of the air intake. “I expect and aim to have stove temperatures of 350-450 C when plenty of wood is burning for cooking purposes. I think that a more moderate temperature would be adequate to take the chill and pesty condensation away in a tiny tent overnight.” Also, when burning pure charcoal at a slow rate it, fortunately, does not make inefficient and smelly smoke, creosote and soot to foul things up.

Holding fire and restarting a fire with charcoal. Lastly, in the context of an overnight stove, a persistent bed of hot coals in the bottom of a barrel stove will also be invaluable. It maintains a source of heat for; starting pyrolysis and maintaining flue draft for rapidly re-kindling the burning of the next load of wood when refuelling is required.

Combustion temperatures and flame dynamics. A review by Browne 1958 indicates that wood smoke needs to reach a very high-temperature threshold between 350-750 C to sustain combustion. In contrast, charcoal can burn between 100 and 250 C. This means that charcoal can burn slowly at almost any low rate, providing there is some oxygen available.

As an example, in my home heating stove, at bedtime, I cover a large chunk of burning charcoal/wood with a deep layer of ash. This allows oxygen to diffuse slowly through the ash to maintain minimal combustion conditions in an insulated environment. In the morning, I uncover the charcoal (somewhat reduced in size) and simply blow air at it with my USB fire blower. This rapidly starts the fire for the new day. This characteristic of charcoal may help us to achieve ‘or fudge’ the ‘holy grail’.

The contrast between a barrel stove and my current stoves. My previously described tent stoves rely on a delicate balance of burning wood with charcoal in a tiny inverted J-burner to sustain a clean and continuous burn. “They certainly are not chuck-it-in-and- let-it burn stoves.” In their defence, they still do hold fire well in their tiny ash/charcoal beds, but they do require appropriate tinder placement and brief flame to ignite the smoke if a long smoky phase is to be avoided, particularly with damp wood.

Importantly and unfortunately, the stoves reverse burn or ‘run backwards’ (the fuel tube becomes a de facto flue pipe). This is NOT good in a confined tent, particularly when sleeping and deadly odourless carbon monoxide can be the gas produced.

A barrel stove as an overnight tent stove. I thought that a simple-barrel-stove might work as a miniature tent stove for backpacking and may make the task of fuel preparation more tolerant and somewhat easier. I also wished to check that a miniature barrel stove could be clean-burning and self-regulating as with the large barrel stoves. I also wanted to be sure that my distant memory of the performance of the Chalet Stove was not a simple case of; “fondness making my mind grow absent”.

Barrel stove downsizing for shed/workshop heating. Way back, I lived in a shed while I built my house. To heat the shed, I managed to make a scaled-down barrel stove from an old milk can.

It was inspired by the Chalet Stove and was done in frustration with a cute little cheap cast iron potbelly stove. The potbelly could only be loaded with a pathetic quantity of wood that was tedious to prepare from my abundant fallen tree branches. It burned away so quickly (and inefficiently).

To salvage some of my pride and get some value from my mistake I used the biggish overhanging cast-iron stovetop from the potbelly stove to make a much better stove. The stovetop had a lovely oval flue pipe connector and round fuelling port and cover plate.

I cut off part of the neck of large commercial milk can so that it fitted closely to the overhanging lip of the potbelly stovetop.

I sat the can on a wide bed of fire bricks and also packed the bottom of the stove with a 50-70mm thick layer of found clay.

When cold, the stovetop could be easily removed to fill the stove with big ugly pieces of wood before lighting. I put a simple cone made of sheet metal into the round hole in the stovetop.

The stove worked very well with no further tinkering. Sometimes, if required, I supplemented the batch loaded fuel with smaller pieces of fuel that would fit through the stovetop. The wall of the milk can had no refractory linining (As the Ski Chalet stove did), so the combustion temperature probably was not so high. However, the wall of the stove got up to red heat which was delightfully hot and much hotter than the Chalet stove.

I took this experience as an indication that successful miniaturization of a barrel stove may be possible. Also, similar Pocketrocket stoves (in the references above) are also scalable.

Effective scaling down of both the above stoves to small tent stove size was still an open question.

The simple-barrel-stove tests

The following experimental tent stoves are very simple (‘like me”) and are cheap (“but not like me”) and are made from a Nescafe Blend 43- 500g coffee tins or similar Baby Formula tins that are fabricated from tin-plated steel. I hoped they may demonstrate that a miniature simple-barrel-stove can support clean, efficient, stable, effortless, and robust combustion over a long period of time with found bush fuel with less than ideal shapes and composition.

It is time for an ode to Baby Formula to lighten up a bit;

They tried for a baby with no luck,
Then saw a baby formula on a big truck,
I’m not faking, the next line your mistaken,
Alas, in bed, on the formula they were not struck.

I made a series of experimental stoves from these tins. While they were only cheap experimental stoves, they could be used by those with a small budget for a tent stove. The stove body may have a short life, depending on how it is cared for. “But if you drink enough coffee or try for enough babies you will have replacement stove bodies.”

I have found that extreme stove heat can destroy tin plate particularly in very small stoves where the charcoal burning phase by necessity must be close the the metal. However, I find that rust is the ‘real killer’ and tin plate can survive a long time if cared for by cleaning, drying and coating with a trace of cooking oil during storage.

Experiment 1- Pre-loaded barrel-stove

The first stove had two 40mm dia holes cut in the bottom of the can, one for a flue pipe and the other for air entry. The lidded end of the tin became the stove bottom.

I also made a small stainless steel cone to mimic the air inlet of the above Ski Chalet stove. It can be seen in some of the later photos, but for most of the time, I did not use it. However, it worked well when starting or restarting a nearly empty stove, as it could direct a strong current of air some distance to where it was required. While it increased the air velocity, it reduced the airflow rate.

The experiments started with testing a pre-loaded micro simple-drum-stove. The stove had success, as I expected, with regard to easy, clean and hot burning. However, it failed to provide long protracted burning.

Fuel holding capacity. The stove could hold about 800g of dense hardwood sticks. On the basis of my previous stove experience, I expected it to burn about 400g/h and I thought that this load maybe would last ~2h.

Stove start-up. The start-up was easy, using some waxies and flame drizzler sticks. During the start-up, the stove smoked a little. It was not as much as most tent stoves do, but it was not up to my expectations. I put the air cone in place and this increased the amount of smoke.

Discussion. At first, this seems counterintuitive when you see the vague gentle burning below the open hole change to savage burning concentrated at the outlet of the cone. I think this can be explained by the cone increasing the air velocity. This meant that the air penetrated further into the fuel and made more smoke. Also, at the same time, the cone reduced the airflow so there was less air to completely burn the extra smoke. Consequently, I discontinued using it for regular running. “That was a quick end to a good idea.”

Immediately without the cone, the combustion was clean and hot. I could tell that the burn rate was so high that the anticipated 2 hour burn time was just a dream. I estimate that the burn would have lasted 0.5h without refuelling intervention. This was equivalent to 1600g/h which is about 4 times greater than my previous tent stoves.

“The heat that was given off from the substantial stove surface was exciting and was consistent with this estimation. The top half of the simple-barrel-stove reached 450C+ at times and seldom went below 350C. “This result was not leading to the ‘holey grail’, but it was an exciting lesson about powerful combustion.”

I then resorted to putting long and thick fuel stick in through the air hole to supplement the pre-packed fuel. With this configuration, the cone could not be used anyway.

Subsequently, I tested various modifications to the fuel feeding arrangements that would allow large long fuel sticks to continuously and somewhat automatically feed-in for protracted burning.

Experiment 2- Supplementing a pre-filled barrel-stove with a single large fuel stick

Stove refuelling. Adding large fuel sticks was an efficient way of adding fuel and it burnt well when amongst a bed of charcoal. However, with wet dense wood, that I would need to use for fuel in our alpine camping conditions, it would be likely that the charcoal could run low before the fuel stick collapsed into charcoal chunks to sustain the charcoal bed and allow the stick to move fresh wood down into the pyrolysis zone. This would eventually cause a burner stall.

Discussion. In the above photo and others, you will see my smallest cooking pot/mug (700ml capacity) beside the stove. While there was no safe place to put the pot on the stove, it heated up to hot drink temperatures even when sat beside the least hot part of the stove. This type of stove should have good cooking potential when designed to accommodate a pot in a hot spot.

Experiment 3- Supplementing a pre-filled barrel-stove with a large split fuel stick

Stove refuelling. From my previous experience, I have found that splitting thick fuel sticks makes them much more combustible, particularly when wet. It means that the stick can be converted to wood gas more rapidly and the resulting thinner stems of charcoal will collapse into charcoal chunks sooner.

More splits make the burning easier, but more splits are more difficult to make when camping. Also, with splitting, they become more splintery and less likely to feed automatically down through the fuel holes (Unlike the perfect selected ‘free-splitting’ sticks that are shown in the photos.).

Discussion. The splitting extended the burn time before they became ‘stranded’ without enough charcoal chunks to sustain wood pyrolysis. However, stranding was inevitable with solid damp fuel sticks with one split.

Experiment 4. A barrel-stove with two stick holes

Stove design. To address the shortcoming of the above stoves, I made another similar stove with two stick holes in the top. These holes were 40mm dia and were close together. This was done so that the two fuel sticks would be close together.

Discussion. This stove configuration worked well and only required a few small sticks and wood chips for starting. It did not have a smokey start-up phase as with the pre-loaded stove. It burnt well with very dry sticks but struggled with thick solid damp wood as described above.

The close proximity of the stick in this design means that the heat radiated radiating from one stick could contribute to the pyrolysis of the other. This improved the stove power and delay the stall time without the need for splitting when using damp fuel sticks.

It also meant that if long sticks were used for extending burn times, any stabilizing support for the upper part of the sticks could be shared by both sticks as shown in the photo below.

I also made some aluminium foil masks to restrict air entry when there was excessive space around the sticks.

In another mode of operation, the efficiency and convenience of a big solid fuel stick could be enjoyed if smaller chunks of wood or light sticks were added through the other hole to indefinitely sustain the combustion of the big long stick. This was both lazy and easy.

“I would consider this a great improvement, just by adding an extra hole and it justified the celebration with the cup of tea.”

Experiment 5. A barrel-stove with two stick holes and one hole cover

Stove design.“I was on a roll.” This is the same stove as in the above experiment, but it has a blanking plate for optional use on one of the fuel holes.

Discussion. The use of a blanking plate made the stove even more versatile. It could have a moderate burn rate (surface ~350-450C) with one stick and substantial supplementary chunks of wood or sticks could still be easily be added to sustain the long fuel sticks combustion without disturbing it.

Also, I noted, if I removed the cover plate the surface temperature dropped by about 100C. “This shows that more air is not always better, particularly if there is little or no smoke coming from the flue pipe. It just dilutes the hot gas and cools it down. Elsewhere, under the heading of ‘Metal protected by smoke?’, I also speculate that the extra oxygen can contribute to quicker destruction of the stove metal.

Experiment 6. A barrel-stove with a hole for one big stick and another for multiple sticks

Stove design. This is the same stove as in the above experiments but it was used with multiple sticks fed into the supplementary fuel port.

Discussion. In this stove configuration, the three thinner sticks will burn relatively quickly and sustain the pyrolysis of the large one so that it can collapse and more quickly contribute to the charcoal bed and a fresh wood load.

This configuration would be good when only damp wood is available. It eliminates or reduces the need for the tedium of cutting and splitting fuel sticks. It is also self-feeding.

“Breaking thinner sticks by hand is quite practical, within a tent, and breaking larger ones over the knee or in a forked stick means that the preparation can be ‘tool-free’.

Experiment 7. A barrel-stove with a big hole for one big stick and other smaller sticks

Stove design. So far, experiments 2-6 in this series have demonstrated incremental improvement in the robustness of the stoves fueling and burn maintenance using two fuel holes. Just to test the limit with the fuel stick holes, I made a stove with a 50 and 40mm diameter holes that overlaped to form an Avacado shape. “I can just hear some of you saying where is this all going….we are back to one hole again?”

Discussion. While the above design worked, it was quite difficult to cover the ‘free air opening’ and the big stick was hard to keep in the optimum position as it had periodic big breakaways of the supporting charcoal column.

I concluded that discrete multiple holes was a better option and I would not rule out the use of 3 or more holes if it left an adequate cooking space on the stovetop.

General discussion and conclusion

Barrel stove scale down. The downdraft barrel stove can perform well when scaled down to a ‘small tent’ size when made from a coffee tin. It provides simple, clean, efficient and robust combustion to give high surface temperatures of about 450C. This concurs with my recollection of the Ski Chalet stove (Although it did not get that hot, probably because of the refractory lining). The coffee tin stove reached similar temperature to my shed stove made from a commercial milk can (without refractory lining).

No ‘holy grail’. An ‘all-night-stove’ was not found. However, there were some valuable lessons about designing better stoves that could burn for longer with less effort. They could be; simple, self-regulating, and have robust combustion. Generous heat could be produced with simple ‘tool-free’ fuel stick preparation without problematic reverse burning. “Just chuck the fuel in the right place and let the stove do the rest.”

Refuelling on-the-fly works. The stove could be kept going indefinitely with one large stick that was supplemented with the strategic addition of small sticks through a supplementary fuel hole. These supplementary sticks could be added as chunks or as a bundle of small long sticks without any need to disturb the large stick.

Stove cleaning and storage. This is important for all stoves whether they are made of titanium, stainless steel or tin plate. Rusting or slow oxidation is the enemy of most stoves. Consequently, cleaning, coating with a protective film of oil and dry storage is important for stove survival.

Vanishing simple barrel stove (downdraft) technology. I am perplexed as to why simple-barrel-stoves are not more prominent in the tent stove zeitgeist on the web. Their simplicity of construction and operation and their ability to provide instant self-regulated combustion heat from a range of fuels (‘anything that will burn’) is outstanding. Have they just been gazumped by ‘rocket stoves? If you have any ideas on this I would appreciate receiving your comments.

Tim

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Addendum 1

In the above experiments, I did not do a test to see if the stove could burn a single block of wood. I got sidetracked with finding ways to extend the burn by adding extra fuel sticks. So I have added experiment 8.

Experiment 8. A barrel-stove burning one big block of wood

I cut a portion of a branch that would just fit into the coffee tin. It weighed 660g and I was surprised that it was not heavier, but part of it was a little rotten.

Starting the stove was a little difficult and I used several fire starters and some small chips of wood to establish a good burn. The burn was not as hot as with multiple sticks, but the surface temperature 250-350C would be acceptable as an overnight tent warmer. The burn lasted over an hour and by 1.5h I started supplementing the fuel with some sticks to maintain the temperature.

The small air cone did not work effectively when the woodblock burnt down. I substituted 20mm dia air tube that was loosely fitted through an aluminium foil disk that covered one of the 40mm dia fuel holes. The tube slipped downwards into a crater was burnt into the block. The second air hole was covered to make more air enter the air tube. The cover could easily be lifted if supplementary fuel sticks were required.

The burning of the single block was possible and the rate of combustion was much lower than my other stoves, but so was the heating temperature. As the old saying goes, “You don’t get nuffin for nuffin”. Even if the burn rate and temperature were acceptable, cutting precise blocks would not be easy and refuelling would be very impractical in a small tent.

Tim

Addendum 2

This stove investigation has eventually inspired the development of my very simple KISS Tent Stove. It is not an ‘overnight-stove’ but it has stellar performance and makes a very sweet winter backpacking load.

Tent stove- a tiny backpacking stove